The Facts On File

DICTIONARYof

ORGANIC CHEMISTRY

The Facts On File

DICTIONARYof

ORGANIC CHEMISTRY

Edited byJohn Daintith

®

The Facts On File Dictionary of Organic Chemistry

Copyright © 2004 by Market House Books Ltd

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Library of Congress Cataloging-in-Publication Data

The Facts on File dictionary of organic chemistry / edited by John Daintith.p. cm.

Includes bibliographical references.ISBN 0-8160-4928-9 (alk. paper).1. Chemistry—Dictionaries. I. Title: Dictionary of organic chemistry. II. Daintith,

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CONTENTS

Preface vii

Entries A to Z 1

Appendixes

I. Carboxylic Acids 233

II. Amino Acids 235

III. Sugars 238

IV. Nitrogenous Bases and

Nucleosides 239

V. The Chemical Elements 241

VI. The Periodic Table 243

VII. The Greek Alphabet 244

VIII. Fundamental Constants 245

IX. Webpages 246

Bibliography 247

vii

PREFACE

This dictionary is one of a series covering the terminology and concepts usedin important branches of science. The Facts on File Dictionary of OrganicChemistry has been designed as an additional source of information for stu-dents taking Advanced Placement (AP) Science courses in high schools. Itwill also be helpful to older students taking introductory college courses.

This volume covers organic chemistry and includes basic concepts, classes ofcompound, reaction mechanisms, and important named organic com-pounds. In addition, we have included a number of compounds that are im-portant in biochemistry, as well as information on certain key biochemicalpathways. The definitions are intended to be clear and informative and,where possible, we have illustrations of chemical structures. The book alsohas a selection of short biographical entries for people who have made im-portant contributions to the field. There are a number of appendixes, in-cluding structural information on carboxylic acids, amino acids, sugars, andnitrogenous bases and nucleosides. There is also a list of all the chemical el-ements and a periodic table. The appendixes also include a short list of use-ful webpages and a bibliography.

The book will be a helpful additional source of information for anyonestudying the AP Chemistry course, especially the section on DescriptiveChemistry. It will also be useful to students of AP Biology.

ACKNOWLEDGMENTS

Contributors

John O. E. Clark B.Sc.Richard Rennie B.Sc., Ph.D.

ABA See abscisic acid.

abscisic acid (ABA) A PLANT HORMONE

once thought to be responsible for theshedding (abscission) of flowers and fruitand for the onset of dormancy in buds(hence its early name, dormin). The com-pound is associated with the closing ofpores (stoma) in the leaves of plants de-prived of water.

absolute alcohol Pure alcohol (ethanol).

absolute configuration A particularmolecular configuration of a CHIRAL mol-ecule, as denoted by comparison with a ref-erence molecule or by some sequence rule.There are two systems for expressing ab-solute configuration in common use: theD–L convention and the R–S convention.See optical activity.

absolute temperature Symbol: T Atemperature defined by the relationship:

T = θ + 273.15where θ is the Celsius temperature. The ab-solute scale of temperature was a funda-mental scale based on Charles’ law, whichapplies to an ideal gas:

V = V0(1 + αθ)where V is the volume at temperature θ, V0the volume at 0, and α the thermal expan-sivity of the gas. At low pressures (wherereal gases show ideal behavior) α has thevalue 1/273.15. Therefore, at θ = –273.15the volume of the gas theoretically be-comes zero. In practice substances becomesolids at these temperatures; however, theextrapolation can be used for a scale oftemperature on which –273.15°C cor-responds to 0° (absolute zero). The scale isalso known as the ideal-gas scale; on it

temperature intervals were called degreesabsolute (°A) or degrees Kelvin (°K), andwere equal to the Celsius degree. It can beshown that the absolute temperature scaleis identical to the currently used thermody-namic temperature scale (on which the unitis the KELVIN).

absolute zero The zero value of ther-modynamic temperature; 0 kelvin or–273.15°C. See absolute temperature.

absorption 1. A process in which a gasis taken up by a liquid or solid, or in whicha liquid is taken up by a solid. In absorp-tion, the substance absorbed goes into thebulk of the material. Solids that absorbgases or liquids often have a porous struc-ture. The absorption of gases in solids issometimes called sorption. There is a dis-tinction between absorption (in which onesubstance is assimilated into the bulk of an-other) and ADSORPTION (which involves at-tachment to the surface). Sometimes it isnot obvious which process is occurring.For example, a porous solid, such as acti-vated CHARCOAL may be said to absorb alarge volume of gas, but the process mayactually be adsorption on the high surfacearea of internal pores in the material.2. The process in which electromagneticradiation, particles, or sound waves loseenergy in passing through a medium. Ab-sorption involves conversion of one formof energy into another.

absorption spectrum See spectrum.

accelerator A substance that increasesthe rate of a chemical reaction. In this sensethe term is synonymous with CATALYST. Itis common to refer to catalysts as ‘acceler-

1

A

acceptor

2

ators’ in certain industrial applications.For example, accelerators are used in theVULCANIZATION of rubber and in the poly-merization of adhesives. Also, in the pro-duction of composite materials usingpolyester resins a distinction is sometimesmade between the catalyst (which initiatesthe polymerization reaction) and the accel-erator (which is an additional substancemaking the catalyst more effective). Theterms promoter and activator are used in asimilar way.

acceptor The atom or group to which apair of electrons is donated in forming aCOORDINATE BOND.

accessory pigment See photosyntheticpigments.

acenaphthene (C12H10) A colorless crys-talline derivative of naphthalene, used inproducing some dyes.

acetal A type of compound formed byreaction of an alcohol with either an al-dehyde or a ketone. The first step in for-mation of an acetal is the formation of an intermediate, known as a hemiacetal.For example, ethanal (acetaldehyde;CH3CHO) reacts with ethanol (C2H5OH)as follows:

CH3CHO + C2H5OH ˆCH(OH)(CH3)(C2H5O)

The hemiacetal has a central carbon atom(from the aldehyde) attached to a hydro-gen, a hydroxyl group, a hydrocarbongroup (CH3), and an alkoxy group(C2H5O). If a ketone is used rather than analdehyde, the resulting hemiacetal containstwo hydrocarbon groups. For example, re-action of the ketone R1COR2 with the al-cohol R3OH is:

R1COR2 + R3OH ˆ CR1R2(OH)(OR3)The formation of a hemiacetal is an exam-ple of NUCLEOPHILIC ADDITION to the car-bonyl group of the aldehyde or ketone. Thefirst step is attack of the lone pair on the Oof the alcohol on the (positively charged) Cof the carbonyl group. This is catalyzed byboth acids and bases. Acid catalysis occursby protonation of the O on the carbonyl,making the C more negative and more sus-ceptible to nucleophilic attack. In basecatalysis the OH– ions from the base affectthe –OH group of the alcohol, making it amore effective nucleophile.

In general, hemiacetals exist only in so-lution and cannot be isolated because theyeasily decompose back to the componentalcohol and aldehyde or ketone. However,some cyclic hemiacetals are more stable.For example, cyclic forms of SUGAR mol-ecules are hemiacetals.

Further reaction of hemiactals with an-other molecule of alcohol leads to a fullacetal. For example:

CH(OH)(CH3)(OC2H5) + C2H5OH ˆCH(CH3)(OC2H5)2

The overall reaction of an aldehyde or ke-tone with an alcohol to give an acetal canbe written:

R1COR2 + R3OH ˆ CR1R2(OR3)2It is also possible to have ‘mixed’ acetalswith the general formula CR1R2(OR3)-(OR4). Note that if the acetal is derivedfrom an aldehyde, then R1 and/or R2 maybe a hydrogen atom. The mechanism offormation of an acetal from a hemiacetal isacid catalyzed. It involves protonation ofthe –OH group of the hemiacetal followedby loss of water to form an oxonium ion,which is attacked by the alcohol molecule.

Formerly it was conventional to use theterms ‘hemiacetal’ and ‘acetal’ for com-pounds formed by reaction between alde-hydes and alcohols. Similar reactionsbetween ketones and alcohols gave rise tocompounds called hemiketals and ketals.Current nomenclature uses ‘hemiacetal’and ‘acetal’ for compounds derived fromeither an aldehyde or a ketone, but reserves‘hemiketal’ and ‘ketal’ for those derivedfrom ketones. In other words, the ketalsare a subclass of the acetals and the

1 2

Acenaphthene

hemiketals are a subclass of the hemiac-etals.

acetaldehyde See ethanal.

acetamide See ethanamide.

acetate See ethanoate.

acetic acid See ethanoic acid.

acetone See propanone.

acetonitrile See methyl cyanide.

acetophenone See phenyl methyl ke-tone.

acetylation See acylation.

acetyl chloride See ethanoyl chloride.

acetylcholine (ACh) A neurotransmit-ter found at the majority of synapses,which occur where one nerve cell meets an-other.

acetylene See ethyne.

acetyl group See ethanoyl group.

acetylide See carbide.

acetyl CoA (acetyl coenzyme A) An im-portant intermediate in cell metabolism,

particularly in the oxidation of sugars,fatty acids, and amino acids, and in certainbiosynthetic pathways. It is formed by thereaction between pyruvate (from GLYCOLY-SIS) and COENZYME A, catalyzed by the en-zyme pyruvate dehydrogenase. The acetylgroup of acetyl CoA is subsequently oxi-dized in the KREBS CYCLE, to yield reducedcoenzymes and carbon dioxide. AcetylCoA is also produced in the initial oxida-tion of fatty acids and some amino acids.Other key roles for acetyl CoA include theprovision of acetyl groups in biosynthesisof fatty acids, terpenoids, and other sub-stances.

acetyl coenzyme A See acetyl CoA.

acetylsalicylic acid See aspirin.

ACh See acetylcholine.

achiral Describing a molecule that doesnot have chiral properties; i.e. one thatdoes not exhibit OPTICAL ACTIVITY.

acid A substance than contains hydro-gen and dissociates in solution to give hy-drogen ions:

HA ˆ H+ + A–

More accurately, the hydrogen ion is sol-vated (a hydroxonium ion):

HA + H2O ˆ H3O+ + A–

Strong acids are completely dissociated inwater. Examples are sulfuric acid and tri-

3

acid

- -

--

NH

S

O

CH3

OHO

P OO

O

HO

O

ONH

CH3H3C

OO

OO

O P O P O CH2 O

NH2

N

NN

N

Acetyl CoA

choloroethanoic acid. Weak acids are onlypartially dissociated. Most organic car-boxylic acids are weak acids. In distinctionto an acid, a base is a compound that pro-duces hydroxide ions in water. Bases are either ionic hydroxides (e.g. NaOH) orcompounds that form hydroxide ions inwater. These may be metal oxides, for ex-ample:

Na2O + H2O → 2Na+ + 2OH–

Ammonia, amines, and other nitrogenouscompounds can also form OH– ions inwater:

NH3 + H2O ˆ NH4+ + OH–

As with acids, strong bases are completelydissociated; weak bases are partially disso-ciated.

This idea of acids and bases is known asthe Arrhenius theory (named for theSwedish physical chemist Svante AugustArrhenius (1859–1927)).

In 1923 the Arrhenius idea of acids andbases was extended by the British chemistThomas Martin Lowry (1874–1936) and,independently, by the Danish physicalchemist Johannes Nicolaus Brønsted(1879–1947). In the Lowry–Brønstedtheory an acid is a compound that can do-nate a proton and a base is a compoundthat can accept a proton. Proton donatorsare called Brønsted acids (or protic acids)and proton acceptors are called Brønstedbases. For example, in the reaction:

CH3COOH + H2O ˆ CH3COO– +H3O+

the CH3COOH is the acid, donating a pro-ton H+ to the water molecule. The water isthe base because it accepts the proton. Inthe reverse reaction, the H3O+ ion is theacid, donating a proton to the baseCH3COO–. If two species are related byloss or gain or a proton they are describedas conjugate. So, in this example,CH3COO– is the conjugate base of the acidCH3COOH and CH3COOH is the conju-gate acid of the base CH3COO–.

In a reaction of an amine in water, forexample:

R3N + H2O ˆ R3NH+ + OH–

The amine R3N accepts a proton fromwater and is therefore acting as a base.R3NH+ is its conjugate acid. Water donatesthe proton to the R3N and, in this case,

water is acting as an acid (H3O+ is its con-jugate base). Note that water can act asboth an acid and a base depending on thecircumstances. It can accept a proton (fromCH3COOH) and donate a proton (toR3N). Compounds of this type are de-scribed as amphiprotic.

One important aspect of theLowry–Brønsted theory is that, because itinvolves proton transfers, it does not nec-essarily have to involve water. It is possibleto describe reactions in nonaqueous sol-vents, such as liquid ammonia, in terms ofacid–base reactions.

A further generalization of the idea ofacids and bases was the Lewis theory putforward, also in 1923, by the US physicalchemist Gilbert Newton Lewis (1875–1946). In this, an acid (a Lewis acid) is acompound that can accept a pair of elec-trons and a base (a Lewis base) is one thatdonates a pair of electrons. In a traditionalacid–base reaction, such as:

HCl + NaOH → NaCl + H2Othe effective reaction is

H+ + OH– → H2OThe OH– (base) donates an electron pair tothe H+ (acid). However, in the Lewistheory acids and bases need not involveprotons at all. For example, ammonia(NH3) adds to boron trichloride (BCl3) toform an adduct:

NH3 + BCl3 → H3NBCl3Here, ammonia is the Lewis base donatinga LONE PAIR of electrons to boron trichlor-ide (the Lewis acid).

The concept of acid–base reactions is animportant generalization in chemistry, andthe Lewis theory connects it to two othergeneral ideas. One is oxidation–reduction:oxidation involves loss of electrons and re-duction involves gain of electrons. Also, inorganic chemistry, it is connected with theidea of electrophile–nucleophile reactions.Acids are ELECTROPHILES and bases are NU-CLEOPHILES. In organic chemistry a numberof inorganic halides, such as AlCl3 andTiCl4, are important Lewis acids, formingintermediates in such processes as theFRIEDEL–CRAFTS REACTION.

acid anhydride A type of organic com-pound containing the group –CO.O.CO–.

acid anhydride

4

Simple acid anhydrides have the generalformula RCOOCOR′, where R and R′ arealkyl or aryl groups. They can be regardedas formed by removing a molecule of waterfrom two molecules of carboxylic acid. Forexample, ethanoic anhydride comes fromethanoic acid:

2CH3COOH – H2O →CH3CO.O.COCH3

A long-chain dicarboxylic acid may alsoform a cyclic acid anhydride, in which the–CO.O.CO– group forms part of a ring.Acid anhydrides can be prepared by reac-tion of an acyl halide with the sodium saltof a carboxylic acid, e.g.:

RCOCl + R′COO–Na+ →RCOOCOR′ + NaCl

Like the acyl halides, they are very reactiveacylating agents. They hydrolyze readily tocarboxylic acids:

RCOOCOR′ + H2O →RCOOH + R′COOH

See also acylation; anhydride.

acid dyes The sodium salts of organicacids used in the dyeing of silk and wool.They are so called because they are appliedfrom a bath acidified with dilute sulfuric orethanoic acid.

acid halide See acyl halide.

acidic Having a tendency to release aproton or to accept an electron pair from adonor. In aqueous solutions the pH is ameasure of the acidity, i.e. an acidic solu-tion is one in which the concentration ofH3O+ exceeds that in pure water at thesame temperature; i.e. the pH is lower than7. A pH of 7 indicates a neutral solution.

acidic hydrogen A hydrogen atom in amolecule that enters into a dissociationequilibrium when the molecule is dissolvedin a solvent. For example, in ethanoic acid(CH3COOH) the acidic hydrogen is theone on the carboxyl group, –COOH:

CH3COOH + H2O ˆCH3COO– + H3O+.

acidity constant See dissociation con-stant.

acid value A measure of the free acidpresent in fats, oils, resins, plasticizers, andsolvents, defined as the number of mil-ligrams of potassium hydroxide requiredto neutralize the free acids in one gram ofthe substance.

acridine (C12H9N) A colorless crys-talline heterocyclic compound with threefused rings. Derivatives of acridine areused as dyes and biological stains.

Acrilan (Trademark) A synthetic fiberthat consists of a copolymer of 1-cyanoethene (acrylonitrile; vinyl cyanide)and ethenyl ethanoate (vinyl acetate). Seeacrylic resin.

acrolein See propenal.

acrylic acid See propenoic acid.

acrylic resin A synthetic resin made bypolymerizing an amide, nitrile, or ester de-rivative of 2-propenoic acid (acrylic acid).Acrylic resins (known as ‘acrylics’) areused in a variety of ways. A common ex-ample is poly(methylmethacrylate), whichis produced by polymerizing methylmethacrylate, CH2:CH(CH3)COOCH3.This is the clear material sold as Plexiglas.Another example is the compound methyl2-cyanoacrylate, CH2:CH(CN)COOCH3.This polymerizes very readily in air and isthe active constituent of ‘superglue’. Inboth these cases there is a double C=Cbond conjugated with the carbonyl C=Obond and the polymerization has a free-radical mechanism. The free election is onthe carbon atom next to the carbonylgroup, which stabilizes the radical. An-other example of an acrylic polymer isformed by free-radical polymerization of

5

acrylic resin

NNN

Acridine

acrylonitrile (CH2:CHCN) to give poly-(acrylonitrile). This is used in syntheticfibers (such as Acrilan). In this case the un-paired electron is on the carbon next to the–CN group. Acrylic resins are also used inpaints.

acrylonitrile See propenonitrile.

actinic radiation Radiation that cancause a chemical reaction; for example,ultraviolet radiation is actinic.

actinomycin Any of a number of antibi-otics produced by certain bacteria. Themain one, actinomycin D (or dactino-mycin), can bind between neighbouringbase pairs in DNA, preventing RNA syn-thesis. It is used in the treatment of somecancers.

action spectrum A graph showing theeffect of different wavelengths of radia-tion, usually light, on a given process. It is often similar to the ABSORPTION SPEC-TRUM of the substance that absorbs the ra-diation and can therefore be helpful inidentifying that substance. For example,the action spectrum of photosynthesis issimilar to the absorption spectrum ofchlorophyll.

activated charcoal See charcoal.

activated complex The partially bondedsystem of atoms in the TRANSITION STATE ofa chemical reaction.

activation energy Symbol: Ea The min-imum energy a system must acquire beforea chemical reaction can occur, regardlessof whether the reaction is exothermic orendothermic. Activation energy is oftenrepresented as an energy barrier that has tobe overcome if a reaction is to take place.See also Arrhenius equation; transitionstate.

activator See accelerator.

active mass See mass action.

active site 1. A site on the surface of a

solid catalyst at which catalytic activity oc-curs or at which the catalyst is particularlyeffective.2. The region of an ENZYME molecule thatcombines with and acts on the substrate. Itconsists of catalytic amino acids arrangedin a configuration specific to a particularsubstrate or type of substrate. The onesthat are in direct combination are the con-tact amino acids. Other amino acids maybe further away but still play a role in theaction of the enzyme. These are auxilliaryamino acids. Binding of a regulatory com-pound to a separate site, known as the AL-LOSTERIC SITE, on the enzyme molecule maychange this configuration and hence the ef-ficiency of the enzyme activity.

activity 1. Symbol: a Certain thermody-namic properties of a solvated substanceare dependent on its concentration (e.g. itstendency to react with other substances).Real substances show departures fromideal behavior and a corrective concentra-tion term – the activity – has to be intro-duced into equations describing realsolvated systems.2. Symbol: A The average number of atomsdisintegrating per unit time in a radioactivesubstance.

activity coefficient Symbol: f A meas-ure of the degree of deviation from idealityof a dissolved substance, defined as:

a = fc

acrylonitrile

6

ener

gy

A ... B ... C

AB C

AB C

H

reaction coordinate

Activation energy

where a is the activity and c the concentra-tion. For an ideal solute f = 1; for real sys-tems f can be less or greater than unity.

acyclic Describing a compound that isnot cyclic (i.e. a compound that does notcontain a ring in its molecules).

acyl anhydride See acid anhydride.

acylating agent See acylation.

acylation Any reaction that introducesan acyl group (RCO–) into a compound.Acylating agents are compounds such asacyl halides (RCOX) and acid anhydrides(RCOOCOR), which react with such nu-cleophiles as H2O, ROH, NH3, andRNH2. In these reactions a hydrogen atomof a hydroxyl or amine group is replacedby the RCO– group. In acetylation theacetyl group (CH3CO–) is used. In benzoy-lation the benzoyl group (C6H5CO–) isused. Acylation is used to prepare crys-talline derivatives of organic compounds toidentify them (e.g. by melting point) andalso to protect –OH groups in synthetic re-actions.

acyl group The group of atoms RCO–.

acyl halide (acid halide) A type of or-ganic compound of the general formulaRCOX, where X is a halogen (acyl chlo-ride, acyl bromide, etc.).

Acyl halides can be prepared by the re-action of a carboxylic acid with a halo-genating agent. Commonly, phosphorushalides are used (e.g. PCl5) or a sulfur di-halide oxide (e.g. SOCl2):

RCOOH + PCl5 → RCOCl + POCl3 + HCl

RCOOH + SOCl2 → RCOCl + SO2 + HCl

The acyl halides have irritating vaporsand fume in moist air. They are very reac-tive to the hydrogen atom of compoundscontaining hydroxyl (–OH) or amine(–NH2) groups. For example, the acylhalide ethanoyl chloride (acetyl chloride;CH3COCl) reacts with water to give a car-boxylic acid (ethanoic acid):

CH3COCl + H2O → CH3COOH + HClWith an alcohol (e.g. ethanol) it gives anester (ethyl ethanoate):

CH3COCl + C2H5OH →CH3COOC2H5 + HCl

With ammonia it gives an amide(ethanamide; acetamide):

CH3COCl + NH3 →CH3CONH2 + HCl

With an amine (e.g. methylamine) it givesan N-substituted amine (N-methyl ethana-mide)

CH3COCl + CH3NH2 →CH3CONH(CH3)

See also acylation.

addition polymerization See polymer-ization.

addition reaction A reaction in whichadditional atoms or groups of atoms are in-troduced into an unsaturated compound,such as an alkene, alkyne, aldehyde, or ke-tone. A simple example is the addition ofbromine across the double bond in ethene:

H2C:CH2 + Br2 → BrH2CCH2BrAddition reactions can occur by additionof electrophiles or nucleophiles. See elec-trophilic addition; nucleophilic addition.

adduct See coordinate bond.

adenine A nitrogenous base found inDNA and RNA. It is also a constituent ofcertain coenzymes, and when combinedwith the sugar ribose it forms the nucleo-side adenosine found in AMP, ADP, andATP. Adenine has a purine ring structure.See also DNA.

adenosine (adenine nucleoside) A NU-CLEOSIDE formed from adenine linked to D-ribose with a β-glycosidic bond. It is widelyfound in all types of cell, either as the freenucleoside or in combination in nucleic

7

adenosine

N

N N

N

NH2

H

71

3

6

9

Adenine

acids. Phosphate esters of adenosine, suchas ATP, are important carriers of energy inbiochemical reactions.

adenosine diphosphate See ADP.

adenosine monophosphate See AMP.

adenosine triphosphate See ATP.

adiabatic change A change for whichno energy enters or leaves the system. In anadiabatic expansion of a gas, mechanicalwork is done by the gas as its volume in-creases and the gas temperature falls. Foran ideal gas undergoing a reversible adia-batic change it can be shown that

pVγ = K1Tγp1–γ = K2

and TVγ–1 = K3where K1, K2, and K3 are constants and γ isthe ratio of the principal specific heat ca-pacities. Compare isothermal change.

adipic acid See hexanedioic acid.

adjacent Designating atoms or bondsthat are next to each other in a molecule.

ADP (adenosine diphosphate) A nu-cleotide consisting of adenine and ribosewith two phosphate groups attached. Seealso ATP.

adrenalin See epinephrine.

adsorbate A substance that is adsorbedon a surface. See adsorption.

adsorbent Having a tendency to adsorb.As a noun the adsorbent is the substance

on which adsorption takes place. See ad-sorption.

adsorption A process in which a layerof atoms or molecules of one substanceforms on the surface of a solid or liquid. Allsolid surfaces take up layers of gas from thesurrounding atmosphere. The adsorbedlayer may be held by chemical bonds(chemisorption) or by weaker van derWaals forces (physisorption).Compare absorption.

aerobic Describing a biochemical processthat takes place only in the presence of freeoxygen. Compare anaerobic.

aerobic respiration (oxidative metabo-lism) Respiration in which free oxygen isused to oxidize organic substrates to car-bon dioxide and water, with a high yield ofenergy. Carbohydrates, fatty acids, and ex-cess amino acids are broken down yieldingacetyl CoA and the reduced coenzymesNADH and FADH2. The acetyl coenzymeA enters a cyclic series of reactions, theKREBS CYCLE, with the production of car-bon dioxide and further molecules ofNADH and FADH2. NADH and FADH2are passed to the ELECTRON-TRANSPORT

CHAIN (involving cytochromes and flavo-proteins), where they combine with atomsof free oxygen to form water. Energy re-leased at each stage of the chain is used toform ATP during a coupling process. Thesubstrate is completely oxidized and thereis a high energy yield. There is a net pro-duction of 38 ATPs per molecule of glucoseduring aerobic respiration, a yield of about19 times that of anaerobic respiration.Aerobic respiration is therefore the pre-ferred mechanism of the majority of organ-isms. See also oxidative phosphorylation;respiration.

aerosol See sol.

affinity The extent to which one sub-stance reacts with another in a chemicalchange.

afterdamp See firedamp.

adenosine diphosphate

8

N

N N

N

NH2

OHOCH2

OH OH

Adenosine

agent orange A herbicide consisting ofa mixture of two weedkillers (2,4-D and2,4,5-T). It was designed for use in chemi-cal warfare to defoliate trees in areas wherean enemy may be hiding or to destroyenemy crops. Agent orange, so-called fromthe orange-colored canisters in which itwas supplied, was first used by US forcesduring the Vietnam war. It contains tracesof the highly toxic chemical DIOXIN, whichcauses cancers and birth defects.

air gas See producer gas.

alanine See amino acid.

albumen The white of an egg, whichconsists mainly of the protein ALBUMIN.

albumin A soluble protein that occurs inmany animal fluids, such as blood serumand egg white.

alcohol A type of organic compound ofthe general formula ROH, where R is a hy-drocarbon group. Examples of simple alco-hols are methanol (CH3OH) and ethanol(C2H5OH). Alcohols have the –OH groupattached to a carbon atom that is part of analkyl group. If the carbon atom is part of

9

alcohol

primary (ethanol)

secondary (propan-2-ol)

tertiary (2-methylpropan-2-ol)

C

H OH

HCH3

C

H3C OH

HH3C

C

H3C OH

CH3H3C

Alcohol

glucose

glycolysis2 ATP

38 ATP

34 ATP

respiratory chainH2O

½O2

H

2 ATP

Krebscycle

CO2

acetyl CoA

pyruvate

Aerobic respiration

an aromatic ring, as in PHENOL, C6H5OH,the compound does not have the character-istic properties of alcohols. Phenyl-methanol (C6H5CH2OH) does have thecharacteristic properties of alcohols (in thiscase the carbon atom to which the –OH isattached is not part of the aromatic ring).

Alcohols can have more than one –OHgroup; those containing two, three, ormore such groups are described as dihy-dric, trihydric, and polyhydric respectively(as opposed to alcohols containing one–OH group, which are monohydric). Forexample, ethane-1,2-diol (ethylene glycol;(HOCH2CH2OH) is a dihydric alcoholand propane-1,2,3-triol (glycerol;HOCH2CH(OH)CH2OH) is a trihydric al-cohol. Dihydric alcohols are known asdiols; trihydric alcohols as triols, etc. Ingeneral, alcohols are named by using thesuffix -ol with the name of the parent hy-drocarbon.

Alcohols are further classified accord-ing to the environment of the –C–OHgrouping. If the carbon atom is attached totwo hydrogen atoms, the compound is aprimary alcohol. If the carbon atom is at-tached to one hydrogen atom and twoother groups, it is a secondary alcohol. Ifthe carbon atom is attached to three othergroups, it is a tertiary alcohol. Alcohols canbe prepared by:1. Hydrolysis of haloalkanes using aque-

ous potassium hydroxide:RI + OH– → ROH + I–

2. Reduction of aldehydes by nascent hy-drogen (e.g. from sodium amalgam inwater):

RCHO +2[H] → RCH2OHThe main reactions of alcohols are:1. Oxidation by potassium dichromate(VI)

in sulfuric acid. Primary alcohols givealdehydes, which are further oxidized tocarboxylic acids:

RCH2OH → RCHO → RCOOH1. Secondary alcohols are oxidized to ke-

tones.R1R2CHOH → R1R2CO

2. Formation of esters with acids. The re-action, which is reversible, is catalyzedby H+ ions:

ROH + R′COOH ˆR′COOR + H2O

3. Dehydration over hot pumice (400°C)to alkenes:

RCH2CH2OH – H2O → RCH:CH24. Reaction with sulfuric acid. Two types

of reaction are possible. With excessacid at 160°C dehdyration occurs togive an alkene:

RCH2CH2OH + H2SO4 →H2O + RCH2CH2.HSO4

RCH2CH2.HSO4 →RCH:CH2 + H2SO4

4. With excess alcohol at 140°C an ether isformed:

2ROH → ROR + H2OSee also acetal; acylation; Grignardreagent.

aldaric acid See sugar acid.

aldehyde A type of organic compoundwith the general formula RCHO, wherethe –CHO group (the aldehyde group) con-sists of a carbonyl group attached to a hy-drogen atom. Simple examples ofaldehydes are methanal (formaldehyde;HCHO) and ethanal (acetaldehyde; CH3-CHO).

Aldehydes are formed by oxidizing aprimary alcohol; in the laboratory potas-sium dichromate(VI) is used in sulfuricacid. They can be further oxidized to car-boxylic acids. Reduction (using a catalystor nascent hydrogen from sodium amal-gam in water) produces the parent alcohol.For example, oxidation of ethanol(C2H5OH) gives ethanal (acetaldehyde;CH3CHO):

C2H5OH + [O] → CH3CHO + H2OFurther oxidation gives ethanoic acid(acetic acid; CH3COOH):

CH3CHO + [O] → CH3COOHThe systematic method of naming alde-hydes is to use the suffix -al with the

aldaric acid

10

aldehyde group

CR

O

H

Aldehyde

name of the parent hydrocarbon. For ex-ample: methane (CH4) is the parent hydro-carbon of the alcohol methanol (CH3OH),the aldehyde methanal (HCHO), and the carboxylic acid methanoic acid(HCOOH); ethane (C2H6) is the parent hydrocarbon of the alcohol ethanol (C2H5-OH), the aldehyde ethanal (CH3CHO),and the carboxylic acid ethanoic acid(CH3COOH); etc. An older method ofnaming aldehydes is based on the name ofthe related acid. For example, methanoicacid (HCOOH) has the traditional name‘formic acid’ and the related aldehyde(HCHO) is traditionally called ‘formalde-hyde’. Similarly, ethanoic acid (CH3OOH)is commonly known as ‘acetic acid’ and thealdehyde CH3CHO is known as ‘acetalde-hyde’.Reactions of aldehydes are:1. Aldehydes are reducing agents, being

oxidized to carboxylic acids in theprocess. These reactions are used as testsfor aldehydes using such reagents asFEHLING’S SOLUTION and TOLLEN’SREAGENT (silver-mirror test).

2. They form addition compounds withhydrogen cyanide to give cyanohydrins.For example, propanal gives 2-hydroxy-butanonitrile:

C2H5CHO + HCN →C2H5CH(OH)CN

3. They form bisulfite addition compoundswith the hydrogensulfite(IV) ion (bisul-fite; HSO3

–):RCHO + HSO3

– → RCH(OH)(HSO3)4. They undergo condensation reactions

with such compounds as hydrazine, hy-droxylamine, and their derivatives.

5. With alcohols they form hemiacetalsand ACETALS.

6. Simple aldehydes polymerize readily.Polymethanal or methanal trimer can beformed from METHANAL depending onthe conditions. ETHANAL gives ethanaltrimer or ethanal tetramer.

See also Cannizzaro reaction; condensa-tion reaction; ketone.

Alder, Kurt (1902–1958) German or-ganic chemist who is noted for the processknown as the DIELS–ALDER REACTION. Par-ticular cases of the reaction had been

known since the 1900s but Alder and OttoDiels recognized that this mechanism isvery common. They first reported their re-sults in 1928. Alder and Diels shared the1950 Nobel Prize for chemistry for thiswork.

alditol See sugar alcohol.

aldohexose An aldose SUGAR with sixcarbon atoms.

aldol A compound that contains both analdehyde group (–CHO) and an alcoholgroup (–OH). See aldol reaction.

aldol reaction A reaction in which twomolecules of aldehyde combine to give analdol – i.e. a compound containing bothaldehyde and alcohol functional groups.The reaction is base-catalyzed; the reactionof ethanal (acetaldehyde) refluxed withsodium hydroxide gives:

2CH3CHO → CH3CH(OH)CH2CHOThe mechanism is similar to that of theCLAISEN CONDENSATION: the first step is re-moval of a proton to give a carbanion,which subsequently attacks the carbon ofthe carbonyl group on the other molecule:

CH3CHO + OH– → –CH2CHO + H2OCH3CHO + –CH2CHO →CH3CH(OH)CH2CHO.

aldonic acid See sugar acid.

aldopentose An aldose SUGAR with fivecarbon atoms.

aldose A SUGAR containing an aldehydegroup (CHO) or a potential aldehydegroup.

algin See alginic acid.

alginic acid (algin; (C6H8O6)n) A yel-low-white organic solid that is found inbrown algae. It is a complex polysaccha-ride and produces, in even very dilute solu-tions, a viscous liquid. Alginic acid hasvarious uses, especially in the food industryas a stabilizer and texture agent.

11

alginic acid

alicyclic compound An aliphatic cycliccompound, such as cyclohexane or cyclo-propane.

aliphatic compound An organic com-pound with properties similar to those ofthe alkanes, alkenes, and alkynes and theirderivatives. Most aliphatic compoundshave an open chain structure but some,such as cyclohexane and sucrose, haverings (these are described as alicyclic). The term is used in distinction to ARO-MATIC COMPOUNDS, which are similar tobenzene.

alizarin (1,2-dihydroxyanthraquinone)An important orange-red organic com-pound used in the dyestuffs industry toproduce red lakes. It occurs naturally in theroot of the plant madder and may also besynthesized from anthraquinone.

alkali A water-soluble strong base.Strictly the term refers to the hydroxides ofthe alkali metals (group 1) only, but incommon usage it refers to any soluble base.Thus borax solution may be described asmildly alkaline.

alkaloid One of a group of natural or-ganic compounds found in plants. Theycontain oxygen and nitrogen atoms; mostare poisonous. However, they include anumber of important drugs with character-istic physiological effects, e.g. morphine,codeine, caffeine, cocaine, and nicotine.

alkane A type of hydrocarbon with gen-eral formula CnH2n+2. Alkanes are satu-rated compounds, containing no double ortriple bonds. Systematic names end in -ane:methane (CH4) and ethane (C2H6) are typ-ical examples. The alkanes are fairly unre-active (their former name, the paraffins,means ‘small affinity’). In ultraviolet radia-tion they react with halogens to give a mix-ture of substitution products. This involvesa free-radical chain reaction and is impor-tant as a first step in producing other com-pounds from alkanes. There are a numberof ways of preparing specific alkanes:1. From a sodium salt of a carboxylic acid

treated with sodium hydroxide:

RCOO–Na+ + NaOH → RH + Na2CO32. By reduction of a haloalkane with

nascent hydrogen from the action ofethanol on a zinc–copper couple:

RX + 2[H] → RH + HX3. By the WURTZ REACTION – i.e. sodium in

dry ether on a haloalkane:2RX + 2Na → 2NaX + RR

4. By the KOLBÉ ELECTROLYTIC METHOD:RCOO– → RR

5. By refluxing a haloalkane with magne-sium in dry ether to form a GRIGNARD

REAGENT:RI + Mg → RMgI

5. With acid this gives the alkane:RMgI + H → RH

The main source of lower molecularweight alkanes is natural gas (for methane)and crude oil.

alkene A type of aliphatic hydrocarboncontaining one or more double bonds inthe molecule. Alkenes with one doublebond have the general formula CnH2n. Thealkenes are unsaturated compounds. Theycan be obtained from crude oil by crackingalkanes. Systematic names end in -ene: ex-amples are ethene (C2H4) and propene(C3H6), both of which are used in plasticsproduction and as starting materials forthe manufacture of many other organicchemicals. The former general name for analkene was olefin.

The methods of synthesizing alkenesare:

alicyclic compound

12

hex-1-ene

CH3 CH2 CH2 CH2 CH2CH =

hex-2-ene

hex-3-ene

CH3 CH2 CH2 CH CH3= CH

CH3 CH2 CHCH = CH2 CH3

Alkene

1. The elimination of HBr from ahaloalkane using an alcoholic solutionof potassium hydroxide:RCH2CH2Br + KOH → KBr + H2O +

RCH:CH22. The dehydration of an alcohol by pass-

ing the vapor over hot pumice (400°C):RCH2CH2OH → RCH:CH2 + H2O

The reactions of simple alkenes include:1. Hydrogenation using a catalyst (usually

nickel at about 150°C):RCH:CH2 + H2 → RCH2CH3

2. Addition reactions with halogen acids togive haloalkanes:

RCH:CH2 + HX → RCH2CH2X2. The addition follows MARKOVNIKOFF’S

RULE.3. Addition reactions with halogens, e.g.

RCH:CH2 + Br2 → RCHBrCH2Br4. Hydration using concentrated sulfuric

acid, followed by dilution and warming:RCH:CH2 + H2O → RCH(OH)CH3

5. Oxidation by cold potassium perman-ganate solutions to give diols:

RCH:CH2 + H2O + [O] →RCH(OH)CH2OH

6. Oxidation to form cyclic epoxides (oxi-ranes). Ethene can be oxidized in airusing a silver catalyst to the cyclic com-pound epoxyethane (C2H4O). Moregenerally peroxy carboxylic acids areused as the oxidizing agent.

7. Polymerization to polyethene (by theZIEGLER PROCESS or PHILLIPS PROCESS).

See also oxo process; ozonolysis.In general, addition to simple alkenes is

ELECTROPHILIC ADDITION. Attack is by anelectrophile on the pi orbital of the alkene.In the case of attack by a halogen acid (e.g.HBr), the initial reaction is by the (positive)hydrogen giving a positively charged inter-mediate ion (carbocation) and a Br– ion.The Br– ion then attacks the intermediatecarbocation. In the case of a halogen (e.g.Br2) the bromine acts as an electrophile toform an initial cyclic positively chargedbromonium ion and a negative Br– ion. TheBr– ion further attacks the bromonium ionto give the substituted product.

alkoxide An organic compound con-taining an ion of the type RO–, where R isan alkyl group. Alkoxides can be made by

the reaction of metallic sodium on an alco-hol. For example, ethanol reacts withsodium to give sodium ethoxide:

2C2H5OH + 2Na → 2C2H5O–Na+) + H2

Alkoxides are ionic compounds containingan alkoxide ion (RO–). They are named ac-cording to the parent alcohol. Thus,methanol (CH3OH) gives methoxidesCH3O–, ethanol (C2H5OH) gives ethoxidesC2H5O–, etc.

alkoxyalkane (diethyl ether) See ether.

alkylbenzene A type of organic hydro-carbon containing one or more alkylgroups substituted onto a benzene ring.Methylbenzene (toluene; C6H5CH3) is thesimplest example. Alkylbenzenes can bemade by a FRIEDEL–CRAFTS REACTION or bythe WURTZ REACTION. Industrially, largequantities of methylbenzene are made fromcrude oil.

Substitution of alkylbenzenes can occurat the benzene ring; the alkyl group directsthe substituent into the 2- or 4-position.Substitution of hydrogen atoms on thealkyl group can also occur.

alkyl group A group obtained by re-moving a hydrogen atom from an alkaneor other aliphatic hydrocarbon. For exam-ple, the methyl group (CH3–) is derivedfrom methane (CH4).

alkyl halide See haloalkane.

alkyl sulfide A THIOETHER with the gen-eral formula RSR′, where R and R′ arealkyl groups.

alkyne A type of hydrocarbon contain-ing one or more triple carbon–carbonbonds in its molecule. Alkynes with onetriple bond have the general formulaCnH2n–2. The alkynes are unsaturated com-pounds. The simplest member of the seriesis ethyne (acetylene; C2H2), which can beprepared by the action of water on calciumdicarbide.

CaC2 + 2H2O → Ca(OH)2 + C2H2The alkynes were formerly called theacetylenes.

13

alkyne

In general, alkynes can be made by thecracking of alkanes or by the action of ahot alcoholic solution of potassium hy-droxide on a dibromoalkane, for example:

BrCH2CH2Br + KOH →KBr + CH2:CHBr + H2O

CH2:CHBr + KOH →CHCH + KBr + H2O

The main reactions of the alkynes are:1. Hydrogenation with a catalyst (usually

nickel at about 150°C):C2H2 + H2 → C2H4C2H4 + H2 → C2H6

2. Addition reactions with halogen acids:C2H2 + HI → H2C:CHI

H2C:CHI + HI → CH3CHI23. Addition of halogens; for example, with

bromine in tetrachloromethane:C2H2 + Br2 → BrHC:CHBr

BrHC:CHBr + Br2 → Br2HCCHBr24. With dilute sulfuric acid at 60–80°C and

mercury(II) catalyst, ethyne formsethanal (acetaldehyde):

C2H2 + H2O → H2C:C(OH)HThis enol form converts to the alde-hyde:

CH3COH5. Ethyne polymerizes if passed through a

hot tube to produce some benzene:3C2H2 → C6H6

6. Ethyne forms unstable dicarbides(acetylides) with ammoniacal solutionsof copper(I) and silver(I) chlorides.

Addition to simple alkynes is ELECTRO-PHILIC ADDITION, as with ALKENES.

allosteric site A part of an enzyme sep-arate from the active site to which a spe-cific effector or modulator can be attached.This attachment is reversible and alters theactivity of the enzyme. Allosteric enzymespossess an allosteric site in addition to theirACTIVE SITE. This site is as specific in its re-lationship to modulators as active sites areto substrates. See active site. Some iron-enzymatic proteins e.g. hemoglobin alsoundergo allosteric effects.

allyl group See propenyl group.

alpha amino acid See amino acid.

alpha helix A highly stable structure in

which peptide chains are coiled to form aspiral. Each turn of the spiral contains ap-proximately 3.6 amino-acid residues. TheR group of these amino-acids extends out-ward from the helix and the helix is heldtogether by hydrogen bonding betweensuccessive coils. If the alpha helix isstretched the hydrogen bonds are brokenbut reform on relaxation. The alpha helixis found in muscle protein and keratin. It isone of the two basic secondary structuresof PROTEINS.

alpha-naphthol test (Molisch’s test) Astandard test for carbohydrates in solution.Molisch’s reagent, alpha-naphthol in alco-hol, is mixed with the test solution. Con-centrated sulfuric acid is added and a violetring at the junction of the two liquids indi-cates the presence of carbohydrates.

alternating copolymer See polymer-ization.

aluminum trimethyl See trimethylalu-minum.

amalgam An alloy of mercury with oneor more other metals. Amalgams may beliquid or solid. An amalgam of sodium(Na/Hg) with water is used as a source ofnascent hydrogen.

amatol A high explosive that consists of

allosteric site

14

•••

•••

• • •

OOOHHH

CCC CCCNNN

NNN

HHH HHHOOO

HHH

CCC

CCCNNN

HHHOOO

CCC

HHHOOO

CCCNNN

HHHCCC

A

hydrogen bond

•••

OOO

amino-acid side A

A

A

Alpha helix

a mixture of ammonium nitrate and TNT(trinitrotoluene).

amide 1. A type of organic compound ofgeneral formulae RCONH2 (primary),(RCO)2NH (secondary), and (RCO)3N(tertiary). Amides are crystalline solids andare basic in nature, some being soluble inwater. They can be formed by reaction ofammonia with acid anhydrides:

(RCO)2O + 2NH3 →RCONH2 + RCOO–NH4

+

They can also be made by reacting ammo-nia with an acyl chloride:

RCOCl + 2NH3 → RCONH2 + NH4ClReactions of amides include:1. Reaction with hot acids to give car-

boxylic acids:RCONH2 + HCl + H2O →

RCOOH + NH4Cl2. Reaction with nitrous acid to give car-

boxylic acids and nitrogen:RCONH2 + HNO2 →RCOOH + N2 + H2O

3. Dehydration by phosphorus(V) oxide togive a nitrile:

RCONH2 – H2O → RCNSee also Hofmann degradation.2. An inorganic salt containing the NH2

–

ion. Ionic amides are formed by the reac-tion of ammonia with certain reactivemetals (such as sodium and potassium).Sodamide, NaNH2, is a common example.

amination The introduction of anamino group (–NH2) into an organic com-pound. An example is the conversion of analdehyde or ketone into an amide by reac-tion with hydrogen and ammonia in thepresence of a catalyst:

RCHO + NH3 + H2 → RCH2NH2 +H2O

amine A compound containing a nitro-gen atom bound to hydrogen atoms or hy-drocarbon groups. Amines have thegeneral formula R3N, where R can be hy-drogen or an alkyl or aryl group. They canbe prepared by reduction of amides ornitro compounds.

Amines are classified according to thenumber of organic groups bonded to thenitrogen atom: one, primary; two, sec-

ondary; three, tertiary. Since amines arebasic they can form the quaternary ion,R3NH+. All three types, plus a quaterniumsalt, can be produced by the HOFMANN

DEGRADATION (which occurs in a sealedvessel at 100°C):

RX + NH3 → RNH3+ X–

RNH3+ X– + NH3 ˆ RNH2 + NH4X

RNH2 + RX → R2NH2+ X–

R2NH2+ X– + NH3 ˆ R2NH + NH4X

R2NH + RX → R3NH+ X–

R3NH+ X– + NH3 ˆ R3N + NH4XR3N + RX → R4N+X–

Reactions of amines include:1. Reaction with acids to form salts:

R3N + HX → R3NH+X–

2. Reaction with acyl halides to give N-substituted amides (primary and sec-ondary amines only):RNH2 + R′COCl → R′CONHR + HX

See also amine salt.

15

amine

primary (e thylamine)

secondary (diethylamine)

tertiary (triethylamine)

C2H5

H

H

N

C2H5

C2H5

H

N

C2H5

C2H5

C2H5

N

Amine

amine salt A salt similar to an ammo-nium salt, but with organic groups at-tached to the nitrogen atom. For example,triethylamine ((C2H5)3N) will react withhydrogen chloride to give triethylammo-nium chloride:

(C2H5)3N + HCl → (C2H3)3NH+Cl–

Salts of this type may have four groups onthe nitrogen atom. For example, withchloroethane, tetraethylammonium chlo-ride can be formed:

(C2H5)3N + C2H5Cl → (C2H5)4N+Cl–

Sometimes amine salts are named using thesuffix ‘-ium’. For instance, aniline(C6H5NH2) forms anilinium chlorideC6H5NH3

+Cl–. Often insoluble alkaloidsare used in medicine in the form of theiramine salt (sometimes referred to as the‘hydrochloride’).

amino acid A derivative of a carboxylicacid in which a hydrogen atom in analiphatic acid has been replaced by anamino group. Thus, from ethanoic acid,the amino acid 2-aminoethanoic acid(glycine) is formed. The amino acids of spe-cial interest are those that occur as con-stituents of naturally occurring PEPTIDES

and PROTEINS. These all have the –NH2 and–COOH groups attached to the same car-bon atom; i.e. they are alpha amino acids.All are white, crystalline, soluble in water

(but not in alcohol), and, with the sole ex-ception of the simplest member, all are op-tically active.

In the body the various proteins are as-sembled from the necessary amino acidsand it is important therefore that all theamino acids should be present in sufficientquantities. In adult humans, twelve of thetwenty amino acids can be synthesized bythe body itself. Since these are not requiredin the diet they are known as nonessentialamino acids. The remaining eight cannotbe synthesized by the body and have to besupplied in the diet. They are known as es-sential amino acids.

aminobenzene See aniline.

aminoethane See ethylamine.

amino group The group –NH2.

amino sugar A sugar in which a hy-droxyl group (OH) has been replaced by anamino group (NH2). Glucosamine (fromglucose) occurs in many polysaccharides ofvertebrates and is a major component ofchitin. Galactosamine or chondrosamine(from galactose) is a major component ofcartilage and glycolipids. Amino sugars areimportant components of bacterial cellwalls.

amine salt

16

The amino acids most commonly found in proteins

alaninearginineasparagineaspartic acidcysteineglutamic acidglutamine

glycinehistidine*isoleucine*leucine*lysine*methionine*phenylalanine*

proline**serinethreonine*tryptophan*tyrosine*valine*

* essential amino acids in animal diets** an imino acid derived from pyrollidine

Amino acid: the amino acids in proteins arealpha amino acids. The –COOH group and–NH2 group are on the same carbon atom

H

COOHR C

NH2

AMINO ACIDS MOST COMMONLY FOUND IN PROTEINS

aminotoluine See toluidine.

ammonia (NH3) A colorless gas with acharacteristic pungent odor. On cooling andcompression it forms a colorless liquid,which becomes a white solid on furthercooling. Ammonia is very soluble in water(a saturated solution at 0°C contains 36.9%of ammonia); the aqueous solution is alka-line and contains a proportion of free am-monia. Ammonia is also soluble in ethanol.It reacts with acids to form ammoniumsalts; for example, it reacts with hydrogenchloride to form ammonium chloride:

NH3(g) + HCl(g) → NH4Cl(g)See also amine salt.

ammoniacal Describing a solution inaqueous ammonia.

amount of substance Symbol: n Ameasure of the number of entities presentin a substance. See mole.

AMP (adenosine monophosphate) Anucleotide consisting of adenine, ribose,and phosphate. See ATP.

amphiprotic Able to act as both an ACID

and a base. For example, the amino acidsare amphiprotic because they contain bothacidic (–COOH) and basic (–NH2) groups.See also amphoteric; solvent.

ampholyte ion See zwitterion.

amphoteric A material that can displayboth acidic and basic properties. The termis most commonly applied to the oxidesand hydroxides of metals that can formboth cations and complex anions. For ex-ample, zinc oxide dissolves in acids to formzinc salts and also dissolves in alkalis toform zincates, [Zn(OH)4]2–. Compoundssuch as the amino acids can also be de-scribed as amphoteric, although it is moreusual to use the term AMPHIPROTIC.

amu See atomic mass unit.

amyl group See pentyl group.

amyl nitrite (C5H11ONO) A pale brown

volatile liquid organic compound; a ni-trous acid ester of 3-methylbutanol (iso-amyl alcohol). It is used in medicine as aninhalant to dilate the blood vessels (andthereby prevent pain) in patients withangina pectoris.

amylopectin The water-insoluble frac-tion of STARCH.

amylose A polymer of GLUCOSE; a poly-saccharide sugar that is found in STARCH.

anabolic steroid Any STEROID hormoneor synthetic steroid that promotes growthand formation of new tissue. Anabolicsteroids are used in the treatment of wast-ing diseases. They are also sometimes usedin agriculture to boost livestock produc-tion. People also use them to build up mus-cles, although this is now generally out-lawed in sporting activities.

anabolism All the metabolic reactionsthat synthesize complex molecules frommore simple molecules. See also metabo-lism.

anaerobic Describing a biochemicalprocess that takes place in the absence offree oxygen. Compare aerobic.

anaerobic respiration Respiration inwhich oxygen is not involved. It is found inyeasts, bacteria, and occasionally in muscletissue. In this type of respiration the or-ganic substrate is not completely oxidizedand the energy yield is low. In the absenceof oxygen in animal muscle tissue, glucoseis degraded to pyruvate by GLYCOLYSIS,with the production of a small amount ofenergy and also lactic acid, which may beoxidized later when oxygen becomes avail-able (see oxygen debt). FERMENTATION is anexample of anaerobic respiration, in whichcertain yeasts produce ethanol and carbondioxide as end products. Only two mol-ecules of ATP are produced by this process.Compare aerobic respiration.

analysis The process of determining theconstituents or components of a sample.There are two broad major classes of

17

analysis

analysis, qualitative analysis – essentiallyanswering the question ‘what is it?’ – andquantitative analysis – answering the ques-tion ‘how much of such and such a compo-nent is present?’ There is a large number ofanalytical methods that can be applied, de-pending on the nature of the sample andthe purpose of the analysis. These includegravimetric, volumetric, and systematicqualitative analysis (classical wet meth-ods); and instrumental methods, such aschromatographic, spectroscopic, nuclear,fluorescence, and polarographic tech-niques.

ångstrom Symbol Å A unit of length de-fined as 10–10 meter. The ångstrom wasused for expressing wavelengths of light orultraviolet radiation or for the sizes of mol-ecules; the nanometer is now preferred.

anhydride A compound formed by re-moving water from an acid or, less com-monly, a base. Many nonmetal oxides areanhydrides of acids: for example CO2 is theanhydride of H2CO3 and SO3 is the anhy-

dride of H2SO4. Organic anhydrides areformed by removing H2O from two car-boxylic acid groups, giving compoundswith the functional group –CO.O.CO–.These form a class of organic compoundscalled ACID ANHYDRIDES.

anhydrous Describing a substance thatlacks moisture, or a salt with no water ofcrystallization.

aniline (aminobenzene; phenylamine;C6H5NH2) A colorless oily substancemade by reducing nitrobenzene (C6H5-NO2). Aniline is used for making dyes,pharmaceuticals, and other organic com-pounds.

animal starch See glycogen.

anion A negatively charged ion, formedby addition of electrons to atoms or mol-ecules. In electrolysis anions are attractedto the positive electrode (the anode). Com-pare cation.

ångstrom

18

OC

H3C

HO

OC

HO

H3C

OC

H3C

O

OC

H3C

Anhydride

C

CH2C OH

O

OH

O

H2CC

CH2C

O

O

O

H2C

maleic acid maleic anhydride

Anhydride: a cyclic anhydride

anionic detergent See detergent.

anionic resin An ION-EXCHANGE ma-terial that can exchange anions, such as Cl–

and OH–, for anions in the surroundingmedium. Such resins are used for a widerange of analytical and purification pur-poses.

They are often produced by addition ofa quaternary ammonium group (N(CH3)4

+)or a phenolic group (–OH–) to a stablepolyphenylethene resin. A typical exchangereaction is:

resin–N(CH3)4+Cl– + KOH ˆ

resin–N(CH3)4+OH– + KCl

Anionic resins can be used to separate mix-tures of halide ions. Such mixtures can beattached to the resin and recovered sepa-rately by elution.

annulene A ring compound containingalternating double and single C–C bonds.The compound C8H8, having an eight-membered ring of carbon atoms, is the next

annulene larger than benzene. It is not anAROMATIC COMPOUND because it is not pla-nar and does not obey the Hückel rule.C8H8 is called cyclo-octatetraene. Higherannulenes are designated by the number ofcarbon atoms in the ring. [10]-annuleneobeys the Hückel rule but is not aromaticbecause it is not planar as a result of inter-actions of the hydrogen atoms inside thering. There is evidence that [18]-annulene,which is a stable red solid, has aromaticproperties.

anode In electrolysis, the electrode thatis at a positive potential with respect to thecathode. In any electrical system, such as adischarge tube or electronic device, theanode is the terminal at which electronsflow out of the system.

anomer Either of two isomeric forms ofa cyclic sugar that differ in the dispositionof the –OH group on the carbon next tothe O atom of the ring (the anomeric

19

anomer

[14]-annulene

[18]-annulene

H

[30]-annulene

H

H H

H H

H H

H H

H H

H H

H H

H

H H

H

H

H

HH H

H

H

H

H H

H H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

H

HH

HH

HH

H

H

H HH

H H

Annulene

carbon). Anomers are diastereoisomers.They are designated α– or β– according to whether the –OH is below or above the ring respectively. See illustration atsugar.

anomeric carbon See anomer.

anthocyanin One of a group of water-soluble pigments found dissolved in higherplant cell vacuoles. Anthocyanins are red,purple, and blue and are widely distrib-uted, particularly in flowers and fruits,where they are important in attracting in-sects, birds, etc. They also occur in buds and sometimes contribute to the au-tumn colors of leaves. They are natural pH indicators, often changing from red to blue as pH increases, i.e. acidity de-creases. Color may also be modified bytraces of iron and other metal salts and or-ganic substances, for example cyanin is redin roses but blue in the cornflower. Seeflavonoid.

anthracene (C14H10) A white crystallinesolid used extensively in the manufactureof dyes. Anthracene is found in the heavy-and green-oil fractions of crude oil and isobtained by fractional crystallization. Itsstructure is benzene-like, having three six-membered rings fused together. The reac-tions are characteristic of AROMATIC

COMPOUNDS.

anthracite The highest grade of coal,with a carbon content of between 92% and98%. It burns with a hot blue flame, givesoff little smoke and leaves hardly any ash.

anthraquinone (C6H4(CO)2C6H4) Acolorless crystalline quinone used in pro-ducing dyestuffs such as alizarin.

antibonding orbital See orbital.

anticlinal conformation See confor-mation.

antiknock agent A substance added togasoline to inhibit preignition or ‘knock-ing’. A common example is lead tetraethyl,although use of this is discouraged in manycountries for environmental reasons.

antioxidant A substance that inhibitsoxidation. Antioxidants are added to suchproducts as foods, paints, plastics, andrubber to delay their oxidation by atmos-pheric oxygen. Some work by formingchelates with metal ions, thus neutralizingthe catalytic effect of the ions in the oxida-tion process. Other types remove interme-diate oxygen free radicals. Naturallyoccurring antioxidants can limit tissue orcell damage in the body. These includevitamin E and β-carotene.

antiperiplanar conformation See con-formation.

apoenzyme The protein part of a conju-gate enzyme. It is an enzyme whose cofac-tor has been removed (e.g. via dialysis)rendering it catalytically inactive. Whencombined with its PROSTHETIC GROUP orcoenzyme it forms a complete enzyme(HOLOENZYME).

aprotic See solvent.

aqueous Describing a solution in water.

arene An organic compound containinga benzene ring; i.e. an aromatic hydrocar-bon or a derivative of an aromatic hydro-carbon.

anomeric carbon

20

Anthracene

O

O5

6

7

8 1

2

3

4

Anthraquinone

arginine See amino acid.

aromatic compound An organic com-pound with characteristic chemical reac-tions, usually containing BENZENE rings inits structure. Aromatic compounds, such asbenzene, have a planar ring of atoms linkedby alternate single and double bonds. Thecharacteristic of aromatic compounds isthat their chemical properties are not thoseexpected for an unsaturated compound;they tend to undergo nucleophilic substitu-tion of hydrogen (or other groups) on thering, and addition reactions only occurunder special circumstances.

The explanation of this behavior is thatthe electrons in the double bonds are delo-calized over the ring, so that the six bondsare actually all identical and intermediatebetween single bonds and double bonds.The pi electrons are thus spread in a mo-lecular orbital above and below the ring.The evidence for this delocalization in ben-zene is that the bond lengths between car-bon atoms in benzene are all equal andintermediate in size between single anddouble bond lengths. Also, if two hydrogenatoms attached to adjacent carbon atomsare substituted by other groups, the com-pound has only one structure. If the bondswere different two isomers would exist.Benzene has a stabilization energy of 150kJ mol–1 over the Kekulé structure. It ispossible to characterize aromatic behaviorby detecting a ring current in NMR. Cer-tain heterocyclic molecules, such as PYRI-DINE, also have aromatic properties.

The delocalization of the electrons inthe pi orbitals of benzene accounts for theproperties of benzene and its derivatives,which differ from the properties of alkenesand other aliphatic compounds. The phe-nomenon is called aromaticity. A defini-tion of aromaticity is that it occurs incompounds that obey the Hückel rule: i.e.that there should be a planar ring with atotal of (4n + 2) pi electrons (where n is anyinteger). Using this rule as a criterion cer-tain nonbenzene rings show aromaticity.Such compounds are called nonbenzenoidaromatics. Examples are the cyclopentadi-enyl ion C5H5

– and the tropyllium ionC7H7

+. Other compounds that have a ring

of atoms with alternate double and singlebonds, but do not obey the rule (e.g. cyclo-octatetraene, which has a nonplanar ringof alternating double and single bonds) arecalled pseudoaromatics.

Compare aliphatic compound. See alsoannulene.

aromaticity See aromatic compound.

Arrhenius equation An equation relat-ing the rate constant of a chemical reactionand the temperature at which the reactiontakes place:

k = Aexp(–Ea/RT)where A is a constant, k the rate constant,T the thermodynamic temperature inkelvins, R the gas constant, and Ea the ac-tivation energy of the reaction.

Reactions proceed at different rates atdifferent temperatures, i.e. the magnitudeof the rate constant is temperature depend-ent. The Arrhenius equation is often writ-ten in a logarithmic form, i.e.

logek = logeA – E/2.3RTThis equation enables the activation en-

ergy for a reaction to be determined. It isnamed for the Swedish chemist Svante Au-gust Arrhenius (1859–1927).

Arrhenius theory See acid.

aryl group An organic group derived byremoving a hydrogen atom from an aro-matic hydrocarbon or derivative. Thephenyl group, C6H5–, is the simplest exam-ple.

ascorbic acid See vitamin C.

asparagine See amino acid.

aspartic acid See amino acid.

aspirin (acetylsalicylic acid; C9H8O4) Acolorless crystalline compound made bytreating salicylic acid with ethanoyl hy-dride. It is used as an analgesic and anti-pyretic drug, and small doses areprescribed for adult patients at risk of heartattack or stroke. It should not be given tochildren.

21

aspirin

association The combination of mol-ecules of a substance with those of anotherto form more complex species. An exampleis a mixture of water and ethanol (whichare termed associated liquids), the mol-ecules of which combine via hydrogenbonding.

asymmetric atom See chirality; iso-merism; optical activity.

atactic polymer See polymerization.

atmosphere A unit of pressure definedas 101 325 pascals (atmospheric pressure).The atmosphere is used in chemistry onlyfor rough values of pressure; in particular,for stating the pressures used in high-pressure industrial processes.

atom The smallest part of an elementthat can exist as a stable entity. Atoms con-sist of a small dense positively charged nu-cleus, made up of neutrons and protons,with electrons in a cloud around this nu-cleus. The chemical reactions of an elementare determined by the number of electrons(which is equal to the number of protons inthe nucleus). All atoms of a given elementhave the same number of protons (the pro-ton number). A given element may havetwo or more isotopes, which differ in thenumber of neutrons in the nucleus.

The electrons surrounding the nucleusare grouped into shells – i.e. main orbitsaround the nucleus. Within these main or-bits there may be subshells. These corre-spond to atomic orbitals. An electron in anatom is specified by four quantum num-bers:1. The principal quantum number (n),

which specifies the main energy levels. ncan have values 1, 2, etc. The corre-sponding shells are denoted by letters K,L, M, etc., the K shell (n = 1) being thenearest to the nucleus. The maximumnumber of electrons in a given shell is2n2.

2. The orbital quantum number (l), whichspecifies the angular momentum. For agiven value of n, l can have possible val-ues of n–1, n–2, … 2, 1, 0. For instance,the M shell (n = 3) has three subshells

with different values of l (0, 1, and 2).Sub-shells with angular momentum 0, 1,2, and 3 are designated by letters s, p, d,and f.

3. The magnetic quantum number (m).This can have values –l, –(l – 1) … 0 …+ (l – l), + l. It determines the orientationof the electron orbital in a magneticfield.

4. The spin quantum number (ms), whichspecifies the intrinsic angular momen-tum of the electron. It can have values+½ and –½.

Each electron in the atom has four quan-tum numbers and, according to the Pauliexclusion principle, no two electrons canhave the same set of quantum numbers.This explains the electronic structure ofatoms.

atomicity The number of atoms permolecule of an element. Helium, for exam-ple, has an atomicity of one, nitrogen two,and ozone three.

atomic mass unit (amu) Symbol: u Aunit of mass used for atoms and molecules,equal to 1/12 of the mass of an atom of carbon-12. It is equal to 1.660 33 × 10–27

kg.

atomic number See proton number.

atomic orbital See orbital.

atomic weight See relative atomic mass(r.a.m.).

ATP (adenosine triphosphate) The uni-versal energy carrier of living cells. Energyfrom respiration or, in photosynthesis,from sunlight is used to make ATP fromADP. It is then reconverted to ADP in var-ious parts of the cell by enzymes known asATPases, the energy released being used todrive three main cellular processes: me-chanical work (muscle contraction and cel-lular movement); the active transport ofmolecules and ions; and the biosynthesis ofother molecules. It can also be converted tolight, electricity, and heat.

ATP is a nucleotide consisting of ade-nine and ribose with three phosphate

association

22

groups attached. Hydrolysis of the termi-nal phosphate bond releases energy(30.6 kJ mol–1) and is coupled to anenergy-requiring process. Further hydroly-sis of ADP to AMP sometimes occurs, re-leasing more energy. The pool of ATP issmall, but the faster it is used, the faster itis replenished. ATP is not transportedaround the body, but is synthesized whereit is needed.

atto- Symbol: a A prefix denoting 10–18.For example, 1 attometer (am) = 10–18

meter (m).

autocatalysis See catalyst.

autoclave An apparatus consisting of anairtight container whose contents areheated by high-pressure steam; the con-tents may also be agitated. Autoclaves areused for reactions between gases underpressure in industrial processing and forsterilizing objects.

auxin Any of a group of plant hor-mones, the most common naturally occur-ring one being indole acetic acid, IAA.Auxins are made continually in growingshoot and root tips. Synthetic auxins,cheaper and more stable than IAA, are em-ployed in agriculture, horticulture, andresearch. These include indoles and naph-thyls: e.g. NAA (naphthalene acetic acid)used mainly as a rooting and fruit settinghormone; phenoxyacetic acids, e.g. 2,4-D(2,4-dichlorophenoxyacetic acid) used asweed-killers and modifiers of fruit develop-ment; and more toxic and persistent ben-zoic auxins, e.g. 2,4,5-trichlorobenzoic

acid, also formerly used as herbicides butnow widely restricted.

Avogadro constant (Avogrado number)Symbol: NA The number of particles in onemole of a substance. Its value is 6.022 52 ×1023 mol–1.

Avogadro number See Avogadro con-stant.

Avogadro’s law The principle thatequal volumes of all gases at the same tem-perature and pressure contain equal num-bers of molecules. It is often calledAvogadro’s hypothesis. It is strictly trueonly for ideal gases.

axial conformation See cyclohexane.

azeotrope (azeotropic mixture) A mix-ture of liquids for which the vapor phasehas the same composition as the liquidphase. It therefore boils without change incomposition and, consequently, withoutprogressive change in boiling point.

The composition and boiling points ofazeotropes vary with pressure, indicatingthat they are not chemical compounds.Azeotropes may be broken by distillationin the presence of a third liquid, by chemi-cal reactions, adsorption, or fractionalcrystallization. See constant-boiling mix-ture.

azeotropic distillation A method usedto separate mixtures of liquids that cannotbe separated by simple distillation. Such amixture is called an azeotrope. A solvent isadded to form a new azeotrope with one ofthe components, and this is then removedand subsequently separated in a secondcolumn. An example of the use of azeo-tropic distillation is the dehydration of96% ethanol to absolute ethanol.Azeotropic distillation is not widely usedbecause of the difficulty of finding inex-pensive nontoxic noncorrosive solventsthat can easily be removed from the newazeotrope.

azeotropic mixture See azeotrope.

23

azeotropic mixture

- --

N

N N

N

OCH2O

OOO

OOO

P O P O P O

NH2

OHOH

-

ATP

azide 1. An organic compound of gen-eral formula RN3.2. An inorganic compound containing theion N3

–.

azine An organic heterocyclic com-pound that has a hexagonal ring contain-ing carbon and nitrogen atoms. Pyridine(C5H5N) is the simplest example.

azo compound A type of organic com-pound of the general formula RN:NR′,where R and R′ are aromatic groups. Azocompounds can be formed by coupling aDIAZONIUM COMPOUND with an aromaticphenol or amine. Most are colored becauseof the presence of the azo group –N:N–.

azo dye An important type of dye used

in acid dyes for wool and cotton. The dyesare azo compounds; usually sodium saltsof sulfonic acids.

azo group See azo compound.

azulene (C10H8) A blue crystalline com-pound having a seven-membered ringfused to a five-membered ring. It convertsto naphthalene on heating.

azide

24

1

2

34 5

6

78

Azulene

25

backbiting A process that can occur incertain free-radical POLYMERIZATION reac-tions, in which a radical with an unpairedelectron on the end of the chain convertsinto one in which the unpaired election isnot at the end of the chain. For example,the radical

RCH2CH2CH2CH2CH2 CH2•may convert into

RCH2CH•CH2CH2 CH2CH3Effectively, this involves a transfer of a

hydrogen atom within the molecule. Typi-cally, the free electron moves from the endof the chain to atom five, counting fromthe end. This is because the process in-volves a transition state with a six-mem-bered ring. The new free radical is morestable than the original one. Further poly-merization occurs at the new unpairedelectron leading to the production of poly-mers with butyl (CH3CH2CH2CH2–) sidechains.

Baeyer, Johann Friedrich WilhelmAdolph von (1835–1917) German or-ganic chemist. Baeyer worked mainly in or-ganic synthesis and is noted for his study ofthe dye indigo. He started his work on in-digo in 1865 and continued for 20 years;he determined the structure of indigo in1883. The structure he postulated was cor-

rect (except for the stereochemistry of thedouble bond, which was subsequentlyshown by x-ray crystallography to betrans). Baeyer discovered a number of sub-stances including barbituric acid. His laterinvestigations on ring compounds andpolyacetylenes led him to consider the sta-bility of carbon–carbon bonds in cycliccompounds. This resulted in the Baeyerstrain theory. Baeyer was awarded the1905 Nobel Prize for chemistry for hiswork on indigo and aromatic compounds.

Baeyer–Villiger reaction A type of re-action in which a ketone reacts with a per-oxy acid, with resulting production of anester. For example,

R–CO–R → R–CO–O–R.The reaction involves ‘insertion’ of an oxy-gen atom next to the carbonyl (CO) group.Typical peroxy acids used are trifluoro-perethanoic acid (CF3.CO.O.OH) andmeta-chloroperbenzoic acid (m-CPBA;ClC6H4.CO.O.OH). The reaction was dis-covered in 1899 by the German chemistsA. Baeyer and V. Villiger, and is commonlyused in organic synthesis. In certain caseshydrogen peroxide (H2O2) can be used asthe oxidizing agent. This is sometimesknown as the Dakin reaction. The Baeyer–Villiger reaction is a type of rearrange-

B

C

CH2

CH2

C

C

H

H2

H2

H

CH2R

CH3

C

CH2

CH2

C

H2

H

CH2R

Backbiting

ment. For a peroxy acid X.CO.O.OH,there is an intermediate cation formedR2C+(OH)(O.CO.X). The mechanism in-volves migration of a group R onto theoxygen of the peroxy acid group.

Bakelite (Trademark) A common ther-mosetting synthetic polymer formed by thecondensation of phenol (C6H5OH) andmethanal (formaldehyde, HCOH). It is anexample of a phenolic resin (or phenol–formaldehyde resin), and was one of thefirst useful synthetic polymers. The reac-tion between phenol and methanal occursunder acid conditions and involves elec-trophilic substitution on the benzene ringto give a three-dimensional polymericstructure. Bakelite is named for the Bel-gian-born US chemist Leo Hendrik Baeke-land (1863–1944), who discovered it in1909.

ball mill A device commonly used in thechemical industry for grinding solid ma-terial. Ball mills usually have slowly rotat-ing steel-lined drums containing steel balls.The material is crushed by the tumbling ac-

tion of the balls in the drum. Comparehammer mill.

banana bond (bent bond) In strained-ring compounds the bond angles thatwould be produced by hybridization of or-bitals are not equal to the angles obtainedby joining the atomic centers. In such casesit is sometimes assumed that the bondingorbital is bent or banana-like in shape. Forexample, in cyclopropane the three carbonatoms are arranged in an equilateral trian-gle, and the bond angle is 60°. The sp3 hy-bridization gives an angle of about 104°between the orbitals. Consequently, the or-bitals overlap at an angle, giving a bananabond. The term ‘banana bond’ is also usedin a quite separate sense for a multicenterbond of the type present in electron-deficient compounds such as diborane(B2H6).

band spectrum A SPECTRUM that ap-pears as a number of bands of emitted orabsorbed radiation. Band spectra are char-acteristic of molecules. Often each bandcan be resolved into a number of closelyspaced lines. The different bands corre-

Bakelite

26

CH2

CH2CH2

CH2

OH OH

OH OH

OH

OH

CH2 CH2

CH2

Bakelite

spond to changes of electron orbit in themolecules and the closely spaced lines ineach band, seen under higher resolution,are the result of different vibrational statesof the molecule.

barrel A measurement of volume oftenused in the oil and chemical industries. Onebarrel is equal to 159 liters (about 29 USgallons).

Barton, Sir Derek Harold Richard(1918–98) British organic chemist notedfor his work on the stereochemistry of or-ganic molecules, particularly natural prod-ucts. In a major paper published in 1950 hesuggested that the rates of reactions in iso-mers are strongly influenced by the spatialorientations of their functional groups.This paper initiated the branch of organicchemistry known as conformational analy-sis. Barton studied many natural products,including phenols. In 1959 he developed asimple synthesis for the hormone aldos-terone. He shared the 1969 Nobel Prize forchemistry with Norwegian chemist OddHassell.

base See acid.

base analog An unnatural purine orpyrimidine that can be incorporated intoDNA, causing altered base pairing. Somebase analogs are used therapeutically asanticancer drugs.

base-catalyzed reaction A reactioncatalyzed by bases. Typical base-catalyzedreactions are the CLAISEN CONDENSATION

and the ALDOL REACTION, in which the firststep is abstraction of a proton to give acarbanion.

base pairing The linking together of thetwo helical strands of DNA by bonds be-tween complementary bases, adenine pair-ing with thymine and guanine pairing withcytosine. The specific nature of base pair-ing enables accurate replication of thechromosomes and thus maintains the con-stant composition of the genetic material.In pairing between DNA and RNA theuracil of RNA pairs with adenine.

basic Acting as a base; having a ten-dency to release hydroxide ions (OH–) inaqueous solution. A basic solution has anexcess of OH– ions over H+ ions; i.e. a pHgreater than 7.

batch process A manufacturing processin which the reactants are fed into theprocess in fixed quantities (batches), ratherthan in a continuous flow. At any particu-lar instant all the material, from its prepa-ration to the final product, has reached adefinite stage in the process. Such processespresent problems of automation and in-strumentation and tend to be wasteful ofenergy. For this reason, batch processing isused on an industrial scale only when smallquantities of valuable or strategic materialsare required, e.g. specialist chemicals orpharmaceuticals. Compare continuousprocess.

Beckmann rearrangement A type ofreaction in which the OXIME of a ketone isconverted into an amide using a sulfuricacid catalyst. First discovered by the Ger-man chemist Ernst Beckmann (1853–1923), it is used in the manufacture ofpolyamides (see nylon).

Beckmann thermometer A type ofmercury thermometer designed to measuresmall differences in temperature ratherthan scale degrees. Beckmann thermome-ters have a larger bulb than common ther-mometers and a stem with a small internaldiameter, so that a range of 5°C coversabout 30 centimeters in the stem. The mer-cury bulb is connected to the stem in sucha way that the bulk of the mercury can beseparated from the stem once a particular5° range has been attained. The thermome-ter can thus be set for any particular range.The Beckmann thermometer has com-monly been used for measuring such quan-tities as depression of freezing point andelevation of boiling point.

bent bond See banana bond.

benzaldehyde See benzenecarbalde-hyde.

27

benzaldehyde

benzene (C6H6) A colorless liquid hy-drocarbon with a characteristic odor. Ben-zene is a highly toxic compound andcontinued inhalation of the vapor is harm-ful. It was originally isolated from coal tarand for many years this was the principalsource of the compound. Contemporarymanufacture is from hexane; petroleumvapor is passed over platinum at 500°Cand at a pressure of 10 atmospheres:

C6H14 → C6H6 + 4H2Benzene is the simplest aromatic hydro-

carbon. See aromatic compound. Thestructure of benzene was the subject ofconsiderable speculation in the 19th cen-tury. The basic problem – known as thebenzene problem – was that of reconcilingthe formula of benzene, C6H6, with itschemical reactions. The empirical formulais the same as that of acetylene, C2H2, and

benzene

28

(a) (b) (c) (d)

(e) (f) (g)

Benzene: early structures suggested for benzene: (a) Kekulé (1865); (b) Claus (1867); (c) Dewar(1867); (d) Ladenburg (1869); (e) Kekulé (1865); (f) Armstrong–Baeyr (1887); (g) Thiele (1899)

HH

H H

CC

CC

Benzene: in benzene, 6 p orbitals can combine in different ways to give delocalized molecularorbitals. The one of lowest energy has two donut-shaped areas above and below the ring ofcarbon atoms.

it might be expected that benzene wouldundergo similar reactions. However, ben-zene does not show the usual behavior of acompound containing double or triplebonds.

For example, acetylene adds bromine toyield CHBr:CHBr and eventually CHBr2-CHBr2. Benzene, with an iron bromide cat-alyst, suffers displacement of one of itshydrogen atoms to yield C6H5Br. This typeof activity in which substitution reactionsoccur indicates that benzene might be satu-rated.

Benzene, however, does not always actas a saturated compound. In sunlightbromine is added to give C6H6Cl6 and hy-drogen can also be added with a nickel cat-alyst to yield cyclohexane, C6H12. Anumber of different formulae were put for-ward to try to explain the properties. In1865 the German chemist August Kekulé(1829–96) suggested a structure with alter-nate double and single bonds in a hexago-nal ring. To account for the fact thatbenzene has only three disubstitution prod-ucts, he further proposed that the positionsof the bonds oscillate so that two moleculesare in equilibrium. This structure – theKekulé formula – is the one often used informulae of compounds containing ben-zene rings.

The modern idea of aromaticity isbased not on equilibrium between Kekuléstructures but on RESONANCE betweenthem. The bonds in benzene have charac-ters between double and single bonds: thecarbon atoms are held together by six sin-gle bonds and the remaining six electrons,from the double bonds, are delocalized

over the ring. This is the reason benzenehas all its C-C bonds of the same lengthand undergoes ELECTROPHILIC SUBSTITUTION

reactions.

benzenecarbaldehyde (benzaldehyde;C6H5CHO) A yellow oily ALDEHYDE

with a distinct almondlike odor (the com-pound occurs in almond kernels). Ben-zenecarbaldehyde may be synthesized inthe laboratory by the usual methods ofaldehyde synthesis. It is used as a food fla-voring and in the manufacture of dyes andantibiotics, and can be readily manufac-tured by the chlorination of methylbenzene(toluene) on the methyl group and thesubsequent hydrolysis of dichloromethyl-benzene:

C6H5CH3 + Cl2 → C6H5CHCl2C6H5CHCl2 + 2H2O → C6H5CH(OH)2

+ 2HClC6H5CH(OH)2 → C6H5CHO + H2O

benzenecarbonyl chloride (benzoylchloride; C6H5COCl) A liquid acyl chlo-ride used as a benzoylating agent. See acy-lation.

29

benzenecarbonyl chloride

Cl

Cl

Cl

Cl

Cl

Cl

1,1-dichlorobenzene(o-dichlorobenzene)

1,2-dichlorobenzene(m-dichlorobenzene)

1,3-dichlorobenzene(p-dichlorobenzene)

Benzene: disubstituted derivates of benzene

HC

CH

HC

HC CH

CH

O

Benzenecarbaldehyde (benzaldehyde)

benzenecarbonyl group

30

benzenecarbonyl group (benzoylgroup) The group C6H5CO–.

benzenecarboxylic acid (benzoic acid;C6H5COOH) A white crystalline car-boxylic acid found naturally in someplants. It is used as a food preservative. Thecarboxyl group (–COOH) directs furthersubstitution onto the benzene ring in the 3position.

benzene-1,2-dicarboxylic acid (ph-thalic acid; C6H4(COOH)2) A whitecrystalline aromatic acid. On heating itloses water to form phthalic anhydride,which is used to make dyestuffs and poly-mers.

benzene-1,4-dicarboxylic acid (tereph-thalic acid; C6H4(COOH)2) A colorlesscrystalline organic acid used to produceDacron and other polyesters.

benzene-1,3-diol (resorcinol; C6H4(OH)2)A white crystalline phenol used in the man-ufacture of dyestuffs and celluloid.

benzene-1,4-diol (hydroquinone; quinol;C6H4(OH)2) A white crystalline phenolused in making dyestuffs. See also quinone.

benzene ring The cyclic hexagonalarrangement of six carbon atoms that arecharacteristic of benzene and its deriva-tives. See aromatic compound; benzene.

benzenesulfonic acid (C6H5SO2OH) Awhite crystalline sulfonic acid made by sul-fonation of benzene. Any further substitu-tion onto the benzene ring is directed intothe 3 position.

benzfuran (coumarone; C8H6O) A crys-talline compound having a benzene ringfused to a furan ring.

benzilic acid rearrangement A reac-tion in which benzil (1,2-diphenylethan-1,2-dione) is treated with hydroxide andthen with acid to give benzilic acid (2-hy-droxy-2,2-diphenylethanoic acid):

C6H5.CO.CO.C6H5 →(C6H5)2C(OH).COOH

The reaction, which involves migration ofa phenyl group (C6H5–) from one carbonatom to another, was the first rearrange-ment reaction to be described (by Germanchemist Justus von Liebig in 1828).

benzoic acid See benzenecarboxylicacid.

benzole A mixture of mainly aromatichydrocarbons obtained from coal.

benzopyrene See benzpyrene.

benzoquinone See quinone.

benzoylation The introduction of abenzoyl group (benzenecarbonyl group)into a compound. See acylation.

benzoyl chloride See benzenecarbonylchloride.

benzoyl group See benzenecarbonylgroup.

benzpyrene (benzopyrene; C20H12) Acyclic aromatic hydrocarbon with a struc-ture consisting of five fused benzene rings.It occurs in coal tar and is produced by in-complete combustion of some organiccompounds. Benzpyrene, which is presentin tobacco smoke, has marked carcino-genic properties.

3

2

1

OOO6

5

4

Benzfuran (coumarone)HC

CH

HC

HC CH

C

O

OH

Benzenecarboxylic acid (benzoic acid)

benzpyrrole See indole.

benzyl alcohol See phenylmethanol.

benzyl group The group C6H5CH2–.

benzyne (C6H4) A short-lived interme-diate present in some reactions. The ring ofsix carbon atoms contains two doublebonds and one triple bond (the systematicname is 1,2-didehydrobenzene).

Bergius process A process formerlyused for making hydrocarbon fuels fromcoal. A mixture of powdered coal, heavyoil, and a catalyst was heated with hydro-gen at high pressure.

beta-pleated sheet A type of PROTEIN

structure in which polypeptide chains runclose to each other and are held together byhydrogen bonds at right angles to the mainchain. The structure is folded in regular‘pleats’. Fibres having this type of structureare usually composed of amino acids withshort side chains. The chains may run inthe same direction (parallel) or opposite di-rections (antiparallel). It is one of the twobasic secondary structures of proteins.

bi- Prefix meaning ‘two’. For example,biphenyls are compounds that have twophenyl groups joined together, as inC6H5–C6H5. The prefix is also commonlyused in naming inorganic compounds toindicate the presence of hydrogen; for in-stance, sodium bisulfate (NaHSO4) issodium hydrogensulfate, etc.

bicarbonate See hydrogencarbonate.

bimolecular Describing a reaction or astep in a reaction that involves two mol-ecules, ions, etc. For example the decom-position of hydrogen iodide,

2HI → H2 + I2takes place between two molecules and istherefore a bimolecular reaction. All bi-molecular reactions are second order, butsome second-order reactions are not bi-molecular. See also order.

binary compound A chemical com-

31

binary compound

Benzyne

Benzpyrene

A

A

A

AA

A

A

A

A

A

A

A

A

A

A

A

Beta-pleated sheet

pound formed from only two elements.Water (H2O) and sodium chloride (NaCl)are examples.

bioassay An experimental technique formeasuring quantitatively the strength of abiologically active chemical by its effect ona living organism. For example, the vita-min activity of certain substances can bemeasured using bacterial cultures. The in-crease in bacterial numbers is comparedagainst that achieved with known stan-dards for vitamins.

biochemical oxygen demand (BOD)The amount of oxygen taken from naturalwater by microorganisms that decomposeorganic waste matter in the water. It istherefore a measure of the quantity of or-ganic pollutants present. The biochemicaloxygen demand is determined by measur-ing the amount of oxygen in a sample ofwater, storing the sample, and then makingthe measurement again five days later.

biochemistry The study of chemicalcompounds and reactions occurring in liv-ing organisms.

biodegradable See pollution.

biosynthesis The series of reactions bywhich organisms obtain the various com-pounds needed for life.

biotechnology The application of tech-nology to biological processes for indus-trial, agricultural, and medical purposes.For example, bacteria such as Penicilliumand Streptomycin are used to produce anti-biotics and fermenting yeasts produce alco-hol in beer and wine manufacture. Recentdevelopments in genetic engineering haveenabled the large-scale production of hor-mones, blood serum proteins, and othermedically important products. Geneticmodification of farm crops, and even live-stock, offers the prospect of improved pro-tection against pests, or products withnovel characteristics, such as new flavorsor extended storage properties. See also en-zyme technology.

biotin A water-soluble vitamin generallyfound, together with vitamins in the Bgroup, in the VITAMIN B COMPLEX. It iswidely distributed in natural foods, eggyolk, kidney, liver, and yeast being goodsources. Biotin is required as a coenzymefor carboxylation reactions in cellular me-tabolism.

biphenyl (C6H5C6H5) An organic com-pound having a structure in which twophenyl groups are joined by a C–C bond.See also polychlorinated biphenyl.

bipyridyl See dipyridyl.

bisulfite addition compound See alde-hyde.

bitumen See tar.

biuret (H2NCONHCONH2) A color-less crystalline organic compound made byheating urea (carbamide). It is used in achemical test for proteins. See also protein;urea.

bivalent (divalent) Having a valence oftwo.

block copolymer See polymerization.

boat conformation See cyclohexane.

BOD See biochemical oxygen demand.

boiling The process by which a liquid isconverted into a gas or vapor by heating atits boiling point. At this temperature thevapor pressure of the liquid is equal to theexternal pressure, and bubbles of vaporcan form within the liquid. The boilingpoint is always the same for a particularliquid at a given pressure (for referencepurposes usually taken as standard pres-sure). See also elevation of boiling point.

boiling point-composition diagram Adiagram for a two-component liquid sys-tem representing both the variation of theboiling point and the composition of thevapor phase as the liquid-phase composi-tion is varied.

bioassay

32

Boltzmann constant Symbol: k Theconstant 1.380 54 J K–1, equal to the gasconstant (R) divided by the Avogadro con-stant (NA).

bomb calorimeter A device for measur-ing the energy released during the combus-tion of substances (e.g. foods and fuels). Itconsists of a strong sealed insulated con-tainer in which a known amount of thesubstance is ignited in an atmosphere ofpure oxygen. The substance undergoescomplete combustion at constant volumeand the resultant rise in temperature can beused to calculate the energy released by thereaction. Such energy values (calorific val-ues) are often quoted in joules per kilogram(J kg–1), formerly in calories.

bond See chemical bond.

bond energy The energy involved informing a chemical bond. For methane, forinstance, the energy of the C–H bond is onequarter of the energy involved for theprocess

C + 4H → CH4

It is thus one quarter of the heat of atom-ization.

The bond dissociation energy is a dif-ferent quantity to the bond energy. It is theenergy required to break a particular bondin a compound, e.g.:

CH4 → CH3 + HMore formally, the bond enthalpy can

be used.

bonding orbital See orbital.

bond length The length of a chemicalbond; the distance between the centers ofthe nuclei of two atoms joined by a chemi-cal bond. Bond lengths may be measuredby electron or x-ray diffraction.

Bosch process The reactionCO + H2O → CO2 + H2

using WATER GAS passed over a hot catalyst.It was used by Carl Bosch (1874–1940) toproduce hydrogen for the Haber processfor making ammonia.

Boyle’s law At a constant temperature,the pressure of a fixed mass of a gas is in-versely proportional to its volume: i.e.

pV = Kwhere K is a constant. The value of K de-pends on the temperature and on the na-ture of the gas. The law holds strictly onlyfor ideal gases. Real gases follow Boyle’slaw at low pressures and high tempera-tures. See gas laws.

Brady’s reagent See 2,4-dinitrophenyl-hydrazine.

Bragg equation An equation used todeduce the crystal structure of a materialusing data obtained from x-rays directed atits surface. The conditions under which acrystal will reflect a beam of x-rays withmaximum intensity is:

nλ = 2dsinθwhere θ is the angle of incidence and re-flection (the Bragg angle) that the x-raysmake with the crystal planes, n is a smallinteger, λ is the wavelength of the x-rays,and d is the distance between the crystalplanes.

branched chain See chain.

bromine A deep red, moderately reac-tive element (symbol Br) belonging to thehalogens; i.e. group 17 (formerly VIIA) ofthe periodic table. Bromine is a liquid atroom temperature (mercury is the onlyother element with this property). It occursin small amounts in seawater, salt lakes,and salt deposits but is much less abundantthan chlorine. A number of organo-bromine compounds are important com-mercially. At one time the main use ofbromine was as 1,2-dibromoethane. Thiswas added to gasoline to combine with thelead produced by decomposition of theantiknock agent lead tetraethyl. This usehas declined with the reduction in use ofleaded gasoline for environmental reasons.Quantities of bromine are used in poly-brominated diphenyl ethers (PBDEs),which are effective flame retardents inplastics. A number of bromine HALONS arealso important.

33

bromine

bromoethane (ethyl bromide; C2H5Br)A colorless volatile compound, used as arefrigerant. It can be made from ethene andhydrogen bromide.

bromoform See tribromomethane.

bromomethane (methyl bromide;CH3Br) A colorless volatile compoundused as a solvent. It can be made frommethane and bromine.

Brønsted acid See acid.

Brønsted base See acid.

buckminsterfullerene An allotrope ofcarbon containing clusters of 60 carbonatoms bound in a highly symmetric poly-hedral structure. The C60 polyhedron has acombination of pentagonal and hexagonalfaces similar to the panels on a soccer ball.The molecule was named for the Americanarchitect Richard Buckminster Fuller(1895–1983) because its structure resem-bles a geodesic dome (invented by Fuller).The C60 polyhedra are informally calledbucky balls. The original method ofmaking the allotrope was to fire a high-power laser at a graphite target. This also produces less stable carbon clusters,such as C70. It can be produced moreconveniently using an electric arc betweengraphite electrodes in an inert gas. Theallotrope is soluble in benzene, from which it can be crystallized to give yellowcrystals. This solid form is known as ful-lerite.

The discovery of buckminsterfullereneled to a considerable amount of researchinto its properties and compounds. Partic-ular interest has been shown in trappingmetal ions inside the carbon cage to form enclosure compounds. Buckminster-fullerene itself is often simply calledfullerene. The term also applies to deriva-tives of buckminsterfullerene and to simi-lar cluster (e.g. C70). Carbon structuressimilar to that in C60 can also form smalltubes, known as bucky tubes.

bucky ball See buckminsterfullerene.

bucky tube See buckminsterfullerene.

buffer A solution in which the pH re-mains reasonably constant when acids oralkalis are added to it; i.e. it acts as a bufferagainst (small) changes in pH. Buffer solu-tions generally contain a weak acid andone of its salts derived from a strong base;e.g. a solution of ethanoic acid and sodiumethanoate. If an acid is added, the H+ reactswith the ethanoate ion (from dissociatedsodium ethanoate) to form undissociatedethanoic acid; if a base is added the OH–

reacts with the ethanoic acid to form waterand the ethanoate ion. The effectiveness ofthe buffering action is determined by theconcentrations of the acid–anion pair:

K = [H+][CH3COO–]/[CH3COOH]where K is the dissociation constant.

Phosphate, oxalate, tartrate, borate,and carbonate systems can also be used forbuffer solutions.

bumping Violent boiling of a liquidcaused when bubbles form at a pressureabove atmospheric pressure.

Bunsen burner A gas burner consistingof a vertical metal tube with an adjustableair-inlet hole at the bottom. Gas is allowedinto the bottom of the tube and the gas–airmixture is burnt at the top. With too littleair the flame is yellow and sooty. Correctlyadjusted, the burner gives a flame with apale blue inner cone of incompletely burntgas, and an almost invisible outer flamewhere the gas is fully oxidized and reachesa temperature of about 1500°C.

burette A piece of apparatus used forthe addition of variable volumes of liquidin a controlled and measurable way. Theburette is a long cylindrical graduated tubeof uniform bore fitted with a stopcock anda small-bore exit jet, enabling a drop of liq-uid at a time to be added to a reaction ves-sel. Similar devices are used to introducemeasured volumes of gas at regulated pres-sure in the investigation of gas reactions.

buta-1,3-diene (butadiene; CH2:CH-CH:CH2) A colorless gas made by cat-alytic dehydrogenation of butane. It is used

bromoethane

34

in the manufacture of synthetic rubber.Buta-1,3-diene is conjugated and to someextent the pi electrons are delocalized overthe whole of the molecule. The moleculecan exist in cis and trans forms. See alsoDiels–Alder reaction.

butadiene See buta-1,3-diene.

butanal (butyraldehyde; C3H7CHO) Acolorless liquid aldehyde.

butane (C4H10) A gaseous alkane ob-tained either from the gaseous fraction ofcrude oil or by the ‘cracking’ of heavierfractions. It is the fourth member of the ho-mologous series of alkanes. Butane is easilyliquefied under pressure and its main use isas a portable supply of fuel (bottle gas). Itis also used in the industrial production ofbuta-1,3-diene. The isomeric hydrocarbonCH3CH(CH3)CH3 (2-methylpropane) isknown as isobutane.

butanedioic acid (succinic acid) A crys-talline carboxylic acid, HOOC(CH2)2-COOH, that occurs in amber and certainplants. It forms during the fermentation ofsugar (sucrose).

butanoic acid (butyric acid; C3H7-COOH) A colorless liquid carboxylicacid. Esters of butanoic acid are present inbutter.

butanol (butyl alcohol; C4H9OH) Ei-ther of two alcohols that are derived frombutane: the primary alcohol butan-1-ol(CH3(CH2)2CH2OH) and the secondaryalcohol butan-2-ol (CH3CH(OH)CH2-CH3). Both are colorless volatile liquidsused as solvents.

butanone (methyl ethyl ketone; CH3-COC2H5) A colorless volatile liquid ke-tone. It is manufactured by the catalyticoxidation of butane and used as a solvent.

butenedioic acid Either of two isomers.Transbutenedioic acid (fumaric acid) is acrystalline compound found in certainplants. Cisbutenedioic acid (maleic acid) isused in the manufacture of synthetic resins.

It can be converted into the trans isomer byheating at 120°C. The cis form, whenstrongly heated, loses water to give a cyclicacid anhydride (maleic anhydride).

Butlerov, Aleksandr Mikhailovich(1828–86) Russian organic chemist whowas one of the main pioneers of the con-cept of structure for compounds. He firstput forward his ideas in 1861. Butlerovpredicted that tertiary alcohols exist andsynthesized tertiary butanol. In 1876 heproposed the basics of the concept of tau-tomerism. He also studied formaldehyde(methanal) and its polymerization to vari-ous sugars.

butyl alcohol See butanol.

butyl group The straight-chain alkylgroup CH3(CH2)2CH2–.

butyl rubber A type of synthetic rubbermade by copolymerizing isobutylene (2-methylpropene, CH3:C(CH3)2) with smallamounts of isoprene (methylbuta-1,3-diene, CH3:C(CH3)CH:CH2).

Before the introduction of tubeless tiresbutyl rubber was used for inner tubesbecause it is impervious to air. Subse-quently halogenated butyl rubbers weredeveloped (halobutyls), which could be

35

butyl rubber

HO.OC H

CO.OH

C

CH

H CO.OH

CO.OHC

C

Hcis (maleic)

trans (fumaric)

Butenedioic acid

cured at higher temperature and vulcan-ized with other rubbers. Both chlorobutylsand bromobutyls are manufactured. Thesetypes of rubber are used in tubeless tiresbonded to the inner surface of the tire.Other uses are in sealants, hoses, and pondliners.

butyraldehyde See butanal.

butyric acid See butanoic acid.

by-product A substance obtained dur-ing the manufacture of a main chemicalproduct. For example, propanone was for-merly manufactured from propan-1-ol, butis now obtained as a by-product in themanufacture of phenol by the CUMENE

PROCESS.

butyraldehyde

36

37

cadaverine An amine, H2N[CH2]5NH2,produced from lysine in decaying meat andfish.

caffeine An alkaloid found in certainplants, especially tea and coffee. It is astimulant of the central nervous systemand a diuretic. The systematic name is1,3,7-trimethylxanthine.

cage compounds See clathrate.

Cahn–Ingold–Prelog system See CIPsystem.

calciferol See vitamin D.

calcium acetylide See calcium carbide.

calcium carbide (calcium acetylide; cal-cium dicarbide; Ca C2) A white solid thatcan be produced by heating coke with cal-cium oxide at high temperature (2000°C).It contains the dicarbide ion –C≡C– and re-acts with water to give ethyne (acetylene;C2H2):

CaC2 + 2H2O → Ca(OH)2 + C2H2Formerly it was an important source ofethyne.

calcium dicarbide See calcium carbide.

calixarene See host–guest chemistry.

calorie Symbol: cal A unit of energy ap-proximately equal to 4.2 joules. It was for-merly defined as the energy needed to raisethe temperature of one gram of water byone degree Celsius. Because the specificthermal capacity of water changes withtemperature, this definition is not precise.

The mean or thermochemical calorie(calTH) is defined as 4.184 joules. The in-ternational table calorie (calIT) is defined as4.1868 joules. Formerly the mean caloriewas defined as one hundredth of the heatneeded to raise one gram of water from0°C to 100°C, and the 15°C calorie as theheat needed to raise it from 14.5°C to15.5°C.

calorific value The energy content of asubstance, defined as the energy released inburning unit mass. Calorific values aremeasured using a BOMB CALORIMETER. Theyare used to express the efficiency of fuels(in megajoules per kilogram). Calorific val-ues are also applied to foods (in kilojoulesper gram or in calories). Here they measurethe energy produced when the food is oxi-dized in metabolism.

calorimeter A device or apparatus formeasuring thermal properties such as spe-cific heat capacity, calorific value, etc. Seebomb calorimeter.

Calvin, Melvin (1911–97) Americanchemist. Calvin worked out the biologicalmechanisms that occur in photosynthesis.He used techniques such as chromatogra-phy and radioisotopes to study the reac-tions of photosynthesis that do not requirelight. He found that there is a series of re-actions, now known as the Calvin cycle.Calvin summarized his findings in a work entitled The Path of Carbon inPhotosynthesis (1957). Calvin won the1961 Nobel Prize for chemistry for hiswork on photosynthesis. He continued towork on various problems associated withphotosynthesis.

C

Calvin cycle

38

Calvin cycle (reductive pentose-phos-phate cycle) See photosynthesis.

cAMP See cyclic AMP.

camphor (C10H16O) A naturally-occur-ring white organic compound with a char-acteristic penetrating odor. It is a cycliccompound and a ketone, formerly ob-tained from the wood of the camphor treebut now made synthetically. Camphor isused as a platicizer for celluloid and as aninsecticide against clothes moths.

cane sugar See sucrose.

Cannizzaro reaction The reaction ofaldehydes to give alcohols and acid anionsin the presence of strong bases. The alde-hydes taking part in the Cannizzaro reac-tion do not have hydrogen atoms on thecarbon attached to the aldehyde group. Forinstance, in the presence of hot aqueoussodium hydroxide:

NaOH + 2C6H5CHO →C6H5CH2OH + C6H5COO–Na+

This reaction is a disproportionation, in-volving both oxidation (to acid) and reduc-tion (to alcohol). Another example is thereaction of methanal to give methanol andmethanoate ions.

NaOH + 2HCHO → CH3OH +HCOO–Na+

The reaction was first described by theItalian chemist Stanislao Cannizzaro(1826– 1910) in 1853.

canonical form See resonance.

caproic acid See hexanoic acid.

caprolactam (C6H11NO) A white crys-talline substance used in the manufactureof NYLON.

carbamide See urea.

carbanion An intermediate in an or-ganic reaction in which one carbon atomcarries a negative charge. Carbanions maybe formed by abstracting a hydrogen ionfrom a C–H bond using a base, e.g. from

ethanal to form –CH2CHO (see aldol reac-tion). They can also be formed fromorganometallic compounds in which thecarbon atom is bonded to an electroposi-tive metal.

carbazole (C12H9N) A white crystallinecompound used in the manufacture ofdyestuffs.

carbene A transient species of the formRR′C:, with two valence electrons that donot form bonds. The simplest example ismethylene, H2C:. Some complex carbenescan be isolated but most are short-lived in-termediates in reactions. In a carbene thecarbon atom has two valence electrons thatare not involved in bonding, and carbenespecies are highly electrophilic. Typicallythey can attack carbon–carbon doublebonds to produce cyclopropane deriva-tives. Also they can attack single bonds ininsertion reactions. For example, they caninsert into O–H bonds:

R–O–H + R2C: → R–O–C(R2)–HThey can also insert into C–H bonds:

R–H + R2C: → R–C(R2)–HReactions like these make carbenes impor-tant ‘reagents’ in organic synthesis and var-ious methods have been developed forgenerating them in the reaction medium.

All carbenes can exist in two possiblestates. The carbon atom has one s orbital

H2C

C C

N

O

CH2C

H2

CH2

H

H2

Caprolactam

NNNHHH

Carbazole

and three p orbitals available for bonding.In carbenes sp2 hybridization occurs andthe carbon atom has three trigonal sp2 hy-brid orbitals in a plane, with one p orbitalat right angles to the plane. Two of thehybrid sp2 orbitals each contain one elec-tron, and form sigma bonds with othercarbon atoms. The two remaining valenceelectrons may be distributed in one of twopossible ways. One, the triplet state, hasone electron in a sp2 orbital and the otherin the p orbital. The other, the singlet state,has both electrons in the sp2 orbital, withan empty p orbital. A triplet-state carbenecan be detected by electron spin resonance(ESR) because there are two unpairedelectrons. Carbenes in the singlet state do not have unpaired electrons and are notdetectable by ESR. Moreover, in a car-bene R2C:, the R–C–R bond angle will belarger if the carbene is in the triplet statethan if it is in the singlet state. In the singletstate the nonbonding electrons form aLONE PAIR, which occupies more space thana single electron. In general, the triplet state(with two unpaired electrons) is more sta-ble than the singlet state. However, there isnot a large energy difference between thetwo states and, depending on the groupsattached to the carbon, the singlet statemay be the more stable for certain car-benes. Also, the state that is actuallyformed as an intermediate may depend onthe mechanism of production. The less sta-ble form of the carbene may be producedand its chemical behavior depends on howquickly it reacts or converts to the morestable state.

The electronic state of the carbene –whether it is singlet or triplet – may haveimportant consequences for how it reacts.For example, a singlet carbene adding to adouble bond to produce a cyclopropanering does so in one stage, so any stereo-chemistry of the double bond is preservedin the product. If the carbene has a tripletstate it acts as a diradical, and the reactionproceeds in two stages. Any stereochem-istry about the double bond will not bepreserved in the final product

carbenium ion See carbocation.

carbide A compound of carbon with amore electropositive element. The carbidesof the elements are classified into:1. Ionic carbides, which contain the car-

bide ion C4–. An example is aluminumcarbide, Al4C3. Compounds of this typereact with water to give methane (theywere formerly also called methanides).The dicarbides are ionic carbon com-pounds that contain the dicarbide ion–C:C–. The best-known example is cal-cium dicarbide, CaC2, also known asCALCIUM CARBIDE, or simply carbide.Compounds of this type give ethynewith water. They were formerly calledacetylides or ethynides. Ionic carbidesare formed with very electropositivemetals. They are crystalline.

2. Covalent carbides, which have giant-molecular structures, as in silicon car-bide (SiC) and boron carbide (B4C3).These are hard high-melting solids.Other covalent compounds of carbon(CO2, CS2, CH4, etc.) have covalentmolecules.

3. Interstitial carbides, which are intersti-tial compounds of carbon with transi-tion metals. Titanium carbide (TiC) is anexample. These compounds are all hardhigh-melting solids, with metallic prop-erties. Some carbides (e.g. nickel carbideNi3C) have properties intermediate be-tween those of interstitial and ionic car-bides.

carbocation An ion with a positivecharge in which the charge is mostly local-ized on a carbon atom. There are twotypes. Carbonium ions have five bonds tothe carbon atom and a complete outer shellof 8 electrons. The simplest example wouldbe the carbonium ion CH5

+, which couldbe regarded as formed by adding H+ tomethane, CH4, in the same way that theammonium ion, NH4

+, is formed from am-monia, NH3. There is, however, a differ-ence between ammonia and methane inthat ammonia has a lone pair of electrons,which it can donate in forming the NH4

+

ion. The carbonium ion CH5+ (and similar

ions) is a transient species, produced in thegas phase by electron bombardment of or-ganic compounds and detected in a mass

39

carbocation

spectrum. Its shape is that of a carbonatom with three hydrogens in a plane andone hydrogen above and one below (a trig-onal bipyramid).

Carbenium ions have three bonds to thecentral carbon and are planar, with thebonds directed toward the corners of a tri-angle (sp2 hybridization). They have sixelectrons in the outer shell of carbon and avacant p orbital. Carbenium ions are im-portant intermediates in a number oforganic reactions, notably the SN1 mecha-nism of NUCLEOPHILIC SUBSTITUTION. It ispossible to produce stable carbenium ionsin salts of the type (C6H5)3C+Cl–, which areorange-red solids. In these the triphenyl-methyl cation is stabilized by delocaliza-tion over the three phenyl groups. It is alsopossible to produce carbenium ions usingSUPERACIDS.

carbocyclic compound A compound,such as benzene or cyclohexane, that con-tains a ring of carbon atoms in its struc-ture.

carbohydrate Any of a class of com-pounds occurring widely in nature andhaving the general formula Cx(H2O)y.(Note that although the name suggests ahydrate of carbon these compounds are inno way hydrates and have no similarities toclasses of hydrates.) Carbohydrates aregenerally divided into two main classes:SUGARS and POLYSACCHARIDES.

Carbohydrates are both stores of en-ergy and structural elements in living sys-tems; plants having typically 15%carbohydrate and animals about 1% car-bohydrate. The body is able to build uppolysaccharides from simple units (an-abolism) or break the larger units down tomore simple units for releasing energy (ca-tabolism).

carbolic acid A former name for phenol(hydroxybenzene; C6H5OH).

carbon The first element of group 14(formerly IVA) of the periodic table. Car-bon is a universal constituent of living mat-ter and the principal deposits of carboncompounds are derived from living

sources; i.e., carbonates (chalk and lime-stone) and fossil fuels (coal, oil, and gas). Italso occurs in the mineral dolomite. The el-ement forms only 0.032% by mass of theEarth’s crust. Minute quantities of elemen-tal carbon also occur as the allotropesgraphite and diamond. A third allotrope,BUCKMINSTERFULLERENE (C60), also exists.

Naturally occurring carbon has the iso-topic composition 12C (98.89%), 13C(1.11%) and 14C (minute traces in theupper atmosphere produced by slow neu-tron capture by 14N atoms). 14C is used forradiocarbon dating because of its longhalf-life of 5730 years.

carbonate A salt of carbonic acid (con-taining the ion CO3

2–).

carbonation 1. The solution of carbondioxide in a liquid under pressure, as incarbonated soft drinks.2. The addition of carbon dioxide to com-pounds, e.g. the insertion of carbon diox-ide into Grignard reagents.

carbon black A finely divided form ofcarbon produced by the incomplete com-bustion of such hydrocarbon fuels as nat-ural gas or petroleum oil. It is used as ablack pigment in inks and as a filler forrubber in tire manufacture.

carbon cycle The circulation of carboncompounds in the environment, one of themajor natural cycles of an element. Carbondioxide in the air is used by green plants inphotosynthesis (in which it is combinedwith water to form sugars and starches).Plants are eaten by animals which exhalecarbon dioxide, or when plants and ani-mals die their remains decompose with theproduction of carbon dioxide. Some plantsare burned or converted to fossil fuelswhich are burned, again with the forma-tion of carbon dioxide.

carbon dating (radiocarbon dating) Amethod of dating – measuring the age of(usually archaeological) materials that con-tain matter of living origin. It is based onthe fact that 14C, a beta emitter of half-lifeapproximately 5730 years, is being formed

carbocyclic compound

40

continuously in the atmosphere as a resultof cosmic-ray action. The 14C becomes in-corporated into living organisms. Afterdeath of the organism the amount of radio-active carbon decreases exponentially byradioactive decay. The ratio of 12C to 14Cis thus a measure of the time elapsed sincethe death of the organic material. Themethod is most valuable for specimens ofup to 20 000 years old, though it has beenmodified to measure ages up to 70 000years. For ages of up to about 8000 yearsthe carbon time scale has been calibratedby dendrochronology; i.e. by measuringthe 12C:14C ratio in tree rings of knownage.

carbon dioxide (CO2) A colorlessodorless nonflammable gas formed whencarbon burns in excess oxygen. It is alsoproduced by respiration. Carbon dioxide ispresent in the atmosphere (0.03% byvolume) and is converted in plants to car-bohydrates by photosynthesis. In the la-boratory it is made by the action of diluteacid on metal carbonates. Industrially, it isobtained as a by-product in certain pro-cesses, such as fermentation or the manu-facture of lime. The main uses are as arefrigerant (solid carbon dioxide, calleddry ice) and in fire extinguishers and car-bonated drinks. Increased levels of carbondioxide in the atmosphere from the com-

bustion of fossil fuels are thought to con-tribute to the greenhouse effect.

Carbon dioxide is the anhydride of theweak acid carbonic acid, which is formedin water:

CO2 + H2O ˆ H2CO3

carbon disulfide (CS2) A colorless poi-sonous flammable liquid made frommethane (natural gas) and sulfur. The purecompound is virtually odorless, but CS2usually has a revolting smell because of thepresence of other sulfur compounds. It isused as a solvent and in the production ofxanthates in making viscose rayon.

carbon fibers Fibers of graphite, whichare used, for instance, to strengthen poly-mers. They are made by heating stretchedtextile fibers and have an orientated crystalstructure.

carbonic acid (H2CO3) A dibasic acidformed in small amounts in solution whencarbon dioxide dissolves in water:

CO2 + H2O ˆ H2CO2It forms two series of salts: hydrogencar-bonates (HCO3

–) and carbonates (CO32–).

The pure acid cannot be isolated.

carbonium ion See carbocation.

carbonize (carburize) To convert an or-

41

carbonize

organiccompoundsin animals

feedingfeedingfeedingorganiccompoundsin green plants

fossil fuels(coal and peat)

combustioncombustioncombustionrespirationrespirationrespirationand decayand decayand decayphotosynthesisphotosynthesisphotosynthesis

CO2free carbondioxide in theatmosphere

respirationrespirationrespirationand decayand decayand decay

Carbon cycle

carbon monoxide

42

ganic compound into carbon by incom-plete oxidation at high temperature.

carbon monoxide (CO) A colorlessflammable toxic gas formed by the incom-plete combustion of carbon. In the labora-tory it can be made by dehydratingmethanoic acid with concentrated sulfuricacid:

HCOOH – H2O → COIndustrially, it is produced by the oxida-tion of carbon or of natural gas, or by thewater-gas reaction. It is a powerful reduc-ing agent and is used in metallurgy.

Carbon monoxide is neutral and onlysparingly soluble in water. It is not the an-hydride of methanoic acid, although underextreme conditions it can react withsodium hydroxide to form sodiummethanoate. It forms metal carbonyls withtransition metals, and its toxicity is due toits ability to form a complex with hemo-globin (in preference to oxygen).

carbon tetrachloride See tetrachloro-methane.

carbonyl A complex in which carbonmonoxide ligands are coordinated to ametal atom. A common example is tetra-carbonyl nickel(0), Ni(CO)4.

carbonyl chloride (phosgene; COCl2) A colorless toxic gas with a choking smell.It is used as a chlorinating agent and tomake polyurethane plastics and insecti-cides; it was formerly employed as a wargas.

carbonyl group The group –C=O. It

occurs in aldehydes (RCO.H), ketones(RR′CO), carboxylic acids (RCO.OH),and in carbonyl complexes of transitionmetals. The group is polar, with negativecharge on the oxygen.

carboxyhemoglobin A complex formedwhen carbon monoxide coordinates to theiron atom in hemoglobin molecules. Theproduct is very stable and hemoglobin hasa much greater affinity for carbon monox-ide than for oxygen. The toxic effect of car-bon monoxide is due to its ability to blockhemoglobin as an oxygen carrier.

carboxylate ion The ion –COO–, pro-duced by ionization of a carboxyl group. Ina carboxylate ion the negative charge isgenerally delocalized over the O–C–Ogrouping and the two C–O bonds have thesame length, intermediate between that ofa double C=O and a single C–O.

carboxyl group The organic group–CO.OH, present in carboxylic acids.

carboxylic acid A type of organic com-pound containing the CARBOXYL GROUP.Simple carboxylic acids have the generalformula RCOOH. Many carboxylic acidsoccur naturally in plants and (in the formof esters) in fats and oils, hence the alter-native name fatty acids. Carboxylic acidswith one COOH group are monobasic,those with two, dibasic, and those withthree, tribasic. The methods of preparationare:1. Oxidation of a primary alcohol or an

aldehyde:RCH2OH + 2[O] → RCOOH + H2O

CR

O

O_

CR

O_

O

CR

O

O

_

Carboxylate ion

2. Hydrolysis of a nitrile using dilute hy-drochloric acid:

RCN + HCl + 2H2O → RCOOH +NH4Cl

The acidic properties of carboxylic acidsare due to the carbonyl group, which at-tracts electrons from the C–O and O–Hbonds. The CARBOXYLATE ION formed,R–COO–, is also stabilized by delocaliza-tion of electrons over the O–C–O group-ing.

Other reactions of carboxylic acids in-clude the formation of ESTERS and the reac-tion with phosphorus(V) chloride to formACYL HALIDES.

carburize See carbonize.

carbylamine reaction See isocyanidetest.

carcinogen Any substance that causesliving tissues to become cancerous. Chemi-cal carcinogens include many organic com-pounds, e.g. hydrocarbons in tobaccosmoke, as well as inorganic ones, e.g. as-bestos. Carcinogenic physical agents includeultraviolet light, x-rays, and radioactivematerials. Some viruses (e.g. hepatitis B)are also carcinogens. Many carcinogensare mutagenic, i.e. they cause changes inthe DNA; dimethylnitrosamine, for exam-ple, methylates the bases in DNA. A poten-tial carcinogen may therefore be identifiedby determining whether it causes muta-tions.

Carius method A method in quantita-tive analysis for determining the amountsof halogens, phosphorus, and sulfur in or-ganic compounds. The compound isheated with concentrated nitric acid andsilver nitrate in a sealed tube. The silvercompounds produced are separated andweighed.

Carnot cycle The idealized reversiblecycle of four operations occurring in a per-fect heat engine. These are the successiveadiabatic compression, isothermal expan-sion, adiabatic expansion, and isothermalcompression of the working substance.The cycle returns to its initial pressure, vol-

ume, and temperature, and transfers en-ergy to or from mechanical work. The effi-ciency of the Carnot cycle is the maximumattainable in a heat engine. See Carnot’sprinciple.

Carnot’s principle (Carnot theorem)The principle that efficiency of any heat en-gine cannot be greater than that of a re-versible heat engine operating over thesame temperature range. It follows directlyfrom the second law of thermodynamics,and means that all reversible heat engineshave the same efficiency, independent ofthe working substance. If heat is absorbedat temperature T1 and given out at T2, thenthe Carnot efficiency is (T1–T2)/T1.

Carnot theorem See Carnot’s principle.

carotene A carotenoid pigment, exam-ples being lycopene and α- and β-carotene.The latter compounds are important in an-imal diets as a precursor of vitamin A. Seecarotenoids; photosynthetic pigments.

carotenoid Any of a group of yellow,orange, or red pigments comprising theCAROTENES and XANTHOPHYLLS. They arefound in all photosynthetic organisms,where they function mainly as accessorypigments in photosynthesis, and in someanimal structures, e.g. feathers. They con-tribute, with anthocyanins, to the autumncolors of leaves since the green pigmentchlorophyll, which normally masks thecarotenoids, breaks down first. They arealso found in some flowers and fruits, e.g.tomato. Carotenoids have three absorptionpeaks in the blue-violet region of the spec-trum.

Carotenes are hydrocarbons. The mostwidespread is β-carotene. This is the or-ange pigment of carrots whose molecule issplit into two identical portions to yieldvitamin A during digestion in vertebrates.Xanthophylls resemble carotenes but con-tain oxygen. See also photosynthetic pig-ments.

Carothers, Wallace Hume (1896–1937)American organic chemist. Carothers isbest known for having discovered nylon.

43

Carothers, Wallace Hume

He also produced neoprene, a syntheticrubber, in 1931. He did so by treating viny-lacetylene with hydrochloric acid. Thisproduced the monomer chlorobutadienewhich readily polymerizes to give the poly-mer neoprene. In the search to find artificalversions of silk and cellulose he used manytypes of condensation polymers. In 1935he discovered the polyamide usuallyknown as nylon by the condensation ofadipic acid and hexamethylenediamine.Carothers suffered from depression andcommitted suicide in 1937. He thereforedid not live to see the commercial produc-tion of nylon in 1940.

carrier gas The gas used to carry thesample in GAS CHROMATOGRAPHY.

casein A phosphorus-containing proteinthat occurs in milk and cheese. It is easilydigested by young mammals and is theirmajor source of protein and phosphorus.The protein has been used for making cer-tain items, such as billiard balls and but-tons, and has also been used to make glue.These uses have declined because of com-petition from synthetic polymers.

catabolism All the metabolic reactionsthat break down complex molecules tosimpler compounds. The function of cata-bolic reactions is to provide energy. Seealso metabolism.

catalyst A substance that alters the rateof a chemical reaction without itself beingchanged chemically in the reaction. Thecatalyst can, however, undergo physicalchange; for example, large lumps of cata-lyst can, without loss in mass, be convertedinto a powder. Small amounts of catalystare often sufficient to increase the rate ofreaction considerably. A positive catalystincreases the rate of a reaction and a nega-tive catalyst reduces it. Homogeneous cat-alysts are those that act in the same phaseas the reactants (i.e. in gaseous and liquidsystems). For example, nitrogen(II) oxidegas will catalyze the reaction between sul-fur(IV) oxide and oxygen in the gaseousphase. Heterogeneous catalysts act in a dif-ferent phase from the reactants. For exam-

ple, finely divided nickel (a solid) will cat-alyze the hydrogenation of oil (liquid).

In increasing a reaction rate a catalystprovides a new pathway for which the rate-determining step has a lower activation en-ergy than in the uncatalyzed reaction. Acatalyst does not change the products in anequilibrium reaction and their concentra-tion is identical to that in the uncatalyzedreaction; i.e. the position of the equilib-rium remains unchanged. The catalyst sim-ply increases the rate at which equilibriumis attained.

In autocatalysis, one of the products ofthe reaction itself acts as a catalyst. In thistype of reaction the reaction rate increaseswith time to a maximum and finally slowsdown. For example, in the hydrolysis ofethyl ethanoate, the ethanoic acid pro-duced catalyzes the reaction.

catalytic converter A device fitted tothe exhaust system of gasoline-fuelled vehi-cles to remove pollutant gases from the ex-haust. It consists of a honeycomb structure(to provide maximum area) coated withplatinum, palladium, and rhodium cata-lysts. Such devices can convert carbonmonoxide to carbon dioxide, oxides of ni-trogen to nitrogen, and unburned fuel tocarbon dioxide and water.

catalytic cracking The conversion,using a catalyst, of long-chain hydrocar-bons from the refining of petroleum intomore useful shorter-chain compounds suchas those occurring in kerosene and gaso-line.

catalytic reaction A chemical reactionthat occurs at a measurable rate only in thepresence of a catalyst.

catechol (1,2-dihydroxybenzene) Acolourless crystalline PHENOL. It is used inphotographic developing.

catecholamine Any of a group of im-portant amines that contain a catechol ringin their molecules. They are neurotransmit-ters and hormones. Examples are epineph-rine and norepinephrine.

carrier gas

44

catenation The formation of chains ofatoms in molecules.

cathode In electrolysis, the electrodethat is at a negative potential with respectto the anode. In any electrical system, suchas a discharge tube or electronic device, thecathode is the terminal at which electronsenter the system.

cation A positively charged ion, formedby removal of electrons from atoms ormolecules. In electrolysis, cations are at-tracted to the negatively charged electrode(the cathode). Compare anion.

cationic detergent See detergent.

cationic resin An ION-EXCHANGE ma-terial that can exchange cations, such as H+

and Na+, for ions in the surroundingmedium. Such resins are used for a widerange of purification and analytical pur-poses.

They are often produced by adding asulfonic acid group (–SO3

–H+) or a car-boxylate group (–COO–H+) to a stablepolyphenylethene resin. A typical exchangereaction is:

resin–SO3–H+ + NaCl = resin–SO3

–Na+

+ HClThey have been used to great effect to sep-arate mixtures of cations of similar sizehaving the same charge. Such mixtures canbe attached to cationic resins and progres-sive elution will recover them in order ofdecreasing ionic radius.

cell A system having two plates (elec-trodes) in a conducting liquid (electrolyte).An electrolytic cell is used for producing achemical reaction by passing a currentthrough the electrolyte (i.e. by electrolysis).A voltaic (or galvanic) cell produces ane.m.f. by chemical reactions at each elec-trode. Electrons are transferred to or fromthe electrodes, giving each a net charge.

cellulose A POLYSACCHARIDE (C6H10O5)nof glucose, which is the main constituent ofthe cell walls of plants. It is obtained fromwood pulp.

cellulose acetate (cellulose ethanoate) Apolymeric substance made by acetylatingcellulose using a mixture of ethanoic acid,ethanoic anhydride, and sulfuric acid. It isused in plastics, in acetate film, and inacetate rayon.

cellulose ethanoate See cellulose ac-etate.

cellulose trinitrate (guncotton; nitrocel-lulose) A highly flammable substancemade by treating cellulose with a nitric–sulfuric acid mixture. Cellulose trinitrate isused in explosives and in lacquers. It is anester of nitric acid (i.e. not a true nitrocompound).

Celsius scale A temperature scale inwhich the temperature of melting pure iceis taken as 0° and the temperature of boil-ing water 100° (both at standard pressure).The degree Celsius (°C) is equal to thekelvin. This was known as the centigradescale until 1948, when the present namebecame official. It is named for the Swedishastronomer Anders Celsius (1701–44).Celsius’ original scale (1742) was inverted(i.e. had 0° as the steam temperature and100° as the ice temperature). See also tem-perature scale.

centi- Symbol: c A prefix denoting 10–2.For example, 1 centimeter (cm) = 10–2

meter (m).

centigrade scale See Celsius scale.

centrifugal pump A device commonlyused for transporting fluids around achemical plant. Centrifugal pumps usuallyhave a set of blades rotating inside a fixedcircular casing. As the blades rotate, thefluid is impelled out of the pump along apipe. Centrifugal pumps do not producehigh pressures but they have the advantageof being relatively cheap because they aresimple in design, have no valves, and workat high speeds. In addition they are notdamaged if a blockage develops. Comparedisplacement pump.

45

centrifugal pump

centrifuge An apparatus for rotating acontainer at high speeds, used to increasethe rate of sedimentation of suspensions orthe separation of two immiscible liquids.See also ultracentrifuge.

CFC Chlorofluorocarbon. See halocar-bon.

c.g.s. system A system of units that usesthe centimeter, the gram, and the second asthe base mechanical units. Much early sci-entific work used this system, but it hasnow almost been abandoned in favor of SI

UNITS.

chain When two or more atoms formbonds with each other in a molecule, achain of atoms results. This chain may be astraight chain, in which each atom is addedto the end of the chain, or it may be abranched chain, in which the main chain ofatoms has one or more smaller side chainsbranching off it.

chain reaction A self-sustaining chemi-cal reaction consisting of a series of steps,each of which is initiated by the one beforeit. An example is the reaction between hy-drogen and chlorine:

Cl2 → 2Cl•H2 + Cl• → HCl + H•H• + Cl2 → HCl + Cl•

2H• → H2

2Cl• → Cl2The first stage, chain initiation, is the dis-sociation of chlorine molecules into atoms;this is followed by two chain propagationreactions. Two molecules of hydrogenchloride are produced and the ejected chlo-rine atom is ready to react with more hy-drogen. The final steps, chain termination,stop the reaction. Chain reactions are im-portant in certain types of free-radicalPOLYMERIZATION reactions.

chair conformation See cyclohexane.

chalcogens The elements of group 16 ofthe periodic table: oxygen, sulfur, sele-nium, tellurium, and polonium.

charcoal An amorphous form of carbonmade by heating wood or other organicmaterial in the absence of air. Activatedcharcoal is charcoal heated to drive off ab-sorbed gas. It is used for absorbing gasesand for removing impurities from liquids.

Chardonnet, Louis-Marie-HilaireBernigaud, Comte de (1839–1924)French organic chemist. Chardonnet is bestknown for inventing rayon. This was thefirst type of artificial silk to be produced. In1884 he produced nitrocellulose (guncot-ton) by treating a pulp made from mul-berry leaves with a mixture of nitric acidand sulphuric acid. The resulting cellulosecompound was dissolved in a mixture ofalcohol and ether, with this solution thenbeing forced into cold water through capil-lary tubes. Nitrocellulose was very inflam-mable, so Chardonnet sought a non-inflammable fibre. This culminated in thedevelopment of rayon in 1889.

Charles’ law For a given mass of gas atconstant pressure, the volume increases bya constant fraction of the volume at 0°Cfor each Celsius degree rise in temperature.The constant fraction (α) has almost thesame value for all gases – about 1/273 –and Charles’ law can be written in the form

V = V0(1 + αvθ)where V is the volume at temperature θ°Cand V0 the volume at 0°C. The constant αvis the thermal expansivity of the gas. For anideal gas its value is 1/273.15.

A similar relationship exists for thepressure of a gas heated at constant vol-ume:

p = p0(1 + αpθ)Here, αp is the pressure coefficient. For anideal gas

αp = αvalthough they differ slightly for real gases.It follows from Charles’ law that for a gasheated at constant pressure,

V/T = Kwhere T is the thermodynamic temperatureand K is a constant. Similarly, at constantvolume, p/T is a constant.

Charles’ volume law is sometimescalled Gay-Lussac’s law after its indepen-dent discoverer.

centrifuge

46

chelate A metal coordination complexin which one ligand coordinates at two ormore points to the same metal ion. The re-sulting complex contains rings of atomsthat include the metal atom. An example ofa chelating agent is 1,2-diaminoethane(H2NCH2CH2NH2), which can coordinateboth its amine groups to the same atom. Itis an example of a bidentate ligand (havingtwo ‘teeth’). EDTA, which can form up tosix bonds, is another example of a chelat-ing agent. The word chelate comes fromthe Greek word meaning ‘claw’.

chemical bond A link between atomsthat leads to an aggregate of sufficient sta-bility to be regarded as an independent mo-lecular species. Chemical bonds includecovalent bonds, electrovalent (ionic)bonds, coordinate bonds, and metallicbonds. Hydrogen bonds and van der Waalsforces are not usually regarded as truechemical bonds.

chemical combination, laws of Agroup of chemical laws developed duringthe late 18th and early 19th centuries,which arose from the recognition of the im-portance of quantitative (as opposed toqualitative) study of chemical reactions.The laws are:1. the law of conservation of mass (matter);2. the law of constant (definite) propor-

tions;3. the law of multiple proportions;4. the law of equivalent (or reciprocal) pro-

portions,These laws played a significant part in Dal-ton’s development of his atomic theory(1808).

chemical conversions See unit processes.

chemical dating A method of usingchemical analysis to find the age of an ar-chaeological specimen in which composi-tional changes have taken place over time.For example, the determination of theamount of fluorine in bone that has beenburied gives an indication of its age be-cause phosphate in the bone has graduallybeen replaced by fluoride ions fromgroundwater. Another dating technique

depends on the fact that, in living organ-isms, amino acids are optically active.After death a slow racemization reactionoccurs and a mixture of L- and D-isomersforms. The age of bones can be accuratelydetermined by measuring the relativeamounts of L- and D-amino acids present.

chemical engineering The branch ofengineering concerned with the design andmaintenance of a chemical plant and itsability to withstand extremes of tempera-ture and pressure, corrosion, and wear. Itenables laboratory processes producinggrams of material to be converted into alarge-scale plant producing tonnes of ma-terial. Chemical engineers plan large-scalechemical processes by linking together theappropriate unit processes and by studyingsuch parameters as heat and mass transfer,separations, and distillations.

chemical equation A method of repre-senting a chemical reaction using chemicalformulae. The formulae of the reactantsare given on the left-hand side of the equa-tion, with the formulae of the products onthe right. The two halves are separated bya directional arrow or arrows (or an equalssign). A number preceding a formula(called a stoichiometric coefficient) indi-cates the number of molecules of that sub-stance involved. The equation mustbalance – that is, the number of atoms ofany one element must be the same on bothsides of the equation.

chemical equilbrium See equilibrium.

chemical formula See formula.

chemical reaction A process in whichone or more elements or chemical com-pounds (the reactants) react to produce adifferent substance or substances (theproducts).

chemical shift See nuclear magnetic res-onance.

chemiluminescence The emission oflight during a chemical reaction.

47

chemiluminescence

chemisorption See adsorption.

chiral Having the property of CHIRALITY.For example, lactic acid is a chiral com-pound because it has two possible struc-tures that cannot be superposed. Seeoptical activity.

chirality The property of existing in left-and right-handed forms; i.e. forms that arenot superposable. In chemistry the term isapplied to the existence of optical isomers.See optical activity.

chirality element A part of a moleculethat causes it to display chirality. The mostcommon type of element is a chirality cen-ter, which is an atom attached to four dif-ferent atoms or groups. This is alsoreferred to as an asymmetric atom. Lesscommonly a molecule may have a chiralityaxis, as in the case of certain substituted al-lenes of the type R1R2C=C=CR3R4. In thisform of compound the R1 and R2 groupsdo not lie in the same plane as the R3 andR4 groups because of the nature of the dou-ble bonds. The chirality axis lies along theC=C=C chain. It is also possible to havemolecules that contain a chirality plane.See optical activity.

chitin A nitrogen-containing heteropoly-saccharide found in some animals and thecell walls of most fungi. It is a polymer ofN-acetylglucosamine. It consists of manyglucose units, in each of which one of thehydroxyl groups has been replaced by anacetylamine group (CH3CONH). Theouter covering of arthropods, the cuticle, isimpregnated in its outer layers with chitin,which makes the exoskeleton more rigid. Itis associated with protein to give auniquely tough yet flexible and light skel-eton, which also has the advantage of beingwaterproof. The chitinous plates are thin-ner for bending and flexibility or thickerfor stiffness as required. The plates cannotgrow once laid down and are broken downat each molt. Chitin is also found in thehard parts of several other groups of ani-mals.

chloral See trichloroethanal.

chloral hydrate See trichloroethanal.

chloramine (NH2Cl) A colorless liquidmade by reacting ammonia with sodiumchlorate(I) (NaOCl). It is formed as an in-termediate in the production of hydrazine.Chloramine is unstable and changes explo-sively into ammonium chloride and nitro-gen trichloride.

chloride See halide.

chlorination 1. Treatment with chlo-rine; for instance, the use of chlorine to dis-infect water.2. See halogenation.

chlorine A green reactive gaseous el-ement belonging to the halogens; i.e. group17 (formerly VIIA) of the periodic table. Itoccurs in sea-water, salt lakes, and under-ground deposits of halite, NaCl. It ac-counts for about 0.055% of the Earth’scrust. Chlorine is strongly oxidizing andcan be liberated from its salts only bystrong oxidizing agents, such as man-ganese(IV) oxide, potassium perman-ganate(VII), or potassium dichromate;note that sulfuric acid is not sufficiently ox-idizing to release chlorine from chlorides.Industrially, chlorine is prepared by theelectrolysis of brine and in some processeschlorine is recovered by the high-tempera-ture oxidation of waste hydrochloric acid.Chlorine is used in large quantities, both asthe element, to produce chlorinated or-ganic solvents, and for the production ofpolyvinyl chloride (PVC), the major ther-moplastic in use today, and in the form ofhypochlorites for bleaching.

The solubility of inorganic metal chlo-rides is such that they are not an environ-mental problem unless the metal ion itselfis toxic but many organochlorine com-pounds are sufficiently stable for the accu-mulated residues of chlorine-containingpesticides to present a severe problem insome areas. This arises because they canaccumulate in food chains and concentratein the tissues of higher animals (see DDT).

chemisorption

48

Symbol: Cl; m.p. –100.38°C; b.p.–33.97°C; d. 3.214 kg m–3 (0°C); p.n. 17;r.a.m. 35.4527.

chloroacetic acid See chloroethanoicacid.

chlorobenzene (monochlorobenzene;C6H5Cl) A colorless liquid made by thecatalytic reaction of chlorine with benzeneor by the RASHIG PROCESS. It can be con-verted to phenol by reaction with sodiumhydroxide under extreme conditions(300°C and 200 atmospheres pressure). Itis also used in the manufacture of other or-ganic compounds.

chloroethane (ethyl chloride; C2H5Cl) A gaseous compound made by the additionof hydrogen chloride to ethene. It is used asa refrigerant and a local anesthetic.

chloroethanoic acid (chloroacetic acid;CH2ClCOOH) A colorless crystallinesolid made by substituting one of the hy-drogen atoms of the methyl group ofethanoic acid with chlorine, using redphosphorus. It is a stronger acid thanethanoic acid because of the electron-with-drawing effect of the chlorine atom.Dichloroethanoic acid (dichloroaceticacid, CHCl2COOH) and trichloroethanoicacid (trichloroacetic acid, CCl3COOH) aremade in the same way. The acid strengthincreases with the number of chlorineatoms present.

chloroethene (vinyl chloride; H2C:CHCl)A gaseous organic compound used in the manufacture of PVC (polyvinyl chlo-ride). Chloroethene is manufactured by thereaction between ethyne and hydrogenchloride using a mercury(II) chloride cata-lyst:

C2H2 + HCl → H2C:CHClAn alternative source, making use of theready supply of ethene, is via dichloro-ethane:

H2C:CH2+ Cl2 → CH2Cl.CH2Cl →H2C:CHCl

chlorofluorocarbon See halocarbon.

chloroform See trichloromethane.

chloromethane (methyl chloride; CH3Cl)A colorless flammable haloalkane gasmade by chlorination of methane. It is usedas a refrigerant and a local anesthetic.

chlorophyll A pigment present in plantsthat acts as a catalyst in the photosynthesisof carbohydrates from carbon dioxide andwater. There are four types, known aschlorophylls a, b, c, and d. The chloro-phylls are PORPHYRINS containing magne-sium.

chloroprene (CH2:CClCH:CH2) A col-orless conjugated diene used in the manu-facture of synthetic chlorinated rubbers(such as neoprene). The systematic name is2–chlorobuta-1,3–diene.

cholecalciferol See vitamin D.

choline An amino alcohol often classi-fied as a member of the vitamin B complex.It can be synthesized in humans fromlecithin by putrefaction in the bowel, but isrequired as an essential nutrient for someanimals and microorganisms. It acts to dis-perse fat from the liver or prevent its excessaccumulation. Its ester acetylcholine func-tions in the transmission of nerve impulses.

chromatography A technique used toseparate or analyze complex mixtures. Anumber of related techniques exist; all de-pend on two phases: a mobile phase, whichmay be a liquid or a gas, and a stationaryphase, which is either a solid or a liquidheld by a solid. The sample to be separatedor analyzed is carried by the mobile phasethrough the stationary phase. Differentcomponents of the mixture are absorbed ordissolved to different extents by the sta-tionary phase, and consequently movealong at different rates. In this way thecomponents are separated. There are manydifferent forms of chromatography de-pending on the phases used and the natureof the partition process between mobileand stationary phases. The main classifica-tion is into column chromatography andplanar chromatography.

49

chromatography

A simple example of column chro-matography is in the separation of liquidmixtures. A vertical column is packed withan absorbent material, such as alumina(aluminum oxide) or silica gel. The sampleis introduced into the top of the columnand washed down it using a solvent. Thisprocess is known as elution; the solventused is the eluent and the sample being sep-arated is the eluate. If the components arecolored, visible bands appear down thecolumn as the sample separates out. Thecomponents are separated as they emergefrom the bottom of the column. In this par-ticular example of chromatography thepartition process is adsorption on the par-ticles of alumina or silica gel. Columnchromatography can also be applied tomixtures of gases. See gas chromatogra-phy. In the other main type of chromatog-raphy, planar chromatography, thestationary phase is a flat sheet of absorbentmaterial. See paper chromatography; thin-layer chromatography.

Components of the mixture are heldback by the stationary phase either by ad-sorption (e.g. on the surface of alumina) orbecause they dissolve in it (e.g. in the mois-ture within chromatography paper).

chromophore A group of atoms in amolecule that is responsible for the color ofthe compound. Usually a chromophore is agroup of atoms having delocalized elec-trons.

cinnamic acid See 3-phenylpropenoicacid.

CIP system (Cahn–Ingold–Prelog system)A method of producing a sequence ruleused in the absolute description ofstereoisomers in the R–S convention (seeoptical activity) or the E-Z CONVENTION.The rule is to consider the atoms that arebound directly to a chiral center (or to adouble bond). The group in which thisatom has the highest proton number hasthe highest priority. So, for example, inHCClBr(NH2), the order of priority is Br > Cl > NH2 > H. If two atoms are thesame, the substituents are considered, with the substituents of highest proton

number taking precedence. So inC(NH2)(NO2)(CH3)(C2H6 ) the order isNO2 > NH2 > C2H6 > CH3. The system isnamed after the British chemists RobertCahn (1899–1981) and Sir Christopher In-gold (1893–1970) and the Bosnian–Swisschemist Vladimir Prelog (1906– ).

cis- Designating an isomer with groupsthat are adjacent. See isomerism.

cis-trans isomerism See isomerism.

citric acid A white crystalline dibasiccarboxylic acid important in plant and an-imal cells. It is present in many fruits, espe-cially citrus fruits. The systematic name is2-hydroxypropane-1,2,3-tricarboxylicacid. The formula is:

HOOCCH2C(OH)(COOH)CH2COOH

Claisen condensation A reaction inwhich two molecules of ester combine togive a keto-ester – a compound containinga ketone group and an ester group. The re-action is base-catalyzed by sodium ethox-ide; the reaction of ethyl ethanoaterefluxed with sodium ethoxide gives:

2CH3.CO.OC2H5 →CH3.CO.CH2.CO.OC2H5 + C2H5OH

The mechanism is similar to that of theALDOL REACTION, the first step being for-mation of a carbanion from the ester:

CH3COC2H5 + –OC2H5 →–CH2COC2H5 + C2H5OH

This attacks the carbon atom of the car-bonyl group on the other ester molecule,forming an intermediate anion that decom-poses to the keto-ester and the ethanoateion. It is named for the German organicchemist Ludwig Claisen (1851–1930) whodescribed it in 1890.

clathrate (enclosure compound) A sub-stance in which small (guest) molecules aretrapped within the lattice of a crystalline(host) compound. Clathrates are formedwhen suitable host compounds are crystal-lized in the presence of molecules of the ap-propriate size. Although the term ‘clathratecompound’ is often used, they are not truecompounds; no chemical bonds areformed, and the guest molecules interact

chromophore

50

by weak van der Waals forces. Theclathrate is maintained by the cagelike lat-tice of the host. The host lattice must bebroken down, for example, by heating ordissolution in order to release the guest.This should be compared with zeolites, inwhich the holes in the host lattice are largeenough to permit entrance or emergence ofthe guest without breaking bonds in thehost lattice. Quinol forms many clathrates,e.g. with SO2; water (ice) forms a clathratewith xenon.

CoA See coenzyme A.

coagulation The association of particles(e.g. in colloids) into clusters. See floccula-tion.

coal A black mineral that consistsmainly of carbon, used as a fuel and as asource of organic chemicals. It is the fos-silized remains of plants that grew in theCarboniferous and Permian periods andwere buried and subjected to high pres-sures underground. There are various typesof coal, classified according to their in-creasing carbon content.

coal gas A fuel gas made by heating coalin a limited supply of air. It consists mainlyof hydrogen and methane, with some car-bon monoxide (which makes the gas highlypoisonous). Coal tar and coke are formedas by-products. Coal gas was a major fuelin some countries in the 19th and early20th century. See coal tar.

coal tar Tar produced by heating coal inthe absence of oxygen. It is a mixture ofmany organic compounds (e.g. benzene,toluene, and naphthalene) and also con-tains free carbon.

cocaine An alkaloid obtained from thedried leaves of a South American shrubErythroxylon coca. It is a stimulant andnarcotic. Its use is restricted in many coun-tries.

codeine A derivative of morphine,methylmorphine. It is less potent than mor-

phine and is used as an analgesic. It is acontrolled substance in the USA.

coenzyme Any of a group of molecules(which are small compared to the size of anenzyme) that enable enzymes to carry outtheir catalytic activity. Examples includenicotinamide adenine dinucleotide (NAD)and ubiquinone (coenzyme Q). Some co-enzymes are capable of catalyzing reac-tions in the absence of an enzyme but therate of reaction is never as high as when acatalyst is present. A coenzyme is not a truecatalyst because it undergoes chemicalchange during the reaction.

coenzyme A (CoA) A complex nu-cleotide containing an active –SH groupthat is readily acetylated to CoAS–COCH3(acetyl CoA). Acetyl CoA is the source ofthe two-carbon units that feed into theKREBS CYCLE. It is produced from glycolysisand the breakdown of fatty acids and someamino acids. It is also a key intermediate inthe biosynthesis of lipids and other ana-bolic reactions.

coenzyme Q See ubiquinone.

cofactor A nonprotein substance thathelps an enzyme to carry out its activity.Cofactors may be cations or organic mol-ecules, known as coenzymes. Unlike en-zymes they are, in general, stable to heat.When a catalytically active enzyme forms acomplex with a cofactor a holoenzyme isproduced. An enzyme without its cofactoris termed an apoenzyme.

coherent units A system or subset ofunits (e.g. SI units) in which the derivedunits are obtained by multiplying or divid-ing together base units, with no numericalfactor involved.

collagen The protein of fibrous connec-tive tissues, present in bone, skin, and car-tilage. It is the most abundant of all theproteins in the higher vertebrates. Collagencontains about 35% glycine, 11% alanine,12% proline and small percentages ofother amino acids. The amino acid se-quence is remarkably regular with almost

51

collagen

every third residue being glycine. Collagenis chemically inert (and insoluble) whichsuggests that its reactive side groups areimmobilized by ionic bonding. Collagenfibrils are highly complex and have a vari-ety of orientations depending on the bio-logical function of the particular type ofconnective tissue. The secondary structureof collagen is that of a triple helix of pep-tide chains. Its tertiary structure is one ofthree alpha helices in a ‘super helix’, whichis responsible for its high tensile strengthand therefore its role in support tissues.

colligative properties A group of prop-erties of solutions that depends on thenumber of particles present, rather thanthe nature of the particles. Colligativeproperties include:1. The lowering of vapor pressure.2. The elevation of boiling point.3. The lowering of freezing point.4. Osmotic pressure.

The explanation of these closely relatedphenomena depends on intermolecularforces and the kinetic behavior of the par-ticles, which is qualitatively similar tothose used in deriving the kinetic theory ofgases.

collimator An arrangement for produc-ing a parallel beam of radiation for use in aspectrometer or other instrument. A sys-tem of lenses and slits is utilized.

colloid A heterogeneous system inwhich the interfaces between phases,though not visibly apparent, are importantfactors in determining the system proper-ties. The three important attributes of col-loids are:1. They contain particles, commonly made

up of large numbers of molecules, form-ing the distinctive unit or disperse phase.

2. The particles are distributed in a contin-uous medium (the continuous phase).

3. There is a stabilizing agent, which has anaffinity for both the particle and themedium; in many cases the stabilizer is apolar group.

Particles in the disperse phase typicallyhave diameters in the range 10–6–10–4 mm.

Milk, rubber, and emulsion paints are typ-ical examples of colloids. See also sol.

colorimetric analysis Quantitativeanalysis in which the concentration of acolored solute is measured by the intensityof the color. The test solution can be com-pared against standard solutions.

column chromatography See chro-matography; gas chromatography.

combustion A reaction with oxygenwith the production of heat and light. Thecombustion of solids and liquids occurswhen they release flammable vapor, whichreacts with oxygen in the gas phase. Com-bustion reactions usually involve a com-plex sequence of free-radical chainreactions. The light is produced by excitedatoms, molecules, or ions. In highly lumi-nous flames it comes from small incandes-cent particles of carbon.

Sometimes the term is also applied toslow reactions with oxygen, and also to re-actions with other gases (for example, cer-tain metals ‘burn’ in chlorine).

complex (coordination compound) Atype of compound in which molecules orions form coordinate bonds with a metalatom or ion. The coordinating species(called ligands) have lone pairs of elec-trons, which they can donate to the metalatom or ion. They are molecules such asammonia or water, or negative ions such asCl– or CN–. The resulting complex may beneutral or it may be a complex ion. For ex-ample:

Cu2+ + 4NH3 → [Cu(NH3)4]2+

Fe3+ + 6CN– → [Fe(CN)6]3–

Fe2+ + 6CN– → [Fe(CN)6]4–

The formation of such coordinationcomplexes is typical of transition metals.Often the complexes contain unpairedelectrons and are paramagnetic and col-ored. See also chelate; sandwich com-pound.

component One of the separate chemi-cal substances in a mixture in which nochemical reactions are taking place. For ex-ample, a mixture of ice and water has one

colligative properties

52

component; a mixture of nitrogen and oxy-gen has two components. When chemicalreactions occur between the substances in amixture, the number of components is de-fined as the number of chemical substancespresent minus the number of equilibriumreactions taking place. Thus, the system:N2 + 3H2 ˆ 2NH3 is a two-componentsystem.

compound A chemical combination ofatoms of different elements to form a sub-stance in which the ratio of combiningatoms remains fixed and is specific to that

substance. The constituent atoms cannotbe separated by physical means; a chemicalreaction is required for the compounds tobe formed or to be changed. The existenceof a compound does not necessarily implythat it is stable. Many compounds havelifetimes of less than a second. See alsomixture.

concentrated Denoting a solution inwhich the amount of solute in the solvent isrelatively high. The term is always relative;for example, whereas concentrated sulfuric

53

concentrated

R2

NH2

OH

R1

NH3

ammonia

H2NNH2

hydrazine

C6H5NH NH2

phenylhydrazine

HONH2

hydroxlamine

NH2NC

H

C6H5

NNC

OH

NC

Nucleophile Product Name

C

phenylhydrazone

hydroxy amine

hydrazone

oxime

In the above the reactant is R1COR2

R2

R1

R2

R1

R2

R1

Condensation reaction: some reactions of aldehydes and ketones

acid may contain 96% H2SO4, concen-trated potassium chlorate may contain aslittle as 10% KClO3. Compare dilute.

concentration The amount of sub-stance per unit volume or mass in a solu-tion. Molar concentration is amount ofsubstance (in moles) per cubic decimeter(liter). Mass concentration is mass of soluteper unit volume. Molal concentration isamount of substance (in moles) per kilo-gram of solute.

concerted reaction A reaction thattakes place in a single stage rather than asa series of simple steps. In a concerted re-action there is a transition state in whichbonds are forming and breaking at thesame time. An example is the SN2 mecha-nism in NUCLEOPHILIC SUBSTITUTION. Seealso pericyclic reaction.

condensation The conversion of a gasor vapor into a liquid or solid by cooling.

condensation polymerization Seepolymerization.

condensation reaction A reaction inwhich addition of two molecules occursfollowed by elimination of a smaller mol-ecule, usually water. Condensation reac-tions (addition–elimination reactions) arecharacteristic of ALDEHYDES and KETONES

reacting with a range of nucleophiles.There is typically nucleophilic addition atthe C atom of the carbonyl group followedby elimination of water.

conducting polymer A type of organicpolymer that conducts electricity like ametal. Conducting polymers are crystallinesubstances containing conjugated unsatu-rated carbon–carbon bonds. In principle,they provide lighter and cheaper alterna-tives to metallic conductors.

conductiometric titration A titrationin which measurement of the electricalconductance is made continuouslythroughout the addition of the titrant andwell beyond the equivalence point. This isin place of traditional end-point determi-

nation by indicators. The operation is car-ried out in a conductance cell, which is partof a resistance bridge circuit. The methoddepends on the fact that ions have differentionic mobilities, H+ and OH– havingparticularly high values. The method is es-pecially useful for weak acid–strong baseand strong acid–weak base titrations forwhich color-change titrations are unreli-able.

configuration 1. The arrangement ofelectrons about the nucleus of an atom.Configurations are represented by sym-bols, which contain:1. An integer, which is the value of the

principal quantum number (shell num-ber).

2. A lower-case letter representing thevalue of the azimuthal quantum number(l), i.e.

s means l = 0, p means l = 1,d means l = 2, f means l = 3.

3. A numerical superscript giving the num-ber of electrons in that particular set; forexample, 1s2, 2p3, 3d5.

The ground state electronic configuration(i.e. the most stable or lowest energy state)may then be represented as follows, for ex-ample, He, 1s2; N, 1s22s22p5. However, el-ements are commonly abbreviated by usingan inert gas to represent the ‘core’, e.g. Zrhas the configuration [Kr]4d25s2.2. The arrangement of atoms or groups ina molecule.

conformation A particular shape ofmolecule that arises through the normalrotation of its atoms or groups about singlebonds. Any of the possible conformationsthat may be produced is called a conformer(or rotamer), and there will be an infinitenumber of these possibilities, differing inthe angle between certain atoms or groupson adjacent carbon atoms. Sometimes, theterm ‘conformer’ is applied more strictly topossible conformations that have mini-mum energies – as in the case of the boatand chair conformations of CYCLOHEXANE.In considering conformations about a sin-gle bond, it is convenient to consider the di-hedral angle between a bond from onecarbon atom and a bond from the other

concentration

54

55

conjugate acid

carbon. This is the angle between thebonds as viewed along the C–C bond. It isalso called the torsion angle. Conforma-tions are also visualized using Newmanprojection diagrams. In these the moleculeis viewed along a particular bond. Thenearer atoms is represented by a point withbonds drawn to this point. The furtheratom is represented by a large circle, withbonds drawn to the edge of this circle. Aconformation in which bonds on the backatom would be hidden by bonds in front isdrawn slightly displaced. In the case ofethane, the maximum energy is the eclipsedconformation, in which the dihedral angle

is O°. The minimum is the staggered con-formation, with a dihedral angle of 60°.

If different atoms are attached to thecarbon atoms, the conformational analysisis more complicated. For example, thecompound XH2C–CH2X, where X is an-other group (e.g. Me), has conformersknown as syn-periplanar, synclinal (orgauche), anticlinal, and anti-periplanar(see illustration).

Conformational analysis is also impor-tant in the structures of nonplanar rings.See cyclohexane.

conjugate acid See acid.

HH

HH

H

H

H

HH H H

H

C C

H

HH

HH

H

C C

H

HH

H

HH

staggered

eclipsed

Conformation: the conformations of ethane resulting from rotation about the C–C bond. The diagrams on the right are Newman projections, with the molecule viewed along the C–C bond.

0 60 120 180 240

S S

E E

dihedral angle

ener

gy

Conformation: the way in which energychanges with dihedral angle as a result ofrotation about the double bond in ethane. E indicates an eclipsed conformation and

S indicates a staggered conformation.

Y

Z

A B

V

W

XC D

U

Dihedral angle: the angle θ between the twoplanes (UVWX and UVYZ) is the dihedral angle

between lines AC and BD. If AC, BD, and ABare bonds, this angle is also known as the

torsion angle.

56

syn-periplanar synclinal

anticlinal anti-periplanar

anticlinal synclinal

X

HH

X

HH

X

HH

H H

HX

X

HH

XH

H

X

HHX

X

HHH

HH

X

XH

H

HX

H

Conformation: the conformations of a disubstituted ethane CH2XCH2X for rotation about the C–Cbond.

0 60 120 180 240

dihedral angle

ener

gy

syn-periplanar

anticlinal

synclinal

anti-periplanar

anticlinal

synclinal

syn-periplanar

300 360

Conformation: the way in which energy changes with dihedral angle in CH2XCH2X for rotationabout the C–C bond.

conjugate base See acid.

conjugated Describing compounds thathave alternating double and single bondsin their structure. For example, but-1,3-ene(H2C:CHCH:CH2) is a typical conjugatedcompound. In such compounds there is de-localization of the electrons in the doublebonds over part of the molecule.

conjugated protein A protein that onhydrolysis yields not only amino acids butalso other organic and inorganic sub-stances. They are simple proteins com-bined with nonprotein groups (prostheticgroups). See also glycoprotein; lipoprotein;phosphoprotein.

conservation of energy, law of Enun-ciated by Helmholtz in 1847, this lawstates that in all processes occurring in anisolated system the energy of the system re-mains constant. The law does of coursepermit energy to be converted from oneform to another (including mass, since en-ergy and mass are equivalent).

conservation of mass, law of Formu-lated by Lavoisier in 1774, this law statesthat matter cannot be created or destroyed.Thus in a chemical reaction the total massof the products equals the total mass of thereactants (the term ‘mass’ must include anysolids, liquids, and gases – including air –that participate).

constant-boiling mixture A generalobservation for most liquids is that thevapor phase above a liquid is richer in themore volatile component (a deviation fromRaoult’s law). Consequently most liquidmixtures show a regular increase in theboiling point as the liquid is progressivelydistilled. In distillation, a point is reachedat which a constant boiling mixture orAZEOTROPE distills over. Further attemptsto fractionate the distillate do not lead to achange in composition. An example of anazeotropic mixture of minimum boilingpoint is water (b.p. 100°C) and ethanol(b.p. 78.3°C), the azeotrope being 4.4%water and boiling at 78.1°C.

constant composition, law of Seeconstant proportions; law of.

constant proportions, law of Formu-lated by Proust in 1779 after the analysis ofa large number of compounds, the princi-ple that the proportion of each element ina compound is fixed or constant. It followsthat the composition of a pure chemicalcompound is independent of the method ofpreparation. It is also called the law of def-inite proportions and the law of constantcomposition.

continuous phase See colloid.

continuous process A manufacturingprocess in which the raw materials are con-stantly fed into the plant. These react asthey flow through the equipment to give acontinuing flow of product. At any point,only a small amount of material is at a par-ticular stage in the process but material atall stages of the reaction is present. Thefractional distillation of crude oil is an ex-ample of a continuous process. Suchprocesses are relatively easy to automateand can therefore be used to manufacture aproduct cheaply. The disadvantages ofcontinuous processing are that it usuallycaters for a large demand and the plant isexpensive to install and cannot normallybe used to make other things. Comparebatch process.

continuous spectrum A spectrum com-posed of a continuous range of emitted orabsorbed radiation. Continuous spectraare produced in the infrared and visible re-gions by hot solids. See also spectrum.

coordinate bond (dative bond) A co-valent bond in which the bonding pair isvisualized as arising from the donation of alone pair from one species to anotherspecies, which behaves as an electronacceptor. The definition includes suchexamples as the ‘donation’ of the lone pairof the ammonia molecule to H+ (an accep-tor) to form NH4

+ or to Cu2+ to form[Cu(NH3)4]2+.

The donor groups are known as Lewisbases and the acceptors are either hydro-

57

coordinate bond

gen ions or Lewis acids. Simple combina-tions, such as H3N→BF3, are known asadducts. See also complex.

coordination compound See complex.

coordination number The number ofcoordinate bonds formed to a metal atomor ion in a complex.

copolymer See polymerization.

Cornforth, Sir John Warcup (1917– )Australian organic chemist. Cornforth isbest known for his work on the problem ofhow the steroid cholesterol is synthesizedin a cell. To investigate this problem heused the three isotopes of hydrogen – nor-mal hydrogen (H-one), deuterium (H-two)and tritium (H-three) – and observed thedifferent speeds of reactions found withthese isotopes to infer how cholesterol wasformed. He shared the 1975 Nobel Prizefor chemistry with Vladimir PRELOG forthis work. Cornforth has synthesized anumber of other compounds includingalkenes and oxazoles.

corn rule See optical activity.

coumarin (1,2–benzopyrone; C9H6O2) Acolorless crystalline compound with apleasant odor, used in making perfumes.On hydrolysis with sodium hydroxide itforms coumarinic acid.

coumarinic acid See coumarin.

coumarone See benzfuran.

Couper, Archibald Scott (1831–92)

Scottish organic chemist. Couper was thefirst person to recognize that carbon has avalence of four and can combine with it-self. He put forward these views in a paperentitled On a New Chemical Theory whichhe wrote in 1858. He asked CharlesAdolphe Wurtz to present his paper to theFrench Academy. Wurtz delayed doing so.Couper’s work on structure in organicmolecules was eventually published but bythat time August KEKULÉ had publishedsimilar ideas and claimed priority. Couperquarrelled violently with Wurtz, returnedto Scotland and was mentally ill for mostof the rest of his life.

coupling A chemical reaction in whichtwo groups or molecules join together. Anexample is the formation of AZO COM-POUNDS.

covalent bond A bond formed by thesharing of an electron pair between twoatoms. The covalent bond is convention-ally represented as a line, thus H–Cl indi-cates that between the hydrogen atom andthe chlorine atom there is an electron pairformed by electrons of opposite spin im-plying that the binding forces are stronglylocalized between the two atoms. Mol-ecules are combinations of atoms boundtogether by covalent bonds; covalent bond-ing energies are of the order 103 kJ mol–1.

Modern bonding theory treats the elec-tron pairing in terms of the interaction ofelectron (atomic) ORBITALS and describesthe covalent bond in terms of both ‘bond-ing’ and ‘anti-bonding’ molecular orbitals.

covalent crystal A crystal in which theatoms present are covalently bonded. They

coordination compound

58

coumarin coumarinic acid

CHCHCH

COCOCOOOO

H

HHH

COCOCO222HHHOHOHOH

CCC

Coumarin

are sometimes referred to as giant latticesor macromolecules. The best-known com-pletely covalent crystal is diamond.

covalent radius The radius an atom isassumed to have when involved in a cova-lent bond. For homonuclear diatomic mol-ecules (e.g. Cl2) this is simply half themeasured internuclear distance. For het-eroatomic molecules substitutional meth-ods are used. For example, the internucleardistance of bromine fluoride (BrF) is about180 pm, therefore using 71 pm for the co-valent radius of fluorine (from F2) we get109 pm for bromine. The accepted value is114 pm.

creosote A colorless oily liquid contain-ing phenols and distilled from wood tar,used as a disinfectant. The name is alsogiven to creosote oil, a dark brown liquiddistilled from coal tar and used for pre-serving timber. It also consists of phenols,mixed with some methylphenols.

cresol See methylphenol.

Crick, Francis Harry Compton(1916– ) British molecular biologist.Crick is best known for determining thestructure of DNA with James WATSON in1953. Based on a combination of modelbuilding, previous knowledge of the physi-cal and chemical features of DNA and thex-ray diffraction photographs of RosalindFRANKLIN they found the famous doublehelix structure. Together with SydneyBrenner, he worked on the problem of thegenetic code. Crick put forward the Cen-tral Dogma of molecular genetics. This as-serts that genetic information passes fromDNA to RNA protein. It was subsequentlyshown that sometimes information canflow from RNA to DNA. In his later yearsCrick worked on how the mind works. Hegave an account of this work in The As-tonishing Hypothesis (1994). In 1988 hisautobiography What Mad Pursuit waspublished. Crick, Watson, and MauriceWILKINS won the 1962 Nobel Prize formedicine for their work on DNA.

critical point The conditions of temper-ature and pressure under which a liquidbeing heated in a closed vessel becomes in-distinguishable from the gas or vaporphase. At temperatures below the criticaltemperature (Tc) the substance can beliquefied by applying pressure; at tempera-tures above Tc this is not possible. For eachsubstance there is one critical point; for ex-ample, for carbon dioxide it is at 31.1°Cand 73.0 atmospheres.

critical pressure The lowest pressureneeded to bring about liquefaction of a gasat its critical temperature.

critical temperature The temperaturebelow which a gas can be liquefied by ap-plying pressure and above which noamount of pressure is sufficient to bringabout liquefaction. Some gases have criti-cal temperatures above room temperature(e.g. carbon dioxide 31.1°C and chlorine144°C) and have been known in the liquidstate for many years. Liquefaction provedmuch more difficult for those gases (e.g.oxygen –118°C and nitrogen –146°C) thathave very low critical temperatures.

critical volume The volume of onemole of a substance at its critical point.

cross linkage An atom or short chainjoining two longer chains in a polymer.

crown ether A compound that has alarge ring composed of –CH2–CH2–O–units. For example, 18-crown-6 has theformula C12H24O6 (six CH2CH2O units).The rings of these compounds are not pla-nar – the name comes from the shape of themolecule. The oxygen atoms of these cyclicethers can coordinate to central metal ionsor to other positive ions (e.g. NH4

+). Thecrown ethers have a number of uses inanalysis, separation of mixtures, and ascatalysts. Cryptands are similar com-pounds in which the ether chains are linkedby nitrogen atoms to give a three-dimen-sional cage structure. They are similar inaction to crown ethers but generally formmore strongly bound complexes. See alsohost–guest chemistry.

59

crown ether

crude oil See petroleum.

cryoscopic constant See depression offreezing point.

cryptand See crown ether.

crystal A solid substance that has a def-inite geometric shape. A crystal has fixedangles between its faces, which have dis-tinct edges. The crystal will sparkle if thefaces are able to reflect light. The constantangles are caused by the regular arrange-ments of particles (atoms, ions, or mol-ecules) in the crystal. If broken, a largecrystal will form smaller crystals.

In crystals, the atoms, ions, or mol-ecules of the substance form a distinct reg-ular array in the solid state. The faces andtheir angles bear a definite relationship tothe arrangement of these particles.

crystal habit The shape of a crystal. Thehabit depends on the way in which thecrystal has grown; i.e. the relative rates ofdevelopment of different faces.

crystalline Denoting a substance thatforms crystals. Crystalline substances havea regular internal arrangement of atoms,even though they may not exist as geomet-rically regular crystals. For instance, lead(and other metals) are crystalline. Suchsubstances are composed of accumulationsof tiny crystals.

crystallite A small crystal that has thepotential to grow larger. It is often used inmineralogy to describe specimens that con-tain accumulations of many minute crys-tals of unknown chemical composition andcrystal structure.

crystallization The process of formingcrystals. When a substance cools from thegaseous or liquid state to the solid state,crystallization occurs. Crystals will alsoform from a solution saturated with asolute.

crystallography The study of the for-mation, structure, and properties of crys-tals. See also x-ray crystallography.

crystalloid A substance that is not a col-loid and which will therefore not passthrough a semipermeable membrane. Seecolloid; semipermeable membrane.

crystal structure The particular repeat-ing arrangement of atoms, molecules, orions in a crystal. ‘Structure’ refers to the in-ternal arrangement of particles, not the ex-ternal appearance.

crystal system A classification of crys-tals based on the shapes of their unit cell. Ifthe unit cell is a parallelopiped with lengthsa, b, and c and the angles between theseedges are α (between b and c), β (betweena and c), and γ (between a and b), then theclassification is:cubic: a = b = c; α = β = γ = 90°tetragonal: a = b ≠ c; α = β = γ = 90°orthorhombic: a ≠ b ≠ c; α = β = γ = 90°hexagonal: a = b ≠ c; α = β = 90°; γ = 120°trigonal: a = b ≠ c; α = β = γ ≠ 90°monoclinic: a ≠ b ≠ c; α = γ = 90° ≠ βtriclinic: a ≠ b ≠ c; α ≠ β ≠ γThe orthorhombic system is also called therhombic system.

CS gas ((2-chlorobenzylidine-)-malanoni-trile; C6H4ClCH:C(CN)2) A white or-ganic compound that is a nasal irritantused in powder form as a tear gas for riotcontrol.

cumene process An industrial processfor the manufacture of phenol from iso-propylbenzene (cumene), which is itselfmade by passing benzene vapor andpropene over a phosphoric acid catalyst(250°C and 30 atmospheres):

C6H6 + CH2:CH(CH3) →C6H5CH(CH3)2

The isopropylbenzene is oxidized by air toa ‘hydroperoxide’:

C6H5C(CH3)2–O–O–HThis is hydrolyzed by dilute acid to phenol(C6H5OH) and propanone (CH3COCH3),which is a valuable by-product.

curie Symbol: Ci A unit of radioactivity,equivalent to the amount of a given radio-active substance that produces 3.7 × 1010

disintegrations per second, the number of

crude oil

60

disintegrations produced by one gram ofradium.

cyanide See nitrile.

cyanocobalamin (vitamin B12) One ofthe water-soluble B-group of vitamins. Ithas a complex organic ring structure at thecenter of which is a single cobalt atom.Foods of animal origin are the only impor-tant dietary source. A deficiency in humansleads to the development of pernicious an-emia since the vitamin is required for thedevelopment of red blood cells. See alsovitamin B complex.

cyanohydrin An addition compoundformed between an aldehyde or ketone andhydrogen cyanide. The general formula isRCH(OH)(CN) (from an aldehyde) orRR′C(OH)(CN) (from a ketone). Cyano-hydrins are easily hydrolyzed to hydroxy-carboxylic acids. For instance, thecompound 2-hydroxypropanonitrile (CH3-CH(OH)(CN)) is hydrolyzed to 2-hydroxy-propanoic acid (CH3CH(OH)(COOH)).

cyclic AMP (cAMP; adenosine-3′′,5′′-monophosphate) A form of adenosine

monophosphate (see AMP) formed fromATP in a reaction catalyzed by the enzymeadenylate cyclase. It has many functions,acting as an enzyme activator, genetic reg-ulator, chemical attractant, secondary mes-senger, and as a mediator in the activity ofmany hormones, including epinephrine,norepinephrine, vasopressin, ACTH, andthe prostaglandins.

cyclic compound A compound con-taining a ring of atoms. If the atoms form-ing the ring are all the same the compoundis homocyclic; if different atoms are in-volved it is heterocyclic.

cyclization Any reaction in which astraight-chain compound is converted intoa cyclic compound.

cycloaddition See pericyclic reaction.

cycloalkane A saturated cyclic hydro-carbon comprising a ring of carbon atoms,each carrying two hydrogen atoms, generalformula CnH2n. Cyclopropane (C3H6), andcyclobutane (C4H8) both have strainedrings and are highly reactive. Other cy-cloalkanes have similar properties to thealkanes, although they are generally less re-active than their corresponding alkane.

cyclohexadiene-1,4-dione See quinone.

cyclohexane (C6H12) A colorless liquidalkane that is commonly used as a solventand in the production of hexanedioic acid(adipic acid) for the manufacture of nylon.Cyclohexane is manufactured by the refor-mation of longer chain hydrocarbons pre-sent in crude-oil fractions. It is alsointeresting from a structural point of view,

61

cyclohexane

R OH

CNH

OH

CN

R2

R1

C

C

from aldehyde

from ketone

Cyanohydrin

equatorial(lower energy)

axial(higher energy)

X

X

Cyclohexane: axial and equatorial positions

existing as a ‘puckered’ six-membered ring,having all bonds between carbon atoms at109.9° (the tetrahedral angle). The mol-ecule undergoes rapid interconversion be-tween two chair conformations, which areenergetically equivalent, passing through aboat conformation of higher energy. Inpassing from a chair to a boat, the cyclo-hexane ring passes through a half-chairconformation, which is the CONFORMATION

of highest energy. This converts to a twist-boat conformation, which has a higher en-ergy than the chair but lower than the trueboat.

If cyclohexane has a substituent, thereare also two different chair conformations,corresponding to whether the substituent isaxial or equatorial (see illustration).

cyclonite A high explosive made fromhexamine.

cyclo-octatetraene See annulene.

cyclopentadiene A cyclic hydrocarbonmade by cracking petroleum. The mol-ecules have a five-membered ring contain-ing two carbon-carbon double bonds andone CH2 group. It forms the negative cy-clopentadienyl ion C5H5

–, present in SAND-WICH COMPOUNDS and is a nonbenzenoidaromatic. See aromatic compound.

cyclopentadienyl ion See cyclopentadi-ene.

cysteine See amino acid.

cystine A compound formed by the join-ing of two cysteine amino acids through a–S–S– linkage (a cystine link). Bonds of thistype are important in forming and main-taining the tertiary structure of proteins.

cyclonite

62

chair

half chair

twist boat

twist boat

chair

boat

half chair

Cyclohexane: ring conformations

63

cytosine

O

OH OH

HOCH2N

NH2

O

N

Cytidine

N

N

NH2

O

3

1

H

Cytosine

cytokinin One of a class of plant hor-mones concerned with the stimulation ofcell division, nucleic acid metabolism, androot-shoot interactions. Cytokinins areoften purine derivatives: e.g. kinetin (6-fur-furyl aminopurine), an artificial cytokinincommonly used in experiments; and zeatin,found in maize cobs.

cytosine A nitrogenous base found inDNA and RNA. Cytosine has the pyrimi-dine ring structure.

cytidine (cytosine nucleoside) A nucleo-side formed when cytosine is linked to D-ri-bose via a β-glycosidic bond.

cytochrome Any of a group of conju-gated proteins containing heme, that act asintermediates in the electron-transportchain. There are four main classes, desig-nated a, b, c, and d.

64

2,4-D (2,4-dichlorophenoxyacetic acid)A synthetic auxin used as a potent selectiveweedkiller. Monocotyledenous species withnarrow erect leaves (e.g. cereals andgrasses) are generally resistant to 2,4-Dwhile dicotyledenous plants are often verysusceptible. The compound is thus used forcontrolling weeds in cereal crops andlawns. See auxin.

Dalton’s law (of partial pressures) Theprinciple that the pressure of a mixture ofgases is the sum of the partial pressures ofeach individual constituent. The partialpressure of a certain amount of a gas in amixture is the pressure that it would exertif it alone were present in the container.Dalton’s law is strictly true only for idealgases. In real gases there are effects causedby intermolecular forces. It is named forthe British chemist John Dalton (1766–1844), who proposed it in 1803.

dative bond See coordinate bond.

d-block elements The transition el-ements of the first, second, and third longperiods of the periodic table, i.e. Sc to Zn,Y to Cd, and La to Hg. They are so calledbecause in general they have inner d-levelswith configurations of the type (n – 1)dxns2

where x = 1–10.

DDT (dichlorodiphenyltrichloroethane;(ClC6H4)2CH(CCl3)) A colorless crys-talline organic compound that was oncewidely used as an insecticide. It is very sta-ble and tends to accumulate in the soil, andpasses up the food chain to accumulate inthe fatty tissues of carnivorous animals. Itssystematic name is 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane.

deactivation A reduction in the reactiv-ity of a substance or in the activity of a cat-alyst.

de Broglie wave A wave associatedwith a particle, such as an electron or pro-ton. In 1924, Louis de Broglie suggestedthat, since electromagnetic waves can bedescribed as particles (photons), particlesof matter could also have wave properties.The wavelength (λ) has the same relation-ship to momentum (p) as in electromag-netic radiation:

λ = h/pwhere h is the Planck constant. See alsoquantum theory.

debye Symbol: D A unit of electric di-pole moment equal to 3.335 64 × 10–30

coulomb meter. It is used in expressing thedipole moments of molecules. The unit isnamed for the Dutch-born physicalchemist Peter Debye (1884–1966).

Debye–Hückel theory A theory of thebehavior (e.g. conductivity) of ions in di-lute solutions of electrolytes. It assumesthat electrolytes in dilute solution are com-pletely dissociated into ions but takes intoaccount interionic attraction and repul-sion. Agreement between the theory andexperiment occurs only with very dilute so-lutions (less than 10–3M).

deca- Symbol: da A prefix denoting 10.For example, 1 decameter (dam) = 10 me-ters (m).

decahydronaphthalene See decalin.

decalin (decahydronaphthalene; C10H18)A liquid hydrocarbon made by the hydro-

D

65

degrees of freedom

genation of naphthalene at high tem-perature and pressure. There are two iso-mers.

decant To pour off the liquid above asediment.

decay 1. The spontaneous breakdown ofa radioactive isotope. See half life; radioac-tivity.2. The transition of excited atoms, ions,molecules, etc., to a state of lower energy.

deci- Symbol: d A prefix denoting 10–1.For example, 1 decimeter (dm) = 10–1

meter (m).

decomposition The process in which acompound is broken down into com-pounds with simpler molecules.

decrepitation The process in which acrystalline solid emits a crackling noise onheating, usually because of loss of water ofcrystallization.

definite proportions, law of See con-stant proportions; law of.

degassing The removal of dissolved orabsorbed gases from liquids or solids,either on heating or in a vacuum.

degenerate Describing different quan-tum states that have the same energy. Forinstance, the five d orbitals in a transition-metal atom all have the same energy butdifferent values of the magnetic quantumnumber m. Differences in energy occur if a

magnetic field is applied or if the arrange-ment of ligands around the atom is notsymmetrical. The degeneracy is then saidto be ‘lifted’.

De Gennes, Pierre Gilles (1932– )French physicist. De Gennes is a versatiletheoretical physicist who has made impor-tant contributions to the theory of liquidcrystals and polymers. In particular, he hasshown that although these forms of matterdo not have order in the same sense as solidcrystals they do have order that character-izes them. This enabled him to analyzethem by using concepts such as order para-meters and scaling taken from the theory ofphase transitions. De Gennes gave an ac-count of his work on liquid crystals in thebook The Physics of Liquid Crystals(1974) and on polymers in Scaling Con-cepts of Polymer Physics (1979). In 1991he won the Nobel Prize for physics for hiscontributions to liquid crystals and poly-mers.

degradation A type of chemical reac-tion involving the decomposition of a mol-ecule into simpler molecules, usually instages. The HOFMANN DEGRADATION ofamides is an example.

degrees of freedom The independentways in which particles can take up energy.In a monatomic gas, such as helium orargon, the atoms have three translationaldegrees of freedom (corresponding to mo-tion in three mutually perpendicular direc-tions). The mean energy per atom for eachdegree of freedom is kT/2, where k is the

H

H

HH

HH

H

HH

HH

HH

H

H

HH

HH

H

trans-decalin cis-decalin

Decalin

Boltzmann constant and T the thermody-namic temperature; the mean energy peratom is thus 3kT/2.

A diatomic gas has in addition two ro-tational degrees of freedom (about twoaxes perpendicular to the bond) and one vi-brational degree (along the bond). The ro-tations also each contribute kT/2 to theaverage energy. The vibration contributeskT (kT/2 for kinetic energy and kT/2 forpotential energy). Thus, the average energyper molecule for a diatomic molecule is3kT/2 (translation) + kT (rotation) + kT(vibration) = 7kT/2.

Linear triatomic molecules also havetwo significant rotational degrees of free-dom; nonlinear molecules have three. Fornonlinear polyatomic molecules, the num-ber of vibrational degrees of freedom is 3N– 6, where N is the number of atoms in themolecule.

The molar energy of a gas is the averageenergy per molecule multiplied by the Avo-gadro constant. For a monatomic gas it is3RT/2, etc.

dehydration 1. Removal of water froma substance.2. Removal of the elements of water (i.e.hydrogen and oxygen in a 2:1 ratio) from acompound to form a new compound. Anexample is the dehydration of propanol topropene over hot pumice:

C3H7OH → CH3CH:CH2 + H2O

deionization The removal of ions froma solution. The usual method is to use anion-exchange resin. The term is commonlyapplied to the purification of tap water;deionized water is cheaper to produce thandistilled water and is adequate for manyapplications.

deliquescent Describing a solid com-pound that absorbs water from the atmos-phere, eventually forming a solution. Seealso hygroscopic.

delocalization A spreading out ofbonding electrons in a molecule over themolecule. See delocalized bond.

delocalized bond A type of bonding inmolecules that occurs in addition to sigmabonding. The electrons forming the delo-calized bond are not localized between twoatoms; i.e. the electron density of the delo-calized electrons is spread over severalatoms and may spread over the whole mol-ecule.

The electron density of the delocalizedbond is spread by means of a delocalizedmolecular orbital and may be regarded as aseries of pi bonds extending over severalatoms, for example the pi bonds in butadi-ene and the C–O pi bonds in the CARBOXY-LATE ION.

denaturation The changes in structurethat occur when a PROTEIN is heated. Thesechanges are irreversible and affect theproperties of the protein.

denatured alcohol Alcohol (ethanol)that has been contaminated by the additionof small amounts of substances to make itunfit for drinking. Ethanol treated in thisway may still be useful for many purposes(e.g. as a solvent) but not have restrictionsor taxes on its sale.

dendritic growth The growth of crys-tals with a branching habit.

dendritic polymer See supramolecularchemistry.

density Symbol: ρ A property of sub-stances equal to the mass of substance perunit volume. The units are g dm–3, etc. Seealso relative density.

deoxyribonucleic acid See DNA.

deoxyribose See ribose.

depression of freezing point A colliga-tive property of solutions in which thefreezing point of a given solvent is loweredby the presence of a solute. The amount ofthe reduction is proportional to the molalconcentration of the solute. The depressiondepends only on the concentration and isindependent of solute composition. The

dehydration

66

proportionality constant, Kf, is called thefreezing point constant or sometimes thecryoscopic constant. ∆t = KfCM, where ∆t isthe lowering of the temperature and CM isthe molal concentration; the unit of Kf iskelvin kilogram mole–1 (K kg mol–1). Al-though closely related to the property ofboiling-point elevation, the cryogenicmethod can be applied to measurement ofrelative molecular mass with considerableprecision. A known weight of pure solventis slowly frozen, with stirring, in a suitablecold bath and the freezing temperaturemeasured using a Beckmann thermometer.A known weight of solute of known mo-lecular mass is introduced, the solventthawed out, and the cooling process andmeasurement repeated. The addition is re-peated several times and an average valueof Kf for the solvent obtained by plotting ∆tagainst CM. The whole process is then re-peated using the unknown solute and itsrelative molecular mass determined usingthe value of Kf previously obtained.

The effect is applied to more precisemeasurement of relative molecular mass byusing a pair of Dewar flasks (pure solventand solution) and measuring ∆t by meansof thermocouples. The theoretical explana-tion is similar to that for lowering of vaporpressure. The freezing point of the solventis that point at which the curve represent-ing the vapor pressure above the liquidphase intersects the curve representing the

vapor pressure above the frozen solvent.The addition of solute depresses the formercurve but as the solid phase that separatesis always pure solvent (above the eutecticpoint), there is no attendant depression ofthe latter curve. Consequently the point ofintersection is depressed, resulting in alowering of the freezing point. See alsolowering of vapor pressure.

derivative A compound that could beproduced from another compound bychemical reaction. Usually, the term is ap-plied to a compound that has a structuralsimilarity to the parent compound; for ex-ample, chlorobenzene (C6H5Cl) is a deriv-ative of benzene (C6H6).

derived unit A unit defined in terms ofbase units, and not directly from a stan-dard value of the quantity it measures. Forexample, the newton is a unit of force de-fined as a kilogram meter second–2 (kg ms–2). See also SI units.

desiccation Removal of moisture froma substance.

desiccator A piece of laboratory appa-ratus for drying solids or for keeping solidsfree of moisture. Typically, a dessicator isan air-tight container in which is kept a hy-groscopic material (e.g. calcium chloride orsilica gel) to absorb moisture from the at-mosphere.

destructive distillation The process ofheating an organic substance in the ab-sence of air, so that it wholly or partiallydecomposes to produce volatile products,which are subsequently condensed. The de-structive distillation of coal was theprocess for manufacturing coal gas andcoal tar. At one time, methanol was madeby the destructive distillation of wood.

detergent Any of a group of substancesthat improve the cleansing action of sol-vents, particularly water. The majority ofdetergents, including SOAP, have the samebasic structure. Their molecules have a hy-drocarbon chain (tail) that does not attractwater molecules. The tail is said to be hy-

67

detergent

Beckmannthermometer

tube forintroductionof solute

containerfor freezingmixture

Depression of freezing point

drophobic (water hating). Attached to thistail is a small group (head) that readily ion-izes and attracts water molecules. It is saidto be hydrophilic (water loving). Deter-gents reduce the surface tension of waterand thus improve its wetting power. Be-cause the detergent ions have their hy-drophilic heads anchored in the water andtheir hydrophobic tails protruding aboveit, the water surface is broken up, enablingthe water to spread over the material to becleaned and penetrate between the materialand the dirt. With the assistance of agita-tion, the dirt can be floated off. The hy-drophobic tails of the detergent molecules‘dissolve’ in grease and oils. The protrud-ing hydrophilic heads repel each othercausing the oil to roll up and form a drop,which floats off into the water as an emul-sion. More recently synthetic detergents,often derived from petrochemicals, havebeen developed. Unlike soaps these deter-gents do not form insoluble scums withhard water.

Synthetic detergents are of three types.Anionic detergents form ions consisting ofa hydrocarbon chain to which is attachedeither a sulfonate group, –SO2–O–, or asulfate group, –O–SO2–O–. The corre-sponding metal salts are soluble in water.Cationic detergents have organic positiveions of the type RNH3

+, in which R has along hydrocarbon chain. Non-ionic deter-gents are complex chemical compoundscalled ethoxylates. They owe their deter-gent properties to the presence of a numberof oxygen atoms in one part of the mol-ecule, which are capable of forming hydro-gen bonds with the surface watermolecules, thus reducing the surface ten-sion of the water. See also soap.

deuterated compound A compound inwhich one or more 1H atoms have been re-placed by deuterium (2H) atoms.

deuterium Symbol: D, 2H A naturallyoccurring stable isotope of hydrogen inwhich the nucleus contains one proton andone neutron. The atomic mass is thus ap-proximately twice that of 1H; deuterium isknown as ‘heavy hydrogen’. Chemically itbehaves almost identically to hydrogen,

forming analogous compounds, althoughreactions of deuterium compounds areoften slower than those of the correspond-ing 1H compounds. This is made use of inkinetic studies where the rate of a reactionmay depend on transfer of a hydrogenatom.

deuterium oxide (D2O) See heavywater.

deuteron The nucleus of a deuteriumatom.

Dewar flask (vacuum flask) A double-walled container of thin glass with thespace between the walls evacuated andsealed to stop conduction and convectionof energy through it. The glass is often sil-vered to reduce radiation. It is named forthe British chemist and physicist Sir JamesDewar (1842–1923).

Dewar structure A representation ofthe structure of benzene in which there is asingle bond between two opposite cornersof the hexagonal ring and two doublebonds at the sides of the ring. The Dewarstructures contribute to the resonance hy-brid of benzene. It is named for theBritish–American chemist Michael Dewar(1918–nn). The nonplanar compound withthis structure, having two fused four-membered rings, was synthesized in 1963.See benzene.

dextrin A polysaccharide SUGAR pro-duced from starch by the action of amylaseenzymes or by chemical hydrolysis. Dex-trins are used as adhesives.

dextro-form See optical activity.

dextronic acid See gluconic acid.

dextrorotatory See optical activity.

dextrose (grape-sugar) The dextrorota-tory naturally occurring form of GLUCOSE,D-(+)-glucose. Because other stereochemi-cal forms of glucose have no significance inbiological systems the term ‘glucose’ is

deuterated compound

68

often used interchangeably with ’dextrose’in biology.

D-form See optical activity.

1,6-diaminohexane (hexamethylene di-amine; H2N(CH2)6NH2) An organiccompound used as a starting material inthe production of nylon. It is manufacturedfrom cyclohexane. See nylon.

diastereoisomer See isomerism.

diatomic molecule A molecule thatconsists of two atoms. Hydrogen (H2),oxygen (O2), nitrogen (N2), and the halo-gens are examples of diatomic elements.

diazine See pyrazine.

diazole See pyrazole.

diazonium compound A compound ofthe type ArN2

+X–, where Ar is an aromaticgroup and X– a negative ion. Diazoniumsalts are made by diazotization. They canbe isolated but are very unstable, and areusually prepared in solution. The –N2

+

group renders the benzene ring susceptibleto nucleophilic substitution (rather thanelectrophilic substitution). Typical reac-tions are:1. Reaction with water on warming the so-lution:

ArN2+ + H2O → ArOH + N2 + H+

2. Reaction with halogen ions (CuCl cata-lyst for chloride ions):

ArN2+ + I– → ArI + N2

Diazonium ions can also act as elec-trophiles and undergo substitution reactingwith other benzene rings (diazo coupling).See also azo compound.

diazotization The reaction of an aro-matic amine (e.g. aniline) with nitrous acidat low temperatures (below 5°C).

C6H5NH2 + HNO2 → C6H5N+N + OH–

+ H2OThe acid is prepared in situ by reaction be-tween nitric acid and sodium nitrite. Theresulting diazonium ion is susceptible to at-tack by nucleophiles and provides a

method of nucleophilic substitution ontothe benzene ring.

dibasic acid An acid that has two acidichydrogen atoms, such as sulfuric acid.Dibasic acids can give rise to two series ofsalts. For example, sulfuric acid (H2SO4)forms sulfates (SO4

2–) and hydrogensul-fates (HSO–

4).

dibenzo-4-pyrone See xanthone.

1,2-dibromoethane (ethylene dibromide;BrCH2CH2Br) A colorless volatile or-ganic liquid, made by reacting brominewith ethene. It is used as a fuel additive toremove lead (as lead bromide, which is alsovolatile).

dicarboxylic acid An organic acid thathas two carboxyl groups (–COOH). Anexample is hexanedioic acid,HOOC(CH2)4COOH (adipic acid).

dichloroacetic acid See chloroethanoicacid.

dichlorodiphenyltrichloroethane SeeDDT.

dichloroethanoic acid See chloroethanoicacid.

dichromate(VI) A salt containing theion Cr2O7

–. Dichromates are strong oxidiz-ing agents. See potassium dichromate.

Diels, Otto Paul Hermann (1876–1954) German organic chemist. The firstmajor discovery which Diels made was car-bon suboxide (C3O2). He discovered thiscompound in 1906 by dehydrating mal-onic acid with phosphorus pentoxide. Hissecond major discovery was the process ofremoving hydrogen from steroids by heat-ing them with selenium. He used thisprocess on cholesterol. He was able to usethe process he found to determine thestructures of steroids. In 1928 Diels and hiscolleague Kurt ALDER discovered whatcame to be known as the Diels–Alder reac-tion for producing a ring compound froma diene. Diels and Alder shared the 1950

69

Diels, Otto Paul Hermann

Nobel Prize for chemistry for this discov-ery.

Diels–Alder reaction A type of reac-tion in which a conjugated DIENE adds to acompound containing a double C=C bond(called the dienophile) to give a ring com-pound. To be effective, the dienophile hasto have electron-withdrawing groups onthe double bond. The diene has to have acis-conformation or to be able to adopt acis-conformation. The reactants are mixedtogether and heated. The mechanism in-volves a single step in which electronsmove to form different bonds. The reactionis an example of a cycloaddition reaction(see pericyclic reaction). It was named forthe German chemists Otto Diels and KurtAlder (1902–58), who described it in 1928.

diene An organic compound containingtwo carbon–carbon double bonds. In aconjugated diene the two double bonds areseparated by a single C–C bond.

dienophile See Diels–Alder reaction.

diesel fuel A petroleum fraction consist-ing of various alkanes in the boiling range200–350°C, used as a fuel for diesel (com-pression-ignition) engines.

diethylether See ethoxyethane.

diffusion Movement of a gas, liquid, orsolid as a result of the random thermal mo-tion of its particles (atoms or molecules). Adrop of ink in water, for example, willslowly spread throughout the liquid. Diffu-

sion in solids occurs very slowly at normaltemperatures. See also Graham’s law.

dihedral angle See conformation.

dihydrate A crystalline compound withtwo molecules of water of crystallizationper molecule of compound.

dihydric alcohol See diol.

dihydroxypurine See xanthine.

diluent A solvent that is added to reducethe strength of a solution.

dilute Denoting a solution in which theamount of solute is low relative to that ofthe solvent. The term is always relative.

dimensionless units The radian andsteradian in SI units. See SI units.

dimer A compound (or molecule)formed by combination or association oftwo molecules of a monomer. Cyclopenta-diene, for example, exists as a dimer atroom temperature. On heating it dissoci-ates.

dimethylbenzene (xylene; C6H4(CH3)2)An organic hydrocarbon present in thelight-oil fraction of crude oil. It is used ex-tensively as a solvent. There are three iso-meric compounds with this name andformula, distinguished as 1,2-, 1,3-, and1,4-dimethylbenzene according to the posi-tions of the methyl groups on the benzenering.

Diels–Alder reaction

70

butadienebutadienebutadiene maleic acidmaleic acidmaleic acid phthalic acidphthalic acidphthalic acid

HHH

CCC HHH

HHHCCC

HHH

HHHCCC

HHHCCC

COOH

COOHCCC

CCCH

H

H

COOH

COOHCCC

CCC

CCC

H

CCC

CCCCCCH

H

Diels–Alder reaction

2,4-dinitrophenylhydrazine (Brady’s re-agent) An orange solid commonly usedin solution with methanol and sulfuric acidto produce crystalline derivatives by con-densation with aldehydes and ketones. Thederivatives, known as 2,4-dinitrophenyl-hydrazones, can easily be purified by re-crystallization and have characteristicmelting points, used to identify the originalaldehyde or ketone.

dinucleotide A compound of two nu-cleotides linked by their phosphate groups.Important examples are the coenzymesNAD and FAD.

diol (dihydric alcohol; glycol) An alco-hol that has two hydroxyl groups (–OH)per molecule of compound.

1,4-dioxan ((CH2)2O2) A colorless liq-uid cyclic ether. It is an inert compoundmiscible with water used as a solvent.

dioxin Any of a related group of highlytoxic chlorinated compounds. Particularlyimportant is the compound 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD), which isproduced as a by-product in the manufac-ture of 2,4,5-T, and may consequentlyoccur as an impurity in certain types ofweedkiller. The defoliant known as AGENT

ORANGE used in Vietnam contained signifi-cant amounts of TCDD. Dioxins cause askin disease (chloracne) and birth defects.Dioxins have been released into the atmos-phere as a result of explosions at herbicidemanufacturing plants, most notably atSeveso, Italy, in 1976.

dipeptide See peptide.

diphosphane (diphosphine, P2H4) Ayellow liquid that can be condensed outfrom phosphine in a freezing mixture. It ig-nites spontaneously in air.

diphosphine See diphosphane.

dipole A system in which two equal andopposite electric charges are separated by afinite distance. Polar molecules have per-

manent dipoles. Induced dipoles can alsooccur. See also van der Waals force.

dipole moment Symbol: µ A quantita-tive measure of polarity in either a bond(bond moment) or a molecule as a whole(molecular dipole moment). The unit is thedebye (equivalent to 3.34 × 10–30 coulombmeter). Molecules such as HF, H2O, NH3,and C6H5NH2 possess dipole moments;CCl4, N2, C6H6, and PF5 do not.

The molecular dipole moment can beestimated by vector addition of individualbond moments if the bond angles areknown. The possession of a dipole momentpermits direct interaction with electricfields or interaction with the electric com-ponent of radiation.

dipyridyl (bipyridyl) A compoundformed by linking two pyridine rings.There are various isomers, some of whichare used in herbicides.

direct dyes A group of dyes that aremostly azo-compounds derived from ben-zidene or benzidene derivatives. They areused to dye cotton, viscose rayon, andother cellulose fibers directly, using a neu-tral bath containing sodium chloride orsodium sulfate as a mordant.

disaccharide A SUGAR with moleculescomposed of two monosaccharide units.These are linked by a –O– linkage (glyco-sidic link). Sucrose and maltose are exam-ples.

disconnection See retrosynthetic analy-sis.

disperse dyes Water-insoluble dyes,which, when held in fine suspension, canbe applied to acetate rayon fabrics. Thedye, together with a dispersing agent, iswarmed to a temperature of 45–50°C andthe fabric added. By modifying the methodof application it is possible to dye poly-acrylic and polyester fibers. The yellow/orange shades are nitroarylamine deriva-tives and the green to bluish shades arederivatives of 1-amino anthraquinone.

71

disperse dyes

Certain azo compounds are disperse dyesand these give a range of colors.

disperse phase See colloid.

dispersing agent A compound used toassist emulsification or dispersion.

dispersion force A weak type of inter-molecular force. See van der Waals force.

displacement pump A commonly useddevice for transporting liquids and gasesaround chemical plants. It works on theprinciple of the bicycle pump: a pistonraises the pressure of the fluid and, when itis high enough, a valve opens and the fluidis discharged through an outlet pipe. As thepiston moves back the pressure falls andthe cycle continues. Displacement pumpscan be used to generate very high pressuresbut because of the system of valves, theyare more expensive than other types ofpump. Compare centrifugal pump.

displacement reaction A chemical re-action in which an atom or group displacesanother atom or group from a molecule.

disproportionation A chemical reac-tion in which there is simultaneous oxida-tion and reduction of the same compound.The CANNIZZARRO REACTION is an examplein organic chemistry.

dissociation Breakdown of a moleculeinto two molecules, atoms, radicals, orions. Often the reaction is reversible, as inthe ionic dissociation of weak acids inwater:

CH3COOH + H2O ˆ CH3COO– +H3O+

dissociation constant The equilibriumconstant of a dissociation reaction. For ex-ample, the dissociation constant of a reac-tion:

AB ˆ A + Bis given by:

K = [A][B]/[AB]where the brackets denote concentration(activity).

Often the degree of dissociation is used– the fraction (α) of the original compoundthat has dissociated at equilibrium. For anoriginal amount of AB of n moles in a vol-ume V, the dissociation constant is givenby:

K = α2n/(1 – α)V Note that this expression is for dissociationinto two molecules.

Acid dissociation constants (or acidityconstants, symbol: Ka) are dissociationconstants for the dissociation into ions insolution:

HA + H2O ˆ H3O+ + A–

The concentration of water can be taken asunity, and the acidity constant is given by:

Ka = [H3O+][A–]/[HA]The acidity constant is a measure of the

strength of the acid. Base dissociation con-stants (Kb) are similarly defined. The ex-pression:

K = α2n/(1 – α)Vapplied to an acid is known as Ostwald’sdilution law (for the German chemistFriedrich Wilhelm Ostwald (1853–1932),who formulated it in 1888). In particular ifα is small (a weak acid) then K = α2n/V, orα = C√V, where C is a constant. The degreeof dissociation is then proportional to thesquare root of the dilution.

distillation The process of boiling a liq-uid and condensing the vapor. Distillationis used to purify liquids or to separate com-ponents of a liquid mixture. See also de-structive distillation; fractional distillation;steam distillation; vacuum distillation.

distilled water Water that has been pu-rified by distillation, perhaps several times.

diterpene See terpene.

divalent (bivalent) Having a valence oftwo.

Djerassi, Carl (1923– ) Austrian-born American chemist. The first notablework which he and his colleagues at Syn-tex, in Mexico City, performed was to ex-tract cortisone from a vegetable source.Djerassi and his colleagues then investi-gated the steroid hormone progesterone

disperse phase

72

which acts as a natural contraceptive. Theyproduced progesterone artificially in theearly 1950s, thus reducing its price.Djerassi improved the power of proges-terone by removing a particular methylgroup. He used a similar trick with testos-terone. This led to the development of thecontraceptive pill. Djerassi published hisautobiography The Pill, Pigmy Chimps,and Degas Horse in 1992.

D-L convention See optical activity.

DNA (deoxyribonucleic acid) A nucleicacid, mainly found in the chromosomes,that contains the hereditary information oforganisms. The molecule is made up of twoantiparallel helical polynucleotide chainscoiled around each other to give a doublehelix. It is also known as the Watson-Crickmodel after James Watson and FrancisCrick who first proposed this model in1953. Phosphate molecules alternate with

deoxyribose sugar molecules along bothchains and each sugar molecule is alsojoined to one of four nitrogenous bases –adenine (A), guanine (G), cytosine (C), orthymine (T). The two chains are joined toeach other by bonding between bases. Thetwo purine bases (adenine and guanine) al-ways bond with the pyrimidine bases(thymine and cytosine), and the pairing isquite specific: adenine with thymine andguanine with cytosine. The two chains aretherefore complementary. The sequence ofbases along the chain makes up a code –the genetic code – that determines the pre-cise sequence of amino acids in proteins.

DNA is the hereditary material of allorganisms with the exception of RNAviruses. Together with histones (and RNAin some instances) it makes up the chromo-somes of eukaryotic cells. See also RNA.See illustration overleaf.

dodecanoic acid (lauric acid; CH3-(CH2)10COOH) A white crystalline car-boxylic acid, used as a plasticizer and formaking detergents and soaps. Its glyceridesoccur naturally in coconut and palm oils.

donor 1. The atom, ion, or moleculethat provides the pair of electrons in form-ing a covalent bond.2. The impurity atoms used in doping semi-conductors.

dopamine A catecholamine precursorof epinephrine and norepinephrine. Inmammals it is found in highest concentra-tion in the corpus striatum of the brain,where it functions as an inhibitory neuro-transmitter. High levels of dopamine areassociated with Parkinson’s disease in hu-mans.

dormin A former name for abscisic acid.

double bond A covalent bond betweentwo atoms that includes two pairs of elec-trons, one pair being the single bond equiv-alent (the sigma pair) and the otherforming an additional bond, the pi bond (πbond). It is conventionally represented bytwo lines, for example H2C=O. See multi-ple bond; orbital.

73

double bond

A T

T A

G C

C G

T A

G C

A T

A T

C G

base

sugar–phosphatebackbone

hydrogenbond

3.4 nm

DNA: the double helix

74

sugar

sugar

sugar

sugar

sugar

sugar

sugar

sugar

phosphate

phosphate

phosphatephosphate

phosphate

phosphate

KEY

adenine

cytosine

thymine

guanine

Part of the structure of DNA showinghydrogen bonding (dotted lines)between complementary bases

DNA: the structure and bonding

75

dynamite

double helix See DNA.

double salt When equivalent quantitiesof certain salts are mixed in aqueous solu-tion and the solution evaporated, a saltmay form, e.g. FeSO4.(NH4)2SO4.6H2O.In aqueous solution the salt behaves as amixture of the two individuals. These saltsare called double salts to distinguish themfrom complex salts, which yield complexions in solution.

dryers Devices used in chemicalprocesses to remove a liquid from a solidby evaporation. Drying equipment is clas-sified by the method of transferring heat toa wet solid. This can be by direct contactbetween hot gases and the solid (direct dry-ers), heat transfer by conduction through aretaining metallic wall (indirect dryers), orinfrared rays (infrared dryers).

dry ice Solid carbon dioxide, used as arefrigerant.

drying oil A natural oil, such as linseedoil, that hardens in air. Such oils containunsaturated fatty acids, which polymerizeon oxidation.

Dumas’ method 1. A method for de-termining the relative molecular mass of avolatile liquid. The method utilizes a glassbulb with a narrow entrance tube. Thebulb is weighed ‘empty’ (i.e. full of air)then the sample is introduced and the bulbimmersed in a heating bath so that the sam-ple boils and expels all the air. When the

surplus vapor has been expelled, the bulb issealed off, cooled, dried, and weighed. Thetip of the tube is then broken under waterso that the water completely fills the tubeand the whole weighed again. This enablesthe volume of the bulb to be calculatedfrom the known density of water, andknowing the density of air one can com-pute the mass of vapor in a known volumeof the sample.2. A method of finding the amount of ni-trogen in an organic compound by heatingthe compound with copper oxide to con-vert the nitrogen into nitrogen oxides.These are reduced by passing them overhot copper and the volume of nitrogen col-lected is measured.Both methods are named for the Frenchchemist Jean Baptiste André Dumas(1800–84).

dye A coloring material for fabric,leather, etc. Most dyes are now syntheticorganic compounds (the first such was thedye mauve synthesized from aniline in1856 by William Perkin). Dyes are oftenunsaturated organic compounds contain-ing conjugated double bonds – the bondsystem responsible for the color is calledthe chromophore. See also azo compound.

dynamite A high explosive made by ab-sorbing nitroglycerine into an earthy ma-terial such as diatomite (kieselguhr). Solidsticks of dynamite are much safer to handlethan the highly sensitive liquid nitroglycer-ine.

ebonite See vulcanite.

ebulioscopic constant See elevation ofboiling point.

ebullition The boiling or bubbling of aliquid.

eclipsed conformation See conforma-tion.

edta (ethylenediamine tetraacetic acid) Acompound with the formula

(HOOCCH2)2N(CH2)2N(CH2COOH)2It is used in forming chelates of transitionmetals. See chelate.

effervescence The evolution of gas inthe form of bubbles in a liquid.

efflorescence The process in which acrystalline hydrated solid spontaneouslyloses water of crystallization to the air. Apowdery deposit is gradually formed.

elastin A structural protein found inmammalian connective tissues, especiallyin elastic fibers. Glycine is the main com-ponent; proline, alanine, and valine are theother main residues.

elastomer An elastic substance, e.g. anatural or synthetic rubber.

electrochemical equivalent Symbol: zThe mass of an element released from a so-lution of its ion when a current of oneampere flows for one second during ELEC-TROLYSIS.

electrochemical series (electromotive se-ries) A series giving the activities of met-

als for reactions that involve ions in solu-tion. In decreasing order of activity, the se-ries is

K, Na, Ca, Mg, Al, Zn, Fe,Pb, H, Cu, Hg, Ag, Pt, Au

Any member of the series will displace ionsof a lower member from solution. For ex-ample, zinc metal will displace Cu2+ ions:

Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)Zinc has a greater tendency than copper

to form positive ions in solution. Similarly,metals above hydrogen displace hydrogenfrom acids:

Zn + 2HCl → ZnCl2 + H2

The series is based on electrode potentials,which measure the tendency to form posi-tive ions. The series is one of increasingelectrode potential for half cells of the typeMn+|M. Thus, copper (EŠ for Cu2+|Cu = +0.34 V) is lower than zinc (EŠ for Zn2+|Zn= –0.76V). The hydrogen half cell has avalue EŠ = 0.

electrochemistry The study of the for-mation and behavior of ions in solutions. Itincludes electrolysis and the generation ofelectricity by chemical reactions in cells.

electrochromatography See electro-phoresis.

electrocyclic reaction See pericyclic re-action.

electrode Any part of an electrical de-vice or system that emits or collects elec-trons or other charge carriers. An electrodemay also be used to deflect charged parti-cles by the action of the electrostatic fieldthat it produces.

76

E

electrode potential Symbol: E A meas-ure of the tendency of an element to formions in solution. For example, a metal in asolution containing M+ ions may dissolvein the solution as M+ ions; the metal thenhas an excess of electrons and the solutionan excess of positive ions – thus, the metalbecomes negative with respect to the solu-tion. Alternatively, the positive ions maygain electrons from the metal and be de-posited as metal atoms. In this case, themetal becomes positively charged with re-spect to the solution. In either case, a po-tential difference is developed betweensolid and solution, and an equilibriumstate will be reached at which further reac-tion is prevented. The equilibrium value ofthis potential difference would give an in-dication of the tendency to form aqueousions.

It is not, however, possible to measurethis for an isolated half cell – any measure-ment requires a circuit, which sets up an-other half cell in the solution. Therefore,electrode potentials (or reduction poten-tials) are defined by comparison with a hy-drogen half cell, which is connected to thehalf cell under investigation by a saltbridge. The e.m.f. of the full cell can thenbe measured.

In referring to a given half cell the morereduced form is written on the right for ahalf-cell reaction. For the half cell Cu2+|Cu,the half-cell reaction is a reduction:

Cu2+(aq) + 2e → CuThe cell formed in comparison with a hy-drogen electrode is:

Pt(s)H2(g)|H+(aq)|Cu2+(aq)|CuThe e.m.f. of this cell is +0.34 volt meas-ured under standard conditions. Thus, thestandard electrode potential (symbol: EŠ)is +0.34 V for the half cell Cu2+|Cu. Thestandard conditions are 1.0 molar solu-tions of all ionic species, standard pressure,and a temperature of 298 K.

Half cells can also be formed by a solu-tion of two different ions (e.g. Fe2+ andFe3+). In such cases, a platinum electrode isused under standard conditions.

electrolysis The production of chemicalchange by passing electric charge throughcertain conducting liquids (electrolytes).

The current is conducted by migration ofions – positive ones (cations) to the cathode(negative electrode), and negative ones (an-ions) to the anode (positive electrode). Re-actions take place at the electrodes bytransfer of electrons to or from them.

In the electrolysis of water (containing asmall amount of acid to make it conductadequately) hydrogen gas is given off at thecathode and oxygen is evolved at theanode. At the cathode the reaction is:

H+ + e– → H2H → H2

At the anode:OH– → e– + OH

2OH → H2O + O2O → O2

In certain cases the electrode materialmay dissolve. For instance, in the electrol-ysis of copper(II) sulfate solution with cop-per electrodes, copper atoms of the anodedissolve as copper ions

Cu → 2e– + Cu2+

electrolyte A liquid containing positiveand negative ions that conducts electricityby the flow of those charges. Electrolytescan be solutions of acids or metal salts(‘ionic compounds’), usually in water. Al-ternatively they may be molten ionic com-pounds – again the ions can move freelythrough the substance. Liquid metals (inwhich conduction is by free electronsrather than ions) are not classified as elec-trolytes. See also electrolysis.

electrolytic Relating to the behavior orreactions of ions in solution.

electrolytic cell See cell; electrolysis.

electromagnetic radiation Energy prop-agated by vibrating electric and magneticfields. Electromagnetic radiation forms awhole electromagnetic spectrum, depend-ing on frequency and ranging from high-frequency radio waves to low-frequencygamma rays.

Electromagnetic radiation can bethought of as waves (electromagneticwaves) or as streams of photons. The fre-quency and wavelength are related by:

λv = c

77

electromagnetic radiation

where c is the speed of light. The energycarried depends on the frequency.

electromagnetic spectrum See electro-magnetic radiation.

electromotive series See electrochemi-cal series.

electron An elementary particle of nega-tive charge (–1.602 192 × 10–19C) and restmass 9.109 558 × 10–31 kg. Electrons arepresent in all atoms in shells around the nu-cleus.

electron affinity Symbol: A The energyreleased when an atom (or molecule orgroup) gains an electron in the gas phase toform a negative ion. It is thus the energy of:

A + e– → A–

A positive value of A (often in electron-volts) indicates that heat is given out. Oftenthe molar enthalpy is given for this processof electron attachment (∆H). Here theunits are joules per mole (J mol–1), and, bythe usual convention, a negative value indi-cates that energy is released.

electron-deficient compounds Com-pounds in which the number of electronsavailable for bonding is insufficient for thebonds to consist of conventional two-elec-tron covalent bonds. Diborane, B2H6, is anexample in which each boron atom hastwo terminal hydrogen atoms bound byconventional electron-pair bonds and inaddition the molecule has two hydrogenatoms bridging the boron atoms (B–H–B).In each bridge there are only two electrons

for the bonding orbital. See also multicen-ter bond.

electron diffraction A technique usedto determine the structure of substances,principally the shapes of molecules in thegaseous phase. A beam of electrons di-rected through a gas at low pressure pro-duces a series of concentric rings on aphotographic plate. The dimensions ofthese rings are related to the interatomicdistances in the molecules. See also x-raydiffraction.

electron donor See reduction.

electronegative Describing an atom ormolecule that attracts electrons, formingnegative ions. Examples of electronegativeelements include the halogens (chlorineetc.), which readily form negative ions (F–,Cl–, etc.). See also electronegativity.

electronegativity A measure of the ten-dency of an atom in a molecule to attractelectrons to itself. Elements to the right-hand side of the periodic table are stronglyelectronegative (values from 2.5 to 4);those on the left-hand side have low elec-tronegativities (0.8–1.5) and are sometimescalled electropositive elements. Differentelectronegativities of atoms in the samemolecule give rise to polar bonds andsometimes to polar molecules.

As the concept of electronegativity isnot precisely defined it cannot be preciselymeasured and several electronegativityscales exist. Although the actual values dif-fer the scales are in good relative agree-

electromagnetic spectrum

78

ELECTROMAGNETIC SPECTRUM(note: the figures are only approximate)

Radiation Wavelength (m) Frequency (Hz)gamma radiation –10–10 1019–

x-rays 10–12 –10–9 1017 –1020

ultraviolet radiation 10–9 –10–7 1015 –1017

visible radiation 10–7 –10–6 1014 –1015

infrared radiation 10–6 –10–4 1012 –1014

microwaves 10–4 –1 109 –1013

radio waves 1 – –109

ment. See also electron affinity; ionizationpotential.

electronic energy levels See atom; en-ergy level.

electronic transition The demotion orpromotion of an electron between elec-tronic energy levels in an atom or molecule.

electron pair Two electrons in one or-bital with opposing spins (spin paired),such as the electrons in a COVALENT BOND

or LONE PAIR.

electron spin See atom.

electron spin resonance (ESR) A simi-lar technique to nuclear magnetic reso-nance, but applied to unpaired electrons ina molecule (rather than to the nuclei). It isa powerful method of studying free radi-cals. ESR is also used in inorganic chem-istry to study transition-metal complexes.

electron-transport chain (respiratorychain) A chain of chemical reactions in-volving proteins and enzymes, resulting inthe formation of ATP and the transfer ofhydrogen atoms to oxygen to form water.The enzymes and other proteins are, in eu-karyotic cells, located in the inner mem-brane of the mitochondria and are groupedinto discrete complexes. The reduced coen-zyme NADH gives up two electrons to thefirst component in the chain, NADH dehy-drogenase, and two hydrogen ions (H+) aredischarged from the matrix of the mito-chondrion into the intermembrane space.The electrons are transferred along thechain to a carrier molecule (ubiquinone).Ubiquinone passes them to the next com-plex, which contains cytochromes b and c1.Another carrier (cytochrome c) transfersthe electrons to the final complex in thechain. There they act with the enzyme cy-tochrome oxidase to reduce an oxygenatom, which combines with two H+ ions toform water. During this electron transfer, afurther two pairs of H+ ions are pumpedinto the intermembrane space by the com-plexes, making a total of six per molecule

of NADH. If FADH2 is the electron donor,only four H+ ions are pumped across, as itdonates electrons directly to ubiquinone.

The function of electron transport inthe mitochondrion is to provide the energyrequired to phosphorylate ADP to ATP.According to the chemiosmotic theory, theH+ ions in the intermembrane space diffuseback to the matrix through the inner mito-chondrial membrane down a concentra-tion gradient. As they do so they passthrough the protein channel (the F0 unit) ofthe enzyme ATP synthase. The energy re-leased allows the catalytic F1 unit of ATPsynthase to synthesize ATP from ADP andinorganic phosphate. Each pair of H+ ionscatalyzes the formation of one molecule ofATP, so for each NADH molecule, threemolecules of ATP may be synthesized (twomolecules of ATP per molecule of FADH2).A similar mechanism is involved in ATPformation by components of the light reac-tion in photosynthesis. See photosynthesis;oxidative phosphorylation.

electronvolt Symbol: eV A unit of en-ergy equal to 1.602 191 7 × 10–19 joule. Itis defined as the energy required to movean electron charge across a potential differ-ence of one volt. It has been used to meas-ure the kinetic energies of elementaryparticles or ions, or the ionization poten-tials of molecules.

electrophile An electron-deficient ionor molecule that takes part in an organicreaction. The electrophile can be either apositive ion (H+, NO2

+), a molecule thatcan accept an electron pair (SO3, O3), or anelectron-deficient group (e.g. a CARBENE).The electrophile attacks negatively chargedareas of molecules, which usually arisefrom the presence in the molecule of apolar single bond or group or of pi-bonds.Compare nucleophile.

electrophilic addition A reaction in-volving the addition of a small molecule toan unsaturated organic compound, acrossthe atoms joined by a double or triple bondwith an ELECTROPHILE as the initial attack-ing species. The reaction is initiated by theattack of the electrophile on the electron-

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electrophilic addition

rich area of the molecule. The mechanismof electrophilic addition is thought to beionic, as in the addition of HBr to ethene:

H2C:CH2 + H+Br– →H3CCH2

+ + Br– →H3CCH2Br

In the case of higher alkenes (more thantwo carbon atoms) several isomeric prod-ucts are possible. The particular isomerproduced depends on the stability of the al-ternative intermediates and this is summa-rized empirically by MARKOVNIKOFF’S RULE.See also addition reaction.

electrophilic substitution A reactioninvolving substitution of an atom or groupof atoms in an organic compound with anELECTROPHILE as the attacking substituent.Electrophilic substitution is very commonin aromatic compounds, in which elec-trophiles are substituted onto the ring. Anexample is the nitration of benzene:

C6H6 + NO2+ → C6H5NO2 + H+

The nitronium ion (NO2+) is formed by

mixing concentrated nitric and sulfuricacids:

HNO3 + H2SO4 → H2NO3+ + HSO4

–

H2NO3+ → NO2

+ + H2OThe accepted mechanism for a simple

electrophilic substitution on benzene in-volves an intermediate of the formC6H5HNO2

+. See also substitution reac-tion.

electrophoresis The use of an electricfield (produced between two electrodes) tocause charged particles of a colloid to movethrough a solution. The technique is usedto separate and identify colloidal sub-stances such as carbohydrates, proteins,and nucleic acids. Various experimentalarrangements are used. One simple tech-nique uses a strip of adsorbent papersoaked in a buffer solution with electrodesplaced at two points on the paper. Thistechnique is sometimes called electrochro-matography. In gel electrophoresis, used toseparate DNA fragments, the medium is alayer of gel.

electropositive Describing an atom ormolecule that tends to lose electrons, form-ing positive ions. Examples of electroposi-

tive elements include the alkali metals(lithium, sodium, etc.), which readily formpositive ions (Li+, Na+, etc.).

electrovalent bond (ionic bond) Abinding force between the ions in com-pounds in which the ions are formed bycomplete transfer of electrons from one el-ement to another element or radical. Forexample, Na + Cl becomes Na+ + Cl–. Theelectrovalent bond arises from the excessof the net attractive force between the ionsof opposite charge over the net repulsiveforce between ions of like charge. The mag-nitude of electrovalent interactions is of theorder 102–103 kJ mol–1 and electrovalentcompounds are generally solids with rigidlattices of closely packed ions.

element A substance that cannot bechemically decomposed into more simplesubstances. The atoms of an element allhave the same proton number (and thus thesame number of electrons, which deter-mines the chemical activity).

At present there are 114 reported chem-ical elements, although research is continu-ing all the time to synthesize new ones. Theelements from hydrogen (p.n. 1) to ura-nium (92) all occur naturally, with the ex-ception of technetium (43), which isproduced artificially by particle bombard-ment. Technetium and elements with pro-ton numbers higher than 84 (polonium)are radioactive. Radioactive isotopes alsoexist for other elements, either naturally insmall amounts or synthesized by particlebombardment. The elements with protonnumber higher than 92 are the transuranicelements. Neptunium (93) and plutonium(94) both occur naturally in small quanti-ties in uranium ores, but the transuranicsare all synthesized. Thus, neptunium andplutonium are made by neutron bombard-ment of uranium nuclei. Other transuran-ics are made by high-energy collisionprocesses between nuclei. The higher pro-ton number elements have been detectedonly in very small quantities – in somecases, only a few atoms have been pro-duced.

electrophilic substitution

80

elevation of boiling point A colliga-tive property of solutions in which the boil-ing point of a solution is raised relative tothat of the pure solvent. The elevation is di-rectly proportional to the number of solutemolecules introduced rather than to anyspecific aspect of the solute composition.The proportionality constant, kB, is calledthe boiling-point elevation constant orsometimes the ebulioscopic constant. Therelationship is

∆t = kBCMwhere ∆t is the rise in boiling point and CMis the molal concentration; the units of kBare kelvins kilograms moles–1 (K kg mol–1).The property permits the measurement ofrelative molecular mass of involatilesolutes. An accurately weighed amount ofpure solvent is boiled until the temperatureis steady, a known weight of solute ofknown molecular mass is quickly intro-duced, the boiling continued, and the ele-vation measured using a Beckmannthermometer. The process is repeated sev-eral times and the average value of kB ob-tained by plotting ∆t against CM. Thewhole process is then repeated with the un-known material and its relative molecularmass obtained using the value of kB previ-ously obtained.

There are several disadvantages withthis method and it is therefore used largelyfor demonstration purposes. The mainproblem is that the exact amount of solventremaining in the liquid phase is unknownand varies with the rate of boiling. The the-oretical explanation of the effect is identi-cal to that for the lowering of vaporpressure; the boiling points are those tem-peratures at which the vapor pressureequals the atmospheric pressure.

elimination reaction A reaction in-volving the removal of a small molecule,e.g. water or hydrogen chloride, from anorganic molecule to give an unsaturatedcompound. An example is the eliminationof a water molecule from an alcohol toproduce an alkene.

An elimination reaction is often in com-petition with a substitution reaction andthe predominant product will depend onthe reaction conditions. The reaction ofbromoethane with sodium hydroxidecould yield either ethene (by elimination ofHBr) or ethanol (by substitution of the Brwith OH). The former product predomi-nates if the reaction is carried out in an al-coholic solution and the latter if thesolution is aqueous.

eluate See elution.

eluent See elution.

elution The removal of an adsorbedsubstance in a CHROMATOGRAPHY columnor ion-exchange column using a solvent(eluent), giving a solution called the eluate.The chromatography column can selec-tively adsorb one or more componentsfrom the mixture. To ensure efficient re-covery of these components graded elutionis used. The eluent is changed in a regularmanner starting with a nonpolar solventand gradually replacing it by a more polarone. This will wash the strongly polar com-ponents from the column.

Embden–Meyerhoff pathway See gly-colysis.

emission spectrum See spectrum.

81

emission spectrum

Beckmannthermometer

tube forintroducing

solute

heater

condenser

Elevation of boiling point

empirical formula See formula.

emulsion A colloid in which a liquidphase (small droplets with a diameterrange 10–5–10–7 cm) is dispersed or sus-pended in a liquid medium. Emulsions areclassed as lyophobic (solvent-repelling andgenerally unstable) or lyophilic (solvent-attracting and generally stable).

enantiomer (enantiomorph) A com-pound whose structure is not superimpos-able on its mirror image; one of any pair ofoptical isomers. See also isomerism; opticalactivity.

enantiomorph See enantiomer.

encephalin See endorphin.

enclosure compound See clathrate.

endorphin (encephalin; enkephalin) Oneof a group of peptides produced in thebrain and other tissues that are releasedafter injury and have pain-relieving effectssimilar to those of opiate alkaloids, such asmorphine. They include the enkephalins,which consist of just five amino acids.Other larger endorphins occur in the pitui-tary, while some are polypeptides, foundmainly in pancreas, adrenal gland, andother tissues.

endothermic Describing a process inwhich heat is absorbed (i.e. heat flowsfrom outside the system, or the tempera-ture falls). The dissolving of a salt in water,for instance, is often an endothermicprocess. Compare exothermic.

end point See equivalence point; volu-metric analysis.

energy Symbol: W A property of a sys-tem; a measure of its capacity to do work.Energy and work have the same unit: thejoule (J). It is convenient to divide energyinto kinetic energy (energy of motion) andpotential energy (‘stored’ energy). Namesare given to many different forms of energy(chemical, electrical, nuclear, etc.); the onlyreal difference lies in the system under dis-

cussion. For example, chemical energy isthe kinetic and potential energies of elec-trons in a chemical compound.

energy level One of the discrete energiesthat an atom, molecule, ion, etc., can haveaccording to quantum theory. Thus in anatom there are certain definite orbits thatthe electrons can be in, corresponding todefinite electronic energy levels of theatom. Similarly, a vibrating or rotatingmolecule can have discrete vibrational androtational energy levels.

energy profile A diagram that traces thechanges in the energy of a system duringthe course of a reaction. Energy profiles areobtained by plotting the potential energyof the reacting particles against the reac-tion coordinate. To obtain the reaction co-ordinate the energy of the total interactingsystem is plotted against position for themolecules. The reaction coordinate is thepathway for which the energy is a mini-mum.

enkephalin See endorphin.

enol An organic compound containingthe C:CH(OH) group; i.e. one in which ahydroxyl group is attached to one of thecarbon atoms of a double bond betweentwo carbon atoms. See keto–enol tau-tomerism.

enthalpy Symbol: H The sum of the in-ternal energy (U) and the product of pres-sure (p) and volume (V) of a system:

H = U + pVIn a chemical reaction carried out at

constant pressure, the change in enthalpymeasured is the internal energy change plusthe work done by the volume change:

∆H = ∆U + p∆V

entropy Symbol: S In any system thatundergoes a reversible change, the changeof entropy is defined as the heat absorbeddivided by the thermodynamic tempera-ture:

δS = δQ/TA given system is said to have a certain en-tropy, although absolute entropies are sel-

empirical formula

82

dom used: it is change in entropy that isimportant. The entropy of a system meas-ures the availability of energy to do work.

In any real (irreversible) change in aclosed system the entropy always increases.Although the total energy of the system hasnot changed (first law of thermodynamics)the available energy is less – a consequenceof the second law of thermodynamics.

The concept of entropy has beenwidened to take in the general idea of dis-order – the higher the entropy, the moredisordered the system. For instance, achemical reaction involving polymeriza-tion may well have a decrease in entropybecause there is a change to a more orderedsystem. The ‘thermal’ definition of entropyis a special case of this idea of disorder –here the entropy measures how the energytransferred is distributed among the parti-cles of matter.

enzyme A macromolecule that catalyzesbiochemical reactions. Enzymes act with agiven compound (the substrate) to producea complex, which then forms the productsof the reaction. The enzyme itself is un-changed in the reaction; its presence allowsthe reaction to take place. The names ofmost enzymes end in -ase, added to thesubstrate (e.g. lactase) or the reaction (e.g.hydrogenase).

Enzymes are extremely efficient cata-lysts for chemical reactions, and very spe-cific to particular reactions. Most enzymesare proteins. They may have a nonproteinpart (cofactor), which may be an inorganicion or an organic constituent (coenzyme).The mechanism of action of most enzymesappears to be by active sites on the enzymemolecule. The substrate acting with the en-zyme changes shape to fit the active site,and the reaction proceeds. Enzymes arevery sensitive to their environment – e.g.temperature, pH, and the presence of othersubstances. Catalytic activity has also beenfound in some RNA molecules.

enzyme technology (enzyme engi-neering) A branch of biotechnology thatutilizes enzymes for industrial purposes.For example rennet (impure rennin) ismanufactured on a large scale to make

cheese and junkets. Enzymes are also usedto determine the concentration of reactantsor products in specific reactions catalyzedby them.

epimerism A form of isomerism exhib-ited by carbohydrates in which the isomers(epimers) differ in the positions of –OHgroups. The α- and β- forms of glucose areepimers. See sugar.

epinephrine (adrenaline) A hormoneproduced by the adrenal glands. The mid-dle part of these glands, the adrenalmedulla, secretes the hormone, which ischemically almost identical to the transmit-ter substance norepinephrine produced atthe ends of sympathetic nerves. Epineph-rine secretion into the bloodstream instress causes acceleration of the heart, con-striction of arterioles, and dilation of thepupils. In addition, epinephrine produces amarked increase in metabolic rate thuspreparing the body for emergency.

epoxide A type of organic compoundcontaining a three-membered ring contain-ing two carbon atoms and one oxygenatom.

epoxyethane (ethylene oxide; C2H4O) Acolorless gaseous cyclic ether. Epoxyethaneis the simplest EPOXIDE. It is made by oxi-dation of ethene over a silver catalyst. Thering is strained and the compound is con-sequently highly reactive. It polymerizes toproduce epoxy polymers (resins). The com-pound hydrolyzes to give 1,2-ethanediol(CH2(OH)CH2(OH)).

epoxy resin See epoxyethane.

equation See chemical equation.

equation of state An equation that in-terrelates the pressure, temperature, andvolume of a system, such as a gas. The idealgas equation (see gas laws) and the van derWaals equation are examples of equationsof state.

equatorial conformation See cyclo-hexane.

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equatorial conformation

equilibrium In a reversible chemical re-action:

A + B ˆ C + DThe reactants are forming the products:

A + B → C + Dwhich also react to give the original reac-tants:

C + D → A + BThe concentrations of A, B, C, and Dchange with time until a state is reached atwhich both reactions are taking place atthe same rate. The concentrations (or pres-sures) of the components are then constant– the system is said to be in a state of chem-ical equilibrium. Note that the equilibriumis a dynamic one; the reactions still takeplace but at equal rates. The relative pro-portions of the components determine the‘position’ of the equilibrium, which may bedisplaced by changing the conditions (e.g.temperature or pressure).

equilibrium constant In a chemicalequilibrium of the type

xA + yB ˆ zC + wDThe expression:

[A]x[B]y/[C]z[D]wwhere the square brackets indicate concen-trations, is a constant (Kc) when the systemis at equilibrium. Kc is the equilibrium con-stant of the given reaction; its units dependon the stoichiometry of the reaction. Forgas reactions, pressures are often used in-stead of concentration. The equilibriumconstant is then Kp, where Kp = Kc

n. Here nis the number of moles of product minusthe number of moles of reactant; for in-stance, in

3H2 + N2 ˆ 2NH3n is 2 – (1 + 3) = –2.

equivalence point The point in a TITRA-TION at which the reactants have beenadded in equivalent proportions, so thatthere is no excess of either. It differsslightly from the end point, which is theobserved point of complete reaction, be-cause of the effect of the indicator, errors,etc.

equivalent proportions, law of (law ofreciprocal proportions) The principlethat when two chemical elements both

form compounds with a third element, acompound of the first two elements con-tains them in the relative proportions thatthey have in compounds with the third el-ement. For example, the mass ratio of car-bon to hydrogen in methane (CH4) is 12:4;the ratio of oxygen to hydrogen in water(H2O) is 16:2. In carbon monoxide (CO),the ratio of carbon to oxygen is 12:16.

equivalent weight A measure of ‘com-bining power’ formerly used in calcula-tions for chemical reactions. Theequivalent weight of an element is the num-ber of grams that could combine with ordisplace one gram of hydrogen (or 8 gramsof oxygen or 35.5 grams of chlorine). It isthe relative atomic mass (atomic weight)divided by the valence. For a compoundthe equivalent weight depends on the reac-tion considered. An acid, for instance, inacid–base reactions has an equivalentweight equal to its relative molecular mass(molecular weight) divided by the numberof acidic hydrogen atoms.

ergosterol A sterol present in plants. Itis converted, in animals, to vitamin D2 byultraviolet radiation, and is the most im-portant of vitamin D’s provitamins.

Erlenmeyer flask A glass laboratoryflask with conical shape and a narrowneck. It is named for the German chemistRichard Erlenmeyer (1825–1909).

essential amino acid See amino acid.

essential fatty acid A polyunsaturatedfatty acid (see carboxylic acid) required forgrowth and health that cannot be synthe-sized by the body and therefore must be in-cluded in the diet. Linoleic acid and(9,12,15)-linolenic acid are the only essen-tial fatty acids in humans, being requiredfor cell membrane synthesis and fat me-tabolism. Arachidonic acid is essential insome animals, such as the cat, but inhumans it is synthesized from linoleic acid.Essential fatty acids occur mainly in vegetable-seed oils, e.g. safflower-seed andlinseed oils.

equilibrium

84

essential oil Any pleasant-smellingvolatile oil obtained from various plants,widely used in making flavorings and per-fumes. Most consist of terpenes and theyare obtained by steam distillation or sol-vent extraction.

ester A type of organic compoundformed, or regarded as formed, by reactionbetween an alcohol and an acid. If the acidis a carboxylic acid, esters have the generalformula R1COOR2, where R1 and R2 arealkyl or aryl groups. For example, ethanol(C2H5OH) reacts with ethanoic acid(acetic acid; CH3COOH) to give the esterethyl ethanoate (C2H5OCOCH3) alongwith water:

C2H5OH + CH3COOH ˆC2H5OCOCH3 + H2O.

Methanol reacts with propanoic acid togive methyl propanoate:

CH3OH + C2H5COOH ˆCH3OCOC2H5 + H2O

This type of reaction, called esterification,is reversible and, in preparing esters, theequilibrium can be displaced toward theester by using a large excess of alcohol oracid. It can also be displaced by distillingoff the water or by removing it with a de-hydrating agent (e.g. sulfuric acid). Esterscan also be made from alcohols with ACYL

HALIDES or ACID ANHYDRIDES.The reverse reaction of esterification is

hydrolysis. Both esterification and ester hy-drolysis are acid-catalyzed. The mecha-nism involves protonation of the oxygen ofthe carbonyl group, allowing nucleophilicattack by water or alcohol at the carbonatom of the carbonyl group. Ester forma-tion cannot be base-catalyzed but the hy-drolysis can be catalyzed by OH– ions,which attack the carbon atom of the car-bonyl group. This type of hydrolysis isknown as saponification (because it is thereaction used to make SOAP from fats andoils).

Simple esters are volatile compounds,often with pleasant odors. They are used asflavorings. Esters of triols occur as fats andoils. See glyceride.

esterification See ester.

ethanal (acetaldehyde; CH3CHO) Awater-soluble liquid aldehyde used as astarting material in the manufacture of sev-eral other compounds. Ethanal can be pre-pared by the oxidation of ethanol. It ismanufactured by the catalytic oxidation ofethyne with oxygen using copper(II) chlo-ride and palladium(II) chloride as catalysts.The mixture of gases is bubbled through anaqueous solution of the catalysts; the reac-tion involves formation of an intermediateorganometallic complex with Pd2+ ions.With dilute acids ethanal polymerizes toethanal trimer (C3O3H3(CH3)3, formerlycalled paraldehyde), which is a sleep-in-ducing drug. Below 0°C ethanal tetramer isformed (C4O4H4(CH3)4, formerly calledmetaldehyde), which is used as a slug poi-son and a fuel in small portable stoves.

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ethanal

CH3

CH

O

CH

CH3O

O

HC

CH3

CH3

CH

O

HC CH3

O

CH

CH3

O

CHH3C

Ethanal trimer

O

Ethanal tetramer

Ethanal: polymer forms of ethanal

ethanimide (acetamide; CH3CONH2) Acolourless solid crystallizing in the form oflong white crystals with a characteristicsmell of mice. It is made by the dehydrationof ammonium ethanoate or by the actionof ammonia on ethanoyl chloride, ethanoicanhydride, or ethyl ethanoate.

ethane (C2H6) A gaseous alkane ob-tained either from the gaseous fraction ofcrude oil or by the ‘cracking’ of heavierfractions. Ethane is the second member ofthe homologous series of alkanes.

ethanedioic acid (oxalic acid; (COOH)2)A white crystalline organic acid that occursnaturally in rhubarb, sorrel, and otherplants of the genus Oxalis. It is slightly sol-uble in water, highly toxic, and used indyeing and as a chemical reagent.

ethane-1,2-diol (ethylene glycol; glycol;CH2(OH)CH2(OH)) A syrupy organicliquid commonly used as antifreeze and asa starting material in the manufacture ofDacron. The compound is manufacturedfrom ethene by oxidation over suitable cat-alysts to form epoxyethane, with subse-quent hydrolysis to the diol.

ethanoate (acetate) A salt or ester ofethanoic acid (acetic acid). See ethanoicacid.

ethanoic acid (acetic acid; CH3COOH)A colorless viscous liquid organic acid witha pungent odor (it is the acid in vinegar).Below 16.7°C it solidifies to a glassy solid(glacial ethanoic acid). It is made by the ox-idation of ethanol or butane, or by the con-tinued fermentation of beer or wine. It ismade into ethenyl ethanoate (vinyl acetate)for making polymers. Cellulose ethanoate(acetate) is made from ethanoic anhydride.See cellulose acetate.

ethanol (ethyl alcohol; alcohol; C2H5OH)A colorless volatile liquid alcohol. Ethanoloccurs in intoxicating drinks, in which it isproduced by fermentation of a sugar:

C6H12O6 → 2C2H5OH + 2CO2Yeast is used to cause the reaction. Atabout 15% alcohol concentration (by vol-

ume) the reaction stops because the yeast iskilled. Higher concentrations of alcoholare produced by distillation.

Apart from its use in drinks, alcohol is used as a solvent and to form ethanal.Formerly, the main source was by fermen-tation of molasses, but now catalytic hy-dration of ethene is used to manufactureindustrial ethanol.

ethanoyl chloride (acetyl chloride;CH3COCl) A liquid acyl chloride used asan acetylating agent.

ethanoyl group (acetyl group) Thegroup RCO–.

ethene (ethylene; C2H4) A gaseousalkene. Ethene is not normally present inthe gaseous fraction of crude oil but can beobtained from heavier fractions by cat-alytic cracking. This is the principal indus-trial source. The compound is important asa starting material in the organic-chemicalsindustry (e.g. in the manufacture ofethanol) and as the starting material for theproduction of polyethene. Ethene is thefirst member of the homologous series ofalkenes.

ether A type of organic compound con-taining the group –O–. Simple ethers havethe formula R1–O–R2, where R1 and R2 arealkyl or aryl groups, which may or may notbe the same. They are either gases or veryvolatile liquids and are very flammable.The commonest example is ethoxyethane(diethylether; C2H5OC2H5) used formerlyas an anesthetic. Ethers now find applica-tion as solvents. They are prepared in thelaboratory by the dehydration of alcoholswith concentrated sulfuric acid. An excessof alcohol is used to ensure that only onemolecule of water is removed from eachpair of alcohol molecules. They are gener-ally unreactive, but the C–O bond can becleaved by reaction with HI or PCl5.

ethanimide

86

R1

R2

O.. ..

Ether

ethoxyethane (ether; diethylether; C2H5-OC2H5) A colorless volatile liquid. Etheris well known for its characteristic smelland anesthetic properties, also for its ex-treme flammability. It still finds some ap-plication as an anesthetic when moremodern materials are unsuitable; it is alsoan excellent solvent. Its manufacture is anextension of the laboratory synthesis:ethanol vapor is passed into a mixture ofexcess ethanol and concentrated sulfuricacid at 140°C:

C2H5OH + H2SO4 →C2H5.O.SO2.OH + H2O

C2H5O.SO2.OH + C2H5OH →C2H5OC2H5 + H2SO4

ethyl acetate See ethyl ethanoate.

ethyl alcohol See ethanol.

ethylamine (aminoethane; C2H5NH2) Acolorless liquid amine. It can be preparedfrom chloroethane heated with concen-trated aqueous ammonia:

C2H5Cl + NH3 → C2H5NH2 + HClIt is used in manufacturing certain dyes.

ethyl bromide See bromoethane.

ethyl carbamate See urethane.

ethyl chloride See chloroethane.

ethylene See ethene.

ethylenediamine tetraacetic acid Seeedta.

ethylene glycol See ethane-1,2-diol.

ethylene oxide See epoxyethane.

ethyl ethanoate (ethyl acetate; C2H5-OOCCH3) An ester formed from ethanoland ethanoic acid. It is a fragrant liquidused as a solvent for plastics and in flavor-ing and perfumery.

ethyl iodide See iodoethane.

ethyne (acetylene; C2H2) A gaseousalkyne. Traditionally ethyne has found use

in oxy-acetylene welding torches, since itscombustion with oxygen produces a flameof very high temperature. It is also impor-tant in the organic chemicals industry forthe production of chloroethene (vinyl chlo-ride), which is the starting material for theproduction of polyvinyl chloride (PVC),and for the production of other vinyl com-pounds. Formerly, ethyne was manufac-tured by the synthesis and subsequenthydrolysis of calcium dicarbide. Modernmethods increasingly employ the crackingof alkanes.

ethynide See carbide.

eudiometer An apparatus for the volu-metric analysis of gases.

evaporation 1. A change of state fromliquid to gas (or vapor). Evaporation cantake place at any temperature, the rate in-creasing with temperature. Some moleculesin the liquid have enough energy to escapeinto the gas phase (if they are near the sur-face and moving in the right direction). Be-cause these are the molecules with higherkinetic energies, evaporation results in acooling of the liquid.2. A change from solid to vapor, especiallyoccurring at high temperatures close to themelting point of the solid. Thin films ofmetal can be evaporated onto a surface inthis way.

exa- Symbol: E A prefix denoting 1018.

excitation The process of producing anexcited state of an atom, molecule, etc.

excitation energy The energy requiredto change an atom, molecule, etc. from onequantum state to a state with a higher en-ergy. The excitation energy (sometimescalled excitation potential) is the differencebetween two energy levels of the system.

excited state A state of an atom, mol-ecule, or other system, with an energygreater than that of the ground state. Com-pare ground state.

87

excited state

exclusion principle The principle,enunciated by the Austrian–Swiss physicistWolfgang Pauli (1900–58) in 1925, that notwo electrons in an atom can have an iden-tical set of quantum numbers.

exothermic Denoting a chemical reac-tion in which heat is evolved (i.e. heatflows from the system or the temperaturerises). Combustion is an example of anexothermic process. Compare endother-mic.

explosive A substance or mixture thatcan rapidly decompose upon detonationproducing large amounts of heat and gases.The three most important classes of explo-sives are: 1. Propellants which burn steadily, and are

used as rocket fuels.

2. Initiators which are very sensitive andare used in small amounts to detonateless sensitive explosives.

3. High explosives which need an initiator,but are very powerful.

E–Z convention A convention for thedescription of a molecule showing cis-transISOMERISM. In a molecule ABC=CDE,where A, B, D, and E are different groups,the sequence rule (see CIP system) is ap-plied to the pair A and B to find which haspriority and it is similarly applied to thepair C and D. If the two groups of highestpriority are on the same side of the bondthen the isomer is designated Z (from Ger-man zusammen, together). If they are onopposite sides the isomer is designated E(German entgegen, opposite).

exclusion principle

88

89

FAD (flavin adenine dinucleotide) A de-rivative of riboflavin that is a coenzyme in electron-transfer reactions. Its reducedform is written as FADH2. See also flavo-protein.

Fahrenheit scale A temperature scale inwhich the ice temperature is taken as 32°and the steam temperature is taken as 212°(both at standard pressure). The scale isnot used for scientific purposes. To convertbetween degrees Fahrenheit (F) and de-grees Celsius (C) the formula C/5 = (F –32)/9 is used. It is named for the Germanphysicist Gabriel Daniel Fahrenheit (1686–1736) who proposed a scale of this type in1714.

faraday Symbol: F A unit of electriccharge equal to the charge required to dis-charge one mole of a singly-charged ion.One faraday is 9.648 670 × 104 coulombs.The unit is named for the British chemistand physicist Michael Faraday (1791–1867).

fat See glyceride.

fatty acid See carboxylic acid.

Fehling’s solution A solution used totest for the aldehyde group (–CHO). It is afreshly made mixture of copper(II) sulfatesolution with alkaline potassium sodium2,3-dihydroxybutanedioate (tartrate). Thealdehyde, when heated with the mixture, isoxidized to a carboxylic acid, and a redprecipitate of copper(I) oxide and coppermetal is produced. The tartrate is presentto complex with the original copper(II)ions to prevent precipitation of copper(II)

hydroxide. It is named for the Germanchemist H. C. von Fehling (1812–85).

femto- Symbol: f A prefix denoting10–15. For example, 1 femtometer (fm) =10–15 meter (m).

fermentation A chemical reaction pro-duced by microorganisms (molds, bacteria,or yeasts). A common example is the for-mation of ethanol from sugars:

C6H12O6 → 2C2H5OH + 2CO2

ferredoxins A group of red-brown pro-teins found in green plants, many bacteriaand certain animal tissues. They containnonheme iron in association with sulfur atthe active site. They are strong reducingagents (very negative redox potentials) andfunction as electron carriers, for examplein photosynthesis and nitrogen fixation.They have also been isolated from mito-chondria.

ferrocene (Fe(C5H5)2) An orange crys-talline solid. It is an example of a sandwichcompound, in which an iron(II) ion is co-ordinated to two cyclopentadienyl ions.The bonding involves overlap of d orbitalson the iron with the pi electrons in the cy-clopentadienyl ring. The compound canundergo substitution reactions on therings, which have aromatic character. Thesystematic name is di-π-cyclopentadienyliron(II).

filler A solid material used to modify thephysical properties or reduce the cost ofsynthetic compounds, such as rubbers,plastics, paints, and resins. Slate powder,glass fiber, mica, and cotton are all used asfillers.

F

filter See filtration.

filter pump A type of vacuum pump inwhich a jet of water forced through a noz-zle carries air molecules out of the system.Filter pumps cannot produce pressuresbelow the vapor pressure of water. Theyare used in the laboratory for vacuum fil-tration, distillation, and similar techniquesrequiring a low-grade vacuum.

filtrate See filtration.

filtration The process of removing sus-pended particles from a fluid by passing orforcing the fluid through a porous material(the filter). The fluid that passes throughthe filter is the filtrate. In laboratory filtra-tion, filter paper or sintered glass is com-monly used.

fine organic chemicals Carbon com-pounds, such as dyes and drugs, that areproduced only in small quantities. Theirmain requirement is that they must have ahigh degree of purity, often higher than95%. They are manufactured for specialpurposes, e.g. for use in spectroscopy,pharmacology, and electronics.

fine structure Closely spaced lines seenat high resolution in a spectral line or band.Fine structure may be caused by vibrationof the molecules or by electron spin. Hy-perfine structure, seen at very high resolu-tion, is caused by the atomic nucleusaffecting the possible energy levels of theatom.

firedamp Methane present in coalmines.

first-order reaction A reaction inwhich the rate of reaction is proportionalto the concentration of one of the reactingsubstances. The concentration of the react-ing substance is raised to the power one;i.e. rate = k[A]. For example, the decompo-sition of hydrogen peroxide is a first-orderreaction:

rate = k[H2O2]Similarly the rate of decay of radioactivematerial is a first-order reaction:

rate = k[radioactive material]For a first-order reaction, the time for adefinite fraction of the reactant to be con-sumed is independent of the original con-centration. The units of k, the RATE

CONSTANT, are s–1.

Fischer, Emil Hermann (1852–1919)German organic chemist. Fischer studiedmany compounds of biological interest. Heis sometimes referred to as the father ofbiochemistry. In 1874 he discoveredphenylhydrazine. He studied peptides,purines and sugars very thoroughly. Hiswork on purines (a name he coined) led tothe synthesis of many compounds such ascaffeine and purine. In his early work heput forward incorrect structures but by1897 he and his colleagues had establishedthe correct structures. Fischer was awardedthe 1902 Nobel Prize for chemistry for hiswork on purines and sugars.

Fischer, Hans (1881–1945) Germanorganic chemist. Fischer devoted his careerto the study of the molecular structures ofthe biologically significant moleculeshemoglobin, chlorophyll and the bile pig-ment bilirubin. Fischer started investigat-ing hemoglobin in 1921. He showed thatthe iron-containing nonprotein part con-sists of four pyrrole rings surrounding aniron atom. He synthesized this part by1929 and thoroughly investigated the por-phyrins. He won the 1930 Nobel prize forchemistry for this work. He then investi-gated the chlorophylls and demonstratedthat they are substituted porphins sur-rounding a magnesium atom. He alsodemonstrated that bile acids are degradedporphins. In 1944 he synthesized bilirubincompletely.

Fischer projection A way of represent-ing the three-dimensional structure of amolecule in two dimensions. The moleculeis drawn using vertical and horizontallines. Horizontal lines represent bonds thatcome out of the paper. Vertical lines repre-sent bonds that go into the paper (or are inthe plane of the paper). Named for EmilFischer, the convention was formerly used

filter

90

for representing the absolute configurationof SUGARS.

Fischer–Tropsch process A method ofmaking a mixture of hydrocarbons usinghydrogen and carbon monoxide (2:1 ratio)passed over a nickel or cobalt catalyst at atemperature of 200°C. The mixture, whichalso contains alcohols and carbonyl com-pounds, can be distilled to make fuels fordiesel and gasoline engines. It was used forthis by Germany in World War II. Now theprocess is one way of making SNG. It isnamed for the German chemist Franz Fis-cher (1852–1932) and the Czech chemistHans Tropsch (1839–1935), who inventedit in 1933.

Fittig reaction See Wurtz reaction.

flame-ionization detector See gaschromatography.

flare stack A chimney at the top ofwhich unwanted gases are burnt in an oilrefinery or other chemical plant.

flash photolysis A technique for inves-tigating free radicals in gases. The gas isheld at low pressure in a long glass orquartz tube, and an absorption spectrumtaken using a beam of light passing downthe tube. The gas is subjected to a very briefintense flash of light from a lamp outsidethe tube, producing free radicals, which areidentified by their spectra. Measurementsof the intensity of spectral lines can bemade with time using an oscilloscope, andthe kinetics of very fast reactions can thusbe investigated.

flash point The lowest temperature atwhich sufficient vapor is given off by aflammable liquid to ignite in the presenceof a spark. See also ignition temperature.

flavanone A type of flavonoid. Fla-vanone glycosides are found in floweringplants.

flavin A derivative of riboflavin occur-ring in the flavoproteins; i.e. FAD or FMN.

flavin adenine dinucleotide See FAD.

flavin mononucleotide See FMN.

flavone See flavonoid.

flavonoid One of a common group ofplant compounds having the C6–C3–C6chemical skeleton in which C6 is a benzenering. They are an important source of non-photosynthetic pigments in plants. Theyare classified according to the C3 portionand include the yellow chalcones and au-rones; the pale yellow and ivory flavonesand flavonols and their glycosides; the red,blue, and purple anthocyanins and antho-cyanidins; and the colorless isoflavones,catechins, and leukoanthocyanidins. Theyare water soluble and usually located in thecell vacuole. See anthocyanin.

flavonol A plant pigment that modifiesthe effects of certain growth substances.See flavonoid.

91

flavonol

O

O

Flavanone

O

O

Flavonoid

O

OH

O

Flavonol

flavoprotein A conjugated protein inwhich a flavin (FAD or FMN) is joined toa protein component. Flavoproteins are en-zymes in the electron-transport chain.

flocculation (coagulation) The combin-ing of the particles of a finely divided pre-cipitate, such as a colloid, into largerparticles or clumps that sink and are easierto filter off.

flocculent Describing a precipitate thathas aggregated in wooly masses.

Flory, Paul John (1910–85) Americanpolymer chemist. Flory’s early work con-sisted of helping Wallace CAROTHERS to de-velop nylon and neoprene. He began toinvestigate the properties of polymers inthe 1930s. Flory solved the difficulty that apolymer molecule does not have a fixedsize and structure by using statistical tech-niques to calculate a distribution of poly-mer chain lengths. Flory also worked onpolymers in which there are links betweenchains. This led to work on the elasticity ofrubber. Flory summarized his work in theclassic books Principles of Polymer Chem-istry (1953) and Statistical Mechanics ofChain Molecules (1969). Flory won the1974 Nobel Prize for chemistry for hiswork on polymers.

fluid A state of matter that is not a solid– that is, a liquid or a gas. All fluids canflow, and the resistance to flow is the vis-cosity.

fluidization The suspension of a finely-divided solid in an upward-flowing liquidor gas. This suspension mimics many prop-erties of liquids, such as allowing objects to‘float’ in it. Fluidized beds so constructedare important in the chemical industry.

fluorescein A fluorescent dye used as anabsorption indicator. It has a yellow solu-tion with green fluorescence.

fluorescence The absorption of energyby atoms, molecules, etc., followed by im-mediate emission of electromagnetic radia-tion as the particles make transitions to

lower energy states. Compare phosphores-cence.

fluoridation The introduction of smallquantities of fluoride compounds into thewater supply as a public-health measure toreduce the incidence of tooth decay.

fluoride See halide.

fluorination See halogenation.

fluorine A slightly greenish-yellowhighly reactive gaseous element belongingto the halogens (group 17 of the periodictable, formerly VIIA). It occurs notably asfluorite (CaF2) and cryolite (Na3AlF3) buttraces are also widely distributed withother minerals. It is slightly more abundantthan chlorine, accounting for about0.065% of the Earth’s crust. The high re-activity of the element delayed its isolation.Fluorine is now prepared by electrolysis ofmolten KF/HF electrolytes, using copper orsteel apparatus. Its preparation by conven-tional chemical methods is impossible.

Fluorine is strongly electronegative andexhibits a strong electron withdrawing ef-fect on adjacent bonds, thus CF3COOH isa strong acid (whereas CH3COOH is not).Fluorine and hydrogen fluoride are ex-tremely dangerous and should only be usedin purpose-built apparatus; gloves and faceshields should be used when working withhydrofluoric acid and accidental exposureshould be treated as a hospital emergency.

Symbol: F; b.p. –188.14°C; m.p.–219.62°C; d. 1.696 kg m–3 (0°C); p.n. 9;r.a.m. 18.99840.32.

fluorocarbon A compound derivedfrom a hydrocarbon by replacing hydrogenatoms with fluorine atoms. Fluorocarbonsare unreactive and most are stable up tohigh temperatures. They have a variety ofuses – in aerosol propellants, oils andgreases, and synthetic polymers such asPTFE. See also halocarbon.

fluxional molecule A molecule inwhich the constituent atoms change theirrelative positions so quickly at room tem-perature that the normal concept of struc-

flavoprotein

92

ture is inadequate; i.e. no specific structureexists for longer than about 10–2 secondand the relative positions become indistin-guishable. For example ClF3 at –60°C hasa distinct ‘T’ shape but at room tempera-ture the fluorine atoms are visualized asmoving rapidly over the surface of thechlorine atom in a state of exchange andare effectively identical.

FMN (flavin mononucleotide) A deriva-tive of riboflavin that is a coenzyme in electron-transfer reactions. See also flavo-protein.

foam A dispersion of bubbles of gas in aliquid, usually stabilized by a SURFACTANT.Solid foams, such as expanded polystyreneor foam rubber, are made by allowingliquid foams to set.

folic acid (pteroylglutamic acid) One ofthe water-soluble B-group of vitamins. Theprincipal dietary sources of folic acid areleafy vegetables, liver, and kidney. Defi-ciency of the vitamin exhibits itself in ane-mia in a similar manner to vitamin B12deficiency, while deficiency during preg-nancy increases the risk of birth defects inchildren.

Folic acid is important in metabolism invarious coenzyme forms, all of which arespecifically concerned with the transferand utilization of the single carbon (C1)group. Before functioning in this mannerfolic acid must be reduced to either dihy-drofolic acid (FH2) or tetrahydrofolic acid(FH4). It is important in the growth and re-production of cells, participating in thesynthesis of purines and thymine. See alsovitamin B complex.

formaldehyde See methanal.

formalin See methanal.

formate See methanoate.

formic acid See methanoic acid.

formula A representation of a moleculeusing symbols for the atoms. Subscripts in-dicate the numbers of atoms present. The

molecular formula gives the numbers andtypes of atom present. For example,ethanoic acid (acetic acid) has the molecu-lar formula C2H4O2. The empirical for-mula gives the simplest ratios of atoms.Thus, the empirical formula of ethanoicacid is CH2O. This is the formula thatwould be obtained by experimental deter-mination of the amounts of each elementpresent. The molecular formula can then

93

formula

butane

propanol

butadiene

ethanoic acid

cyclohexane

benzene

OH

OH

O

Formula: representations of some simple compounds

be obtained if the relative molecular massis known.

More information is given by the struc-tural formula, which shows how the atomsare joined together. The formula ofethanoic acid is usually written as CH3-COOH, showing that it is formed from amethyl group (CH3–) and a carboxylategroup (–COOH). Sometimes full stops areused in such formulae to divide up thegroups. Often it is necessary to show a ringcompound or to show the disposition ofthe atoms or groups in space (see isomer;optical activity). In such cases a diagram ofthe structure has to be given.

In certain cases symbols are used forgroups of atoms. Common ones are Me formethyl, Et for ethyl, Pr for propyl, Bu forbutyl, and Ph for phenyl. These symbolsare sometimes called organic elements. So,for example, ethanol is EtOH, phenol isPhOH, and ethanoic acid is MeCOOH.

In representing three-dimensionalstructures certain conventions are used.Particular types of projection formulae areused for certain types of compound. Forexample, the FISCHER PROJECTION has beenextensively used for representing the open-chain form of sugars. The NEWMAN PROJEC-TION is used for discussions of rotationabout C–C single bonds (see also confor-mation). More generally, it is conventionalto use a straight line for a single bond in theplane of the paper. A bond coming out ofthe paper is represented as a solid narrowwedge, intended to give the impression ofperspective. A bond into the paper is repre-sented by a dotted or dashed line. Also, or-ganic chemists commonly representstructures without the C or H atoms, ex-cept where these appear in functionalgroups. A hydrocarbon chain is drawn as azig-zag line and the BENZENE ring is drawnas one of the Kekulé structures.

Often a general formula is used to rep-resent a class of compounds. For instanceCnH2n for alkenes. It is also common to usethe symbol R for an organic group. SoRCOOH is any carboxylic acid. When twodifferent groups are needed, R and R′ areused (or R1, R2, R3, etc.). Ar is sometimesused for any aryl group.

formyl group The group HCO–.

fossil fuel A mineral fuel that forms un-derground from the remains of living or-ganisms. Fossil fuels include coal, naturalgas, peat, and petroleum.

fraction A mixture of liquids with simi-lar boiling points collected by fractionaldistillation.

fractional crystallization Crystalliza-tion of one component from a mixture insolution. When two or more substances arepresent in a liquid (or in solution), on cool-ing to a lower temperature one substancewill preferentially form crystals, leavingthe other substance in the liquid (or dis-solved) state. Fractional crystallization canthus be used to purify or separate sub-stances if the correct conditions areknown.

fractional distillation (fractionation) Adistillation carried out with partial reflux,using a long vertical column (fractionatingcolumn). It utilizes the fact that the vaporphase above a liquid mixture is generallyricher in the more volatile component. Ifthe region in which refluxing occurs is suf-ficiently long, fractionation permits thecomplete separation of two or morevolatile liquids. Fractionation is the funda-mental process for producing petroleumfrom crude oil.

Unlike normal reflux, the fractionatingcolumn may be insulated to reduce heatloss, and special designs are used to maxi-mize the liquid-vapour interface.

fractionation See fractional distillation.

Frankland, Sir Edward (1825–99)British chemist. Frankland is best known

formyl group

94

O

OH

HO

HOCH2

OH

OH

Formula: representation of the β-D-anomer ofglucose

for introducing the concept of what is nowknown as valence. In 1852 he noticed thatnitrogen and phosphorus frequently formcompounds in which there are either threeor five atoms of the other elements. Thissuggested to Frankland that each elementhas a definite combining power which issatisfied by a certain number of atoms. Heelaborated what came to be known as thetheory of valence in 1866. In 1864 Frank-land pointed out that the carboxyl group(CO2H) is present in many organic acids.Frankland was also concerned with tech-nological applications of chemistry, no-tably to coal gas and to water purification.

Franklin, Rosalind (1920–58) Britishx-ray crystallographer. Rosalind Franklinis best known for having played a key rolein the discovery of the structure of DNAand for having been portrayed in an un-flattering way by James WATSON in hisbook The Double Helix (1968). Her earlywork was on coal. Her x-ray photographsof DNA in 1952 led Francis CRICK andWatson to postulate the double helix struc-ture of DNA. Franklin subsequently con-firmed the double helix picture using x-raycrystallography. In her later years sheworked on tobacco mosaic virus. Her earlydeath prevented the possibility of her win-ning a Nobel Prize for her work on DNA.

free energy A measure of the ability of asystem to do useful work. See Gibbs func-tion; Helmholtz function.

free radical An atom or group of atomswith a single unpaired electron. Free radi-cals are produced by breaking a covalentbond; for example:

CH3Cl → CH3• + Cl•They are often formed in light-induced

reactions. Most free radicals are extremelyreactive and can be stabilized and isolatedonly under special conditions. They can bestudied by electron spin resonance. See alsocarbene.

freezing The process by which a liquid isconverted into a solid by cooling; the re-verse of melting.

freezing mixtures Two or more sub-stances mixed together to produce a lowtemperature. A mixture of sodium chlorideand ice in water (–20°C) is a common ex-ample.

freezing point The temperature atwhich a liquid is in equilibrium with itssolid phase at standard pressure and belowwhich the liquid freezes or solidifies. Thistemperature is always the same for a par-ticular liquid and is numerically equal tothe melting point of the solid. See also de-pression of freezing point.

Freon (Trademark) Any of a number ofchlorofluorocarbons (CFCs) and fluoro-carbons used as refrigerants. See fluorocar-bon; halocarbon.

Friedel–Crafts reaction A type of reac-tion in which an alkyl or acyl group is sub-stituted on a benzene ring. In Friedel–Crafts alkylation the reactant is ahaloalkane, and an alkylbenzene is pro-duced:

CH3Cl + C6H6 → C6H5CH3 + HClIn Friedel–Crafts acylation the reactant isan acyl halide and the product is an aro-matic ketone:

CH3COCl + C6H6 →C6H5COCH3 + HCl

These reactions occur at about 100°Cusing aluminum chloride as a catalyst. Thisaccepts a lone pair of electrons from thehalogen atom on the haloalkane or acylhalide, which results in a positive charge onthe adjoining carbon atom. The reaction isthen ELECTROPHILIC SUBSTITUTION. It is alsopossible to use alkenes (for alkylation) andacid anhydrides (for acylation). It is namedfor the French chemist Charles Friedel(1832–99) and the US chemist JamesCrafts (1832–99).

frontier orbital Either of two orbitals ina molecule: the highest occupied molecularorbital (the HOMO) or the lowest unoccu-pied molecular orbital (the LUMO). TheHOMO is the orbital with the highest en-ergy level occupied at absolute zero tem-perature. The LUMO is the lowest-energyunoccupied orbital at absolute zero. For a

95

frontier orbital

particular molecule the nature of these twoorbitals is very important in determiningthe chemical properties. Frontier-orbitaltheory, which considers the symmetry ofthese orbitals, has been very successful inexplaining such reactions as the DIELS–ALDER REACTION. See also Woodward–Hoffmann rules.

fructan A polysaccharide made entirelyof fructose residues. They are used as foodstores in many plants.

fructose (fruit sugar; C6H12O6) A SUGAR

found in fruit juices, honey, and canesugar. It is a ketohexose, existing in a pyra-nose form when free. In combination (e.g.in sucrose) it exists in the furanose form.

fruit sugar See fructose.

fucoxanthin A xanthophyll pigment ofdiatoms, brown algae, and golden brownalgae. The light absorbed is used with high efficiency in photosynthesis, theenergy first being transferred to chloro-phyll a. It has three absorption peaks cov-ering the blue and green parts of thespectrum.

fuel cell A type of cell in which fuel isconverted directly into electricity. In one

form, hydrogen gas and oxygen gas are fedto the surfaces of two porous nickel elec-trodes immersed in potassium hydroxidesolution. The oxygen reacts to form hy-droxyl (OH–) ions, which it releases intothe solution, leaving a positive charge onthe electrode. The hydrogen reacts with theOH– ions in the solution to form water,giving up electrons to leave a negativecharge on the other electrode. Large fuelcells can generate tens of amperes. Usuallythe e.m.f. is about 0.9 volt and the effi-ciency around 60%.

Fukui, Kenichi (1918–98) Japanesephysical and theoretical chemist. Fukui isbest known for his work on frontier orbitaltheory, a theory which describes thechanges in molecular orbitals during achemical reaction. He was particularly in-terested in applying frontier orbital theoryto the reactions of methyl radicals. Heshared the 1981 Nobel Prize for chemistrywith Roald HOFFMANN for his work onfrontier orbital theory. He also studied thereaction between nitrogen molecules andtransition metal complexes.

fullerene See buckminsterfullerene.

fullerite See buckminsterfullerene.

fructan

96

Friedel-Crafts acetylation

+ CH3CI

+ CH3COCI

benzene chloromethane methylbenzene(toluene)

benzene ethanoyl chloride phenyl methylketone

C

O

CH3

CH3

Friedel–Crafts methylation

Friedel–Crafts reactions

fuller’s earth A natural clay used as anabsorbent and industrial catalyst.

fumaric acid See butenedioic acid.

functional group A group of atoms in acompound that is responsible for the char-acteristic reactions of the type of com-pound. Examples are:

alcohol –OHalkoxide –ORaldehyde –CHOamide –CO–NH2amine –NH2ketone =COcarboxylic acid –CO.OHester –CO–ORacyl halide –CO.X (X = halogen)nitro compound –NO2sulfonic acid –SO2.OHnitrile –CNdiazonium salt –N2

+

diazo compound –N=N–

fundamental units The units of length,mass, and time that form the basis of mostsystems of units. In SI, the fundamentalunits are the meter, the kilogram, and thesecond.

furan (furfuran; C4H4O) A heterocyclicliquid organic compound. Its five-mem-bered ring contains four carbon atoms andone oxygen atom. The structure is charac-teristic of some monosaccharide sugars (fu-ranoses).

furanose A SUGAR that has a five-membered ring (four carbon atoms andone oxygen atom).

furfuran See furan.

fused Describing a solid that has beenmelted and solidified into a single mass.Fused silica, for example, is produced bymelting sand.

fused ring See ring.

fusel oil A mixture of high-molecular-weight alcohols together with some estersand fatty acids, formed from alcoholic fer-mentation and obtained during distillation.It is used as a source of higher alcohols.

fusion Melting.

97

fusion

98

GA3 See gibberellic acid

GAG See glycosaminoglycan.

galactose (C6H12O6) A SUGAR found inlactose and many polysaccharides. It is analdohexose, isomeric with glucose.

gas The state of matter in which forcesof attraction between the particles of a sub-stance are small. The particles have free-dom of movement and gases and have nofixed shape or volume. The atoms or mol-ecules of a gas are in a continual state ofmotion and are continually colliding witheach other and with the walls of the con-taining vessel. These collisions with thewalls create the pressure of a gas.

gas chromatography A type of CHRO-MATOGRAPHY widely used for the separa-tion and analysis of mixtures. Gaschromatography employs a column packedwith either a solid stationary phase(gas–solid chromatography or GSC) or asolid coated with a nonvolatile liquid(gas–liquid chromatography or GLC). Thewhole column is placed in a thermostatic-ally controlled heating jacket. A volatilesample is introduced into the column usinga syringe, and a carrier gas, such as hydro-gen or nitrogen, is passed through it. Thecomponents of the sample will be carriedalong in this mobile phase. Some of thecomponents will cling more readily to thestationary phase than others, either be-cause they become attached to the solidsurface or because they dissolve in the liq-uid. The time taken for different compo-nents to pass through the column ischaracteristic of a particular compoundand can be used to identify it. The emer-

gent sample is passed through a detector,which registers the presence of the differentcomponents in the carrier gas.

Two types of detector are in commonuse: the katharometer, which measureschanges in thermal conductivity, and theflame-ionization detector, which turns thevolatile components into ions and registersthe change in electrical conductivity. Gaschromatography is also used in other tech-niques to identify the separated compo-nents, as in gas chromatography–massspectroscopy (GCMS) and gas chromatog-raphy infrared (GCIR).

gas chromatography infrared See gaschromatography.

gas chromatography–mass spectros-copy See gas chromatography.

gas constant (universal gas constant)Symbol: R The universal constant8.314 34 J mol–1 K–1 appearing in theequation of state for an ideal gas. See gaslaws.

gas equation See gas laws.

gas laws Laws relating the temperature,pressure, and volume of a fixed mass ofgas. The main gas laws are BOYLE’S LAW

and CHARLES’ LAW. The laws are notobeyed exactly by any real gas, but manycommon gases obey them under certainconditions, particularly at high tempera-tures and low pressures. A gas that wouldobey the laws over all pressures and tem-peratures is a perfect or ideal gas

Boyle’s and Charles’ laws can be com-bined into an equation of state for idealgases:

G

pVm = RTwhere Vm is the molar volume and R themolar gas constant. For n moles of gas

pV = nRTAll real gases deviate to some extent

from the gas laws, which are applicableonly to idealized systems of particles ofnegligible volume with no intermolecularforces. There are several modified equa-tions of state that give a better descriptionof the behavior of real gases, the bestknown being the VAN DER WAALS EQUATION.

gas–liquid chromatography See gaschromatography.

gasohol Alcohol (ethanol) obtained bythe industrial fermentation of sugar for useas a motor fuel. It has been produced on alarge scale in Brazil.

gas oil One of the main fractions ob-tained from petroleum by distillation, usedas a fuel for diesel engines. See diesel fuel;petroleum.

gasoline See petroleum.

gas–solid chromatography See gaschromatography.

Gatterman–Koch reaction A reactionfor substituting a formyl group (HCO–)into a benzene ring of an aromatic hydro-carbon. It is used in the industrial produc-tion of benzaldehyde from benzene:

C6H6 → C6H5CHOThe aromatic hydrocarbon is mixed with aLewis acid, such as aluminum chloride,and a mixture of carbon monoxide and thehydrogen chloride is passed through. Thefirst stage if the production of a H–C≡O+

ion:HCl + CO + AlCl3 → HCO+ + AlCl4–

Copper(I) chloride (CuCl) is also added asa catalyst. The HCO+ ion acts as an elec-trophile in electrophilic substitution on thebenzene ring.

The Gatterman–Koch reaction is usedfor introducing the HCO– group into hy-drocarbons. A variation of the reaction inwhich the HCO– group is substituted intothe benzene ring of a phenol uses hydrogen

cyanide rather than carbon monoxide.Typically, a mixture of zinc cyanide andhydrochloric acid is used, to give zinc chlo-ride (which acts as a Lewis acid) and hy-drogen cyanide. The electrophile in thiscase is protonated hydrogen cyanide:

HCN + HCl + ZnCl2 → HCNH+ +ZnCl3–

The phenol is first substituted to give anIMINE:

C6H5OH → HOC6H4CH=NHThis then hydrolyzes to the aromatic alde-hyde:

HOC6H4CH=NH2 → HOC6H4CHOSimilar reactions using alkyl cyanides (ni-triles) rather than hydrogen cyanide givearomatic ketones. This type of reaction, inwhich a cyanide is used to produce an alde-hyde (or ketone) is often called the Gatter-man reaction. The Gatterman–Kochreaction was reported by the Germanchemist Ludwig Gatterman (1860–1920)in 1897 (with J. C. Koch). The use of hy-drogen cyanide was reported by Gatter-man in 1907.

Gatterman reaction 1. See Sandmeyerreaction.2. See Gatterman–Koch reaction.

gauche conformation See conforma-tion.

gauss Symbol: G The unit of magneticflux density in the c.g.s. system. It is equalto 10–4 tesla.

Gay-Lussac’s law 1. The principle thatgases react in volumes that are in simple ra-tios to each other and to the products ifthey are gases (all volumes measured at thesame temperature and pressure). The lawwas first put forward in 1808 by theFrench chemist and physicist Joseph-LouisGay Lussac (1778–1850).2. See Charles’ law.

GCIR (gas chromatography infrared) Seegas chromatography.

GCMS (gas chromatography–mass spec-troscopy) See gas chromatography.

99

GCMS

gel A lyophilic colloid that is normallystable but may be induced to coagulatepartially under certain conditions (e.g.lowering the temperature). This produces apseudo-solid or easily deformable jellylikemass, called a gel, in which intertwiningparticles enclose the whole dispersingmedium. Gels may be further subdividedinto elastic gels (e.g. gelatin) and rigid gels(e.g. silica gel).

gelatin (gelatine) A pale yellow proteinobtained from the bones, hides, and skinsof animals. It forms a colloidal jelly whendissolved in hot water and is used in thefood industry, to make capsules for variousmedicinal drugs, as an adhesive and sizingmedium, and in photographic emulsions.

gelatine See gelatin.

gel electrophoresis See electrophoresis.

gel filtration A form of column chro-matography in which a gel is used as thestationary medium. Components movethrough pores in the gel at a rate that de-pends on the size of their molecules. Thetechnique is used to separate proteins.

gem positions Positions in a moleculeon the same atom. For example, 1,1-dichloroethane (CH3CHCl2), in whichboth chlorine atoms are on the same car-bon, is a gem dihalide.

general formula See formula.

geometrical isomerism See isomerism.

gibberellic acid (GA3) A common GIB-BERELLIN and one of the first to be discov-ered. Together with GA1 and GA2 it wasisolated from Gibberella fujikuroi, a fun-gus that infects rice seedlings causing ab-normally tall growth.

gibberellin A plant hormone involvedchiefly in shoot extension. Gibberellins arediterpenoids; their molecules have the gib-bane skeleton. More than thirty have been

isolated, the first and one of the most com-mon being gibberellic acid, GA3.

Gibbs free energy See Gibbs function.

Gibbs function (Gibbs free energy) Sym-bol: G A thermodynamic function definedby

G = H – TSwhere H is the enthalpy, T the thermody-namic temperature, and S the entropy. It isuseful for specifying the conditions ofchemical equilibrium for reactions forconstant temperature and pressure (G is aminimum). It is named for the US math-ematician and physicist Josiah WillardGibbs (1839–1903), who first developedthe theory of chemical thermodynamics.See also free energy.

giga- Symbol: G A prefix denoting 109.For example, 1 gigahertz (GHz) = 109 hertz(Hz).

glacial acetic acid See glacial ethanoicacid.

glacial ethanoic acid (glacial aceticacid) Pure water-free ethanoic acid.

GLC Gas–liquid chromatography. Seegas chromatography.

globulin One of a group of proteins thatare insoluble in water but will dissolve inneutral solutions of certain salts. They gen-erally contain glycine and coagulate whenheated. Three types of globulin are foundin blood: alpha (α), beta (β), and gamma(γ). α and β globulins are made in the liverand are used to transport nonprotein ma-terial. γ globulins are made in reticuloen-dothelial tissues, lymphocytes, and plasmacells and most of them have antibody ac-tivity (see immunoglobulin).

glove box A sealed box with gloves fit-ted to ports in one side and having a trans-parent top, used for safety reasons or tohandle materials in an inert or sterile at-mosphere.

gel

100

glucan See glycan.

gluconic acid (dextronic acid; CH2OH-(CHOH)4COOH) A soluble crystallineorganic acid made by the oxidation of glu-cose (using specific molds). It is used inpaint strippers.

glucosan A POLYSACCHARIDE that isformed of glucose units. Cellulose andstarch are examples.

glucose (dextrose; grape sugar;C6H12O6) A monosaccharide occurringwidely in nature as D-glucose. It occurs asglucose units in sucrose, starch, and cellu-lose. It is important to metabolism becauseit participates in energy-storage andenergy-release systems. See also sugar.

glucoside See glycoside.

glue An adhesive, of which there are var-ious types. Aqueous solutions of starch andethyl cellulose are used as pastes for stick-ing paper; traditional wood glue is made byboiling animal bones (see gelatin); quick-drying adhesives are made by dissolvingrubber or a synthetic polymer in a volatilesolvent; and some polymers, such as epoxyresins and polyvinyl acetate (PVA), arethemselves used as glues.

glutamic acid See amino acid.

glutamine See amino acid.

glutathione A tripeptide of cysteine,glutamic acid, and glycine, widely distrib-uted in living tissues. It takes part in manyoxidation–reduction reactions, due to thereactive thiol group (–SH) being easily ox-idized to the disulfide (–S–S–), and acts asan antioxidant, as well as a coenzyme toseveral enzymes.

gluten A mixture of proteins found inwheat flour. It is composed mainly of twoproteins (gliaden and glutelin), the proteinsbeing present in almost equal quantities.Certain people are sensitive to gluten(celiac disease) and must have a gluten-freediet.

glycan A polysaccharide made of morethan 10 monosaccharide residues. A ho-moglycan is made up of a single type ofsugar unit (i.e. > 95%). As a class the gly-cans serve both as structural units (e.g. cel-lulose in plants and chitin in invertebrates)and energy stores (e.g. starch in plants andglycogen in animals). The most commonhomoglycans are made up of D-glucoseunits and called glucans.

glyceraldehyde A simple triose sugarused in determining absolute configura-tion.

glyceride An ester formed between glyc-erol (propane-1,2,3-triol) and one or morecarboxylic acids. Glycerol has three alco-hol groups, and if all three groups haveformed esters, the compound is a triglyc-eride. Naturally occurring fats and oils aretriglycerides of long-chain carboxylic acids(hence the name ‘fatty acid’). The main car-boxylic acids forming glycerides in fats andoils are:1. octadecanoic acid (stearic acid), a satu-

rated acid CH3(CH2)16COOH.2. hexadecanoic acid (palmitic acid), a sat-

urated acid CH3(CH2)14COOH.3. cis-9-octadecenoic acid (oleic acid), an

unsaturated acid.CH3(CH2)7CH:CH(CH2)7C OOH

glycerin See propane-1,2,3-triol.

glycerine (glycerin) See propane-1,2,3-triol.

glycerol See propane-1,2,3-triol.

glyceryl trinitrate See nitroglycerine.

glycine See amino acid.

101

glycine

CHO

CH OH

CH2OH

Glyceraldehyde: Fischer projection ofD-glyceraldehyde

glycogen (animal starch) A polysaccha-ride that is the main carbohydrate store ofanimals. It is composed of many glucoseunits linked in a similar way to starch.Glycogen is readily hydrolyzed in a step-wise manner to glucose itself. It is storedlargely in the liver and in muscle but isfound widely distributed in the body.

glycol See diol.

glycolipid A sugar-containing lipid withone or more sugar residues attached to alipid by a glycosidic link (or ester link inprokaryotes). Glycolipids play an impor-tant structural role in cell membranes,where the sugar residues are always extra-

cellular. Animal glycolipids are derivedfrom sphingosine, an amino alcohol with along unsaturated hydrocarbon chain. Inglycolipids, the amino group of sphingo-sine is joined to a fatty acid chain by anamide bond and the primary hydroxylgroup is linked to a sugar residue. The sim-plest glycolipid (in animal cells) is cere-broside, which has one sugar residue(either glucose or galactose). Glycolipidswith branched chains of sugar residues are known as gangliosides. The fatty acid chain and sphingosine chain arehydrophobic while the sugar residues arehydrophilic, making glycolipids amphipro-tic.

glycogen

102

Production of fructosediphosphate

This stage requires energyand the conversion of ATPto ADP

The 6C fructose diphosphatebreaks into two 3C molecules

This stage yields energy and2 molecules of pyruvate

ADP is converted to ATP.NADH is also formed, andmay participate in subsequentreactions

(starch, glycogen) (glucose)

ATPADP

ATPADP

2 NAD+

2 NADH

2 ADP2 ATP

2 ADP2 ATP

(glucose 1-phosphate)

(glucose 6-phosphate)

fructose 6-phosphate

fructose 1,6-diphosphate

2 x (glyceraldehyde3-phosphate)

2 x (1,3-diphosphoglycerate)

2 x (3-phosphoglycerate)

2 x (2-phosphoglycerate)

2 x (phosphoenolpyruvate)

2 x (pyruvate)

Glycolysis

glycolysis (Embden–Meyerhof pathway;glycolytic pathway) The conversion ofglucose into pyruvate, with the release ofsome energy in the form of ATP. Glycoly-sis occurs in cell cytoplasm. It yields twomolecules of ATP and two of NADH2 permolecule of glucose. In anaerobic condi-tions, breakdown proceeds no further andpyruvate is converted into ethanol or lacticacid for storage or elimination. In aero-bic conditions, glycolysis is followed by the KREBS CYCLE. The rate of glycolysis iscontrolled by the enzyme phosphofructo-kinase, which catalyzes an essentialy irre-versible reaction. There are two otherirreversible reactions catalyzed by hexoki-nase and pyruvate kinase.

glycolytic pathway See glycolysis.

glycoprotein A conjugated proteinformed by the combination of a proteinwith carbohydrate side chains. Certainantigens, enzymes, and hormones are gly-coproteins.

glycosaminoglycan (GAG) One of agroup of compounds, sometimes calledmucopolysaccharides, consisting of longunbranched chains of repeating disaccha-ride sugars, one of the two sugar residuesbeing an amino sugar – either N-acetylglu-cosamine or N-acetylgalactosamine. Thesecompounds are present in connective tis-sue; they include heparin and hyaluronicacid. Most glycosaminoglycans are linkedto protein to form proteoglycans (some-times called mucoproteins). See also glyco-protein.

glycoside A compound in which thering form of a SUGAR is joined to someother organic group. The link in a glyco-side occurs at the anomeric carbon atom. Ifthe hydroxyl group at this carbon is re-moved, the result is a glycosyl group. If thisgroup is joined to another organic groupthrough an oxygen atom, then the resultingcompound is an O-glycoside. The linkagering–O–organic group is a glycosidic link.A simple example of a glycoside would bethe compound formed by replacing theanomeric –OH group in a sugar by a

methoxy group, –OCH3. Such glycosidesare described as alpha or beta according towhether the organic group is below orabove the ring at the anomeric carbon. Ifthe glycosyl group is attached throughsome other atom, it is often described as ppa S-glycoside, C-glycoside, etc. Com-pounds in which the glycosyl group is at-tached through nitrogen (i.e. N-glycosides)are also called glycosylamines. If the sugaris glucose, the compound is a glucoside.Glycosides occur in plants and includemany useful substances. See also nucleo-side.

glycosidic link See glycoside.

glycosylamine See glycoside.

glycosyl group See glycoside.

glyoxylate cycle A modification of theKrebs cycle occurring in some microorgan-isms, algae, and higher plants in regionswhere fats are being rapidly metabolized,e.g. in germinating fat-rich seeds. Acetylgroups formed from the fatty acids arepassed into the glyoxylate cycle, with theeventual formation of mainly carbohy-drates.

graft copolymer See polymerization.

Graham’s law (of diffusion) The princi-ple that gases diffuse at a rate that is in-versely proportional to the square root oftheir density. Light molecules diffuse fasterthan heavy molecules. It is named for theScottish chemist Thomas Graham (1805–69), who reported it in 1829.

gram (gramme; symbol: g) A unit ofmass defined as 10–3 kilogram.

gram-atom See mole.

gram-equivalent The equivalent weightof a substance in grams.

gramme An alternative spelling of gram.

gram-molecule See mole.

103

gram-molecule

granulation A process for enlargingparticles to improve the flow properties ofsolid reactants and products in industrialchemical processes. The larger a particle,and the freer from fine materials in a solid,the more easily it will flow. Dry granula-tion produces pellets from dry materials,which are crushed into the desired size.Wet granulation involves the addition of aliquid to the material, and the resultingpaste is extruded and dried before cuttingto the required size.

grape sugar See glucose.

gravimetric analysis A method ofquantitative analysis in which the final an-alytical measurement is made by weighing.There are many variations in the methodbut in essence they all consist of:1. taking an accurately weighed sample

into solution;2. precipitation as a known compound by

a quantitative reaction;3. digestion and coagulation procedures;4. filtration and washing;5. drying and weighing as a pure com-

pound.Filtration is a key element in the methodand a variety of special filter papers andsinter-glass filters are available.

gray Symbol: Gy The SI unit of absorbedenergy dose per unit mass resulting fromthe passage of ionizing radiation throughliving tissue. One gray is an energy absorp-tion of one joule per kilogram of mass. Theunit is named for the British radiobiologistL. H. Gray (1905–65).

Grignard, François Auguste Victor(1871–1935) Franch organic chemist.Grignard is best remembered for theorganomagnesium compounds known asGrignard reagents which he discovered in1901. He found that these compounds,which have the general formular RMgX,where R is an organic group and X is ahalogen, can be used in the synthesis ofmany types of compounds, including alco-hols, hydrocarbons and carboxylic acids.Grignard shared the 1912 Nobel Prize forchemistry for this work. He started com-

piling his Treatise on Organic Chemistryafter World War I. The first volumes ap-peared in 1935, with others helping tocomplete this multi-volume treatise afterhis death.

Grignard reagent A type of organo-metallic compound with the general for-mula RMgX, where R is an alkyl or arylgroup and X is a halogen (e.g. CH3MgCl).Grignard reagents are prepared by reactingthe haloalkane or haloaryl compound withmagnesium in dry ether:

CH3Cl + Mg → CH3MgClGrignard reagents probably have the

form R2Mg.MgCl2. They are used exten-sively in organic chemistry. With methanala primary alcohol is produced:

RMgX + HCHO → RCH2OH +Mg(OH)X

Other aldehydes give secondary alcohols:RMgX + R′CHO → RR′CHOH +

Mg(OH)XAlcohols and carboxylic acids give hydro-carbons:

RMgX + R′OH → RR′ + Mg(OH)XWater also gives a hydrocarbon:

RMgX + H2O → RH + Mg(OH)XSolid carbon dioxide in acid solution givesa carboxylic acid:

RMgX + CO2 + H2O → RCOOH +Mg(OH)X

They are named for Victor Grignard.

ground state The lowest energy state ofan atom, molecule, or other system. Com-pare excited state.

group 1. In the periodic table, a series ofchemically similar elements that have simi-lar electronic configurations. A group isthus a column of the periodic table. For ex-ample, the alkali metals, all of which haveouter s1 configurations, belong to group 1.See also periodic table.2. (functional group) In organic chemistry,an arrangement of atoms that bestows aparticular type of property on a moleculeand enables it to be placed in a particularclass, e.g. the aldehyde group –CHO. Seealso functional group.

granulation

104

GSC Gas–solid chromatography. Seegas chromatography.

GTP (guanosine triphosphate) A nucleo-side triphosphate occurring in all cells as a coenzyme for various key processes.Often it provides energy by undergoing hy-drolysis to GDP (guanosine diphosphate)and a phosphate group, a reaction cat-alyzed by an enzyme or other componenthaving GTPase activity. In protein synthe-sis, GTP is essential for the assembly of ri-bosomes and elongation of the polypeptidechain. It is also required for the assembly ofmicrotubules, for protein transport withincells, and for the relaying of messages tovarious cell components in signal transduc-tion.

guanidine (iminourea; HN:C(NH2)2) Astrongly basic crystalline organic com-pound which can be nitrated to make apowerful explosive. It is also used in mak-ing dyestuffs, medicines and polymer resins.

guanine A nitrogenous base found in

DNA and RNA. Guanine has a purine ringstructure.

guanosine (guanine nucleoside) A nu-cleoside present in DNA and RNA andconsisting of guanine linked to D-ribose viaa β-glycosidic bond.

guanosine triphosphate See GTP.

gum One of a group of substances thatswell in water to form gels or sticky solu-tions. Similar compounds that produceslimy solutions are called mucilages. Gumsand mucilages are not distinguishablechemically. Most are heterosaccharides,being large, complex, flexible, and oftenhighly-branched molecules.

guncotton See cellulose trinitrate.

105

guncotton

HN

N

N

NH

O

H2N

6543

21 7

89

Guanine

HN

N N

N

H2N

HOCH2

OHOH

O

O

Guanosine

106

habit See crystal habit.

half-chair conformation See cyclo-hexane.

half life For a certain radioactive nu-cleus, the time taken for half the originalnuclei in a sample to decay.

halide A compound containing a halo-gen. The HALOALKANES are examples.

haloalkane (alkyl halide) A type of or-ganic compound in which one or more hy-drogen atoms of an alkane have beenreplaced by halogen atoms. Haloalkanescan be made by direct reaction of thealkane with a halogen. Other methods are:1. Reaction of an alcohol with the halogen

acid (e.g. from NaBr + H2SO4) or withphosphorus halides (red phosphorusand iodine can be used):

ROH + HBr → RBr + H2OROH + PCl5 → RCl + POCl3 + HCl

2. Addition of an acid to an alkene:RCH:CH2 + HBr → RCH2CH2Br

The haloalkanes are much more reactivethan the alkanes, and are useful startingcompounds for preparing a wide range oforganic chemicals. In particular, they un-dergo nucleophilic substitutions in whichthe halogen atom is replaced by some other group (iodine compounds are themost reactive). Some reactions of haloalka-nes are:1. Refluxing with aqueous potassium hy-

droxide to give an alcohol:RI + OH– → ROH + I–

2. Refluxing with potassium cyanide in al-coholic solution to give a nitrile:

RI + CN– → RCN + I–

3. Refluxing with an alkoxide to give anether:

RI + –OR′ → ROR′ + I–

4. Reaction with alcoholic ammonia solu-tion (100°C in a sealed tube) to give anamine:

RI + NH3 → RNH2 + HI5. Boiling with alcoholic potassium hy-

droxide, to eliminate an acid and pro-duce an alkene:

RCH2CH2I + KOH → KI + H2O +RCH:CH2

See also Grignard reagent; Wurtz reaction.

halocarbon A chemical compound thatcontains carbon atoms bound to halogenatoms and (sometimes) hydrogen atoms.The halocarbons include haloalkanes suchas tetrachloromethane (CCl4) and the halo-forms (CHCl3, CHBr3, etc.). There are var-ious types of halocarbon that are useful butare also significant pollutants. For exam-ple, the chlorofluorocarbons (CFCs) con-tain carbon, fluorine, and chlorine. Theyare useful as refrigerants, aerosol propel-lants, and in making rigid plastic foams.However, they are also thought to damagethe ozone layer and an international agree-ment exists to phase out their use. Similarcompounds are the hydrochlorofluorocar-bons (HCFCs), which contain hydrogen aswell as chlorine and fluorine, and the hy-drofluorocarbons (HFCs), which containhydrogen and fluorine.

The halons are a class of halocarbonsthat contain bromine as well as hydrogenand other halogens. Their main use is infire extinguishers. They are, however, sig-nificantly more active than CFCs in theireffect on the ozone layer. The halocarbonsare also thought to contribute to globalwarming.

H

haloform Any of the four compoundsCHX3, where X is a halogen atom (F, fluo-roform; Cl, chloroform; Br, bromoform; I,iodoform). The systematic names are tri-fluoromethane, trichloromethane, tribro-momethane, and triiodomethane.

haloform reaction A reaction of amethyl ketone with NaOX, where X is Cl,Br, or I, to give a haloform. With sodiumchlorate(I), for example:

RCOCH3 + 3NaOCl → RCOCCl3 +3NaOH

RCOCl3 + NaOH → NaOCOR +CHCl3

The reaction can be used to make car-boxylic acids (from the NaOCOR), and isespecially useful when R is an aromaticgroup because the starting ketone,RCOCH3, can be produced byFriedel–Crafts acetylation. See also tri-iodomethane.

halogenating agent A compound usedto introduce halogen atoms into a mol-ecule. Examples are phosphorus trichloride(PCl3) and aluminum trichloride (AlCl3).

halogenation A reaction in which ahalogen atom is introduced into a mol-ecule. Halogenations are specified as chlo-rinations, brominations, fluorinations,etc., according to the element involved.There are several methods.1. Direct reaction with the element using

high temperature or ultraviolet radia-tion:

CH4 + Cl2 → CH3Cl + HCl2. Addition to a double bond:

H2C:CH2 + HCl → C2H5Cl3. Reaction of a hydroxyl group with a

halogenating agent, such as PCl3:C2H5OH → C2H5Cl + OH–

4. In aromatic compounds direct substitu-tion can occur using aluminum chlorideas a catalyst:

2C6H6 + Cl2 → 2C6H5Cl5. Alternatively in aromatic compounds,

the chlorine can be introduced by react-ing the diazonium ion with copper(I)chloride:

C6H5N2+ + Cl– → C6H5Cl + N2

halogens A group of elements (group17, formerly VIIA, of the periodic table)consisting of fluorine, chlorine, bromine,iodine, and the short-lived radioactive el-ement astatine. The halogens all have outervalence shells that are one electron short ofa rare-gas configuration. Because of this,the halogens are characterized by high elec-tron affinities and high electronegativities,fluorine being the most electronegative el-ement known.

A wide range of organic halides isformed in which the C–F bond is charac-teristically resistant to chemical attack; theC–Cl bond is also fairly stable, particularlyin aryl compounds, but the alkyl halogencompounds become increasingly suscepti-ble to nucleophilic attack and generallymore reactive.

halon See halocarbon.

halothane (CHBrClCF3) A colorlessnonflammable liquid halocarbon used as ageneral anesthetic. The systematic name is1-chloro-1-bromo-2,2,2-trifluoroethane.

hammer mill A device used in the chem-ical industry for crushing and grindingsolid materials at high speeds to a specifiedsize. The impact between the particles,grinding plates, and grinding hammers pul-verizes the particles. Hammer mills can beused for a greater variety of soft materialthan other types of grinding equipment.Compare ball mill.

hardening (of oils) The conversion ofliquid plant oils into a more solid form foruse in margarine by hydrogenation using anickel catalyst. In vegetable oils the fattyacids present (as glycerides) contain doublebonds (i.e. they are unsaturated). The hy-drogenation process increases the amountof unsaturated material, increasing themelting point, but still leaves unsaturatedfatty acids. For this reason it is claimed thatmargarines are healthier than animal fats(e.g. butter), which contains saturated fats,because the unsaturated fats are less likelyto lead to cholesterol build-up in the body,and consequent risk of coronary heart dis-ease. However, the hydrogenation process

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hardening

may also affect the nature of the doublebonds. Natural unsaturated fatty acidsmostly have a cis configuration about thedouble bonds. In the hydrogenationprocess, a proportion of these are con-verted into fatty acids with a trans config-uration. Glycerides of these are known astrans-fats. It has been claimed that there isalso a link between trans-fats and coronaryheart disease. See Sabatier–Senderensprocess.

Haworth, Sir Walter Norman (1883–1950) British organic chemist. Haworthwas a pioneer of the study of carbohy-drates, particularly sugars. He showed thatsugar molecules are ring molecules, with apuranose ring consisting of five carbonatoms and an oxygen atom and a furanosering consisting of four carbon atoms andan oxygen atom. He and his colleaguessubsequently investigated the chain struc-tures of various polysaccharides. This es-tablished the structures of cellulose, starchand glycogen. In 1929 he published thebook The Constitution of the Sugars whichsoon became the standard work on thistopic. In 1933, together with his colleagueEdmund Hirst, he synthesized vitamin C(ascorbic acid), having previously estab-lished its structure. Haworth shared the1937 Nobel Prize for chemistry with PaulKARRER.

HCFC Hydrochlorofluorocarbon. Seehalocarbon.

heat Energy transferred as a result of atemperature difference. The term is oftenloosely used to mean internal energy (i.e.the total kinetic and potential energy of theparticles). It is common in chemistry to de-fine such quantities as heat of combustion,heat of neutralization, etc. These are in factmolar enthalpies for the change, given thesymbol ∆HMŠ. The superscript symbol de-notes standard conditions, while the sub-script M indicates that the enthalpy changeis for one mole. The unit is usually the kilo-joule per mole (kJ mol–1). By convention,∆H is negative for an exothermic reaction.Molar enthalpy changes stated for chemi-cal reactions are changes for standard con-

ditions, which are defined as 298 K (25°C)and 101 325 Pa (1 atmosphere). Thus, thestandard molar enthalpy of reaction is theenthalpy change for reaction of substancesunder these conditions producing reactantsunder the same conditions. The substancesinvolved must be in their normal equilib-rium physical states under these conditions(e.g. carbon as graphite, water as the liq-uid, etc.). Note that the measured enthalpychange will not usually be the standardchange. In addition, it is common to spec-ify the entity involved. For instance∆HfŠ(H2O) is the standard molar en-thalpy of formation for one mole of H2Ospecies.

heat exchanger A device that enablesthe heat from a hot fluid to be transferredto a cool fluid without allowing the fluidsto come into contact. The normal arrange-ment is for one of the fluids to flow in acoiled tube through a jacket containing thesecond fluid. Both the cooling and heatingeffect may be of benefit in conserving theenergy used in a chemical plant and in con-trolling the process.

heat of atomization The energy re-quired in dissociating one mole of a sub-stance into atoms. See heat.

heat of combustion The energy liber-ated when one mole of a substance burns inexcess oxygen. See heat.

heat of crystallization The energy lib-erated when one mole of a substance crys-tallizes from a saturated solution of thissubstance.

heat of dissociation The energy re-quired to dissociate one mole of a sub-stance into its constituent elements.

heat of formation The energy changewhen one mole of a substance is formedfrom its elements. See heat.

heat of neutralization The energy lib-erated when one mole of an acid or base isneutralized.

Haworth, Sir Walter Norman

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heat of reaction The energy changewhen molar amounts of given substancesreact completely. See heat.

heat of solution The energy changewhen one mole of a substance is dissolvedin a given solvent to infinite dilution (inpractice, to form a dilute solution).

heavy hydrogen See deuterium.

heavy water Deuterium oxide, D2O.

hecto- Symbol: h A prefix denoting 102.For example, 1 hectometer (hm) = 102

meters (m).

helicate See supramolecular chemistry.

Hell–Volard–Zelinsky reaction A method for the preparation of halogenatedcarboxylic acids using free halogen in thepresence of a phosphorus halide. The halo-genation occurs at the carbon atom adja-cent to the –COOH group. With Br2 andPBr3:

RCH2COOH → RCHBrCOOH →RCBr2COOH

Helmholtz free energy See Helmholtzfunction.

Helmholtz function (Helmholtz free en-ergy) Symbol: F A thermodynamic func-tion defined by

F = U – TSwhere U is the internal energy, T the ther-modynamic temperature, and S the en-tropy. It is a measure of the ability of asystem to do useful work in an isothermalprocess. The function is named for the Ger-man physiologist and physicist HermannLudwig Ferdinand von Helmholtz (1821–94). See also free energy.

heme (haeme) An iron-containing por-phyrin that is the prosthetic group in HEMO-GLOBIN, myoglobin, and some cytochromes.

hemiacetal See acetal.

hemicellulose One of a group of sub-stances that make up the amorphous ma-

trix of plant cell walls together with pecticsubstances (and occasionally, in maturecells, with lignin, gums, and mucilages).They are heteropolysaccharides, i.e. poly-saccharides built from more than one typeof sugar, mainly the hexoses (mannose andgalactose) and the pentoses (xylose andarabinose). They vary greatly in composi-tion between species. In some seeds (e.g.the endosperm of dates) hemicelluloses area food reserve.

hemiketal See acetal.

hemin The hydrochloride form of heme.Hemin is the crystalline form in whichheme can be isolated and studied in the lab-oratory. The iron present is the trivalentstate (iron(III)). Hemin can be made tocrystallize by heating hemoglobin gentlywith acetic acid and sodium chloride. A va-riety of crystal forms are known.

hemocyanin A blue copper-containingblood pigment found in many mollusksand arthropods. Hemocyanin is the secondmost abundant blood pigment after hemo-globin and functions similarly in acting asan oxygen-carrier in the blood.

hemoerythrin A pigment occurring inthe blood of certain invertebrates, similarin structure to HEMOGLOBIN.

hemoglobin The pigment of the redblood cells in humans and other verte-

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hemoglobin

N

N N

CH3

H3C

H3C CH3

COOHCOOH

CH2

CH2

Fe

N

Heme

henry

110

brates that is responsible for the transportof oxygen from the lungs to the tissues. Itconsists of a basic protein, globin, linkedwith four heme groups. Heme is a complexcompound containing an iron atom. Themost important property of hemoglobin isits ability to combine reversibly with onemolecule of oxygen per iron atom to formoxyhemoglobin, which has a bright redcolor. The iron is present in the divalentstate (iron(II)) and this remains unchangedwith the binding of oxygen. There are vari-ations in the polypeptide chains, giving riseto different types of hemoglobins in differ-ent species. The binding of oxygen dependson the oxygen partial pressure; high pres-sure favors formation of oxyhemoglobinand low pressure favors release of oxygen.

henry Symbol: H The SI unit of induc-tance, equal to the inductance of a closedcircuit that has a magnetic flux of oneweber per ampere of current in the circuit.1 H = 1 Wb A–1. It is named for the USphysicist Joseph Henry (1797–1828).

Henry’s law The concentration (C) of agas in solution is proportional to the par-tial pressure (p) of that gas in equilibriumwith the solution, i.e. p = kC, where k is aproportionality constant. The relationshipis similar in form to that for RAOULT’S LAW,which deals with ideal solutions. A conse-quence of Henry’s law is that the ‘volumesolubility’ of a gas is independent of pres-sure. The law is named for the Britishphysician and chemist William Henry(1755–1836), who formulated it in 1801.

heparin A polysaccharide that inhibitsthe formation of thrombin from prothrom-bin and thereby prevents the clotting ofblood. It is used in medicine as an antico-agulant. See polysaccharide.

heptane (C7H16) A colorless liquidalkane obtained from petroleum refining.It is used in gasoline and as a solvent.

hertz Symbol: Hz The SI unit of fre-quency, defined as one cycle per second(s–1). Note that the hertz is used for regu-larly repeated processes, such as vibration

or wave motion. It is named for the Ger-man physicist Heinrich Hertz (1857–94).

Hess’s law A derivative of the first lawof thermodynamics. It states that the totalheat change for a given chemical reactioninvolving alternative series of steps is inde-pendent of the route taken. It is named forthe Russian chemist Germain Henri Hess(1802–50), who proposed it in 1840.

hetero atom See heterocyclic com-pound.

heterocyclic compound A compoundthat has a ring containing more than onetype of atom. Commonly, heterocycliccompounds are organic compounds withat least one atom in the ring that is not acarbon atom. Pyridine and glucose are ex-amples. The noncarbon atom is called ahetero atom. Compare homocyclic com-pound.

heterogeneous Relating to more thanone phase. A heterogeneous mixture, forinstance, contains two or more distinctphases. Heterogeneous catalysis involves acatalyst that is a different phase than thatof the reactants (usually gaseous reactantspassed over a solid catalyst).

heterolysis See heterolytic fission.

heterolytic fission (heterolysis) Thebreaking of a covalent bond so that bothelectrons of the bond remain with one frag-ment. A positive ion and a negative ion areproduced:

RX → R+ + X–

Compare homolytic fission.

heteropolymer See polymerization.

heteropolysaccharide See hemicellu-lose.

hexadecanoate (palmitate) A salt orester of hexadecanoic acid.

hexadecanoic acid (palmitic acid) Acrystalline carboxylic acid:

CH3(CH2)14COOH

It is present as glycerides in fats andoils. See glyceride.

hexamethylenetetramine (hexamine;C6H12N4) A white crystalline organiccompound made by condensing methanalwith ammonia. It is used as a fuel forcamping stoves, in vulcanizing rubber, andas a urinary disinfectant. It can be nitratedto make the high explosive cyclonite.

hexamine See hexamethylenetetramine.

hexane (C6H14) A liquid alkane ob-tained from the light fraction of crude oil.The principal use of hexane is in gasolineand as a solvent.

hexanedioic acid (adipic acid; HOOC-(CH2)4COOH) A colorless crystallineorganic dicarboxylic acid that occurs inrosin. It is used in the manufacture ofNYLON.

hexanoate A salt or ester of hexanoicacid.

hexanoic acid (caproic acid; CH3-(CH2)4COOH) An oily carboxylic acidfound (as glycerides) in cow’s milk andsome vegetable oils.

hexose A SUGAR that has six carbonatoms in its molecules.

hexose monophosphate shunt Seepentose phosphate pathway.

hexyl group The group C5H11CH2–,having a straight chain of carbon atoms.

HFC Hydrofluorocarbon. See halocar-bon.

high-performance liquid chromatog-raphy See HPLC.

histamine An amine formed from theamino acid histidine by decarboxylationand produced mainly in connective tissueas a response to injury or allergic reaction.It causes contraction of smooth muscle,stimulates gastric secretion of hydrochloricacid and pepsin, and dilates blood vessels,which lowers blood pressure and producesinflammation, itching, or allergic symp-toms.

histidine See amino acid.

Hodgkin, Dorothy Mary Crowfoot(1910–94) British x-ray crystallographerDorothy Hodgkin determined the structureof many complex organic molecules. Shewon the 1964 Nobel Prize for chemistryfor this work. Together with Charles Bunn,she published the structure of penicillin in1949. She then started work on the struc-ture of vitamin B12 and found its structurein 1956. This work made use of early elec-tronic computers. In 1969 she determinedthe structure of insulin.

Hoffmann, Roald (1937– ) Polish-born American chemist, Hoffmann is bestknown for his collaboration with RobertWOODWARD in the mid 1960s which led tothe formulation of the Woodward–Hoff-mann rules. These rules stated whichchemical reactions can take place in termsof molecular orbitals. Woodward andHoffmann were largely concerned with or-ganic reactions but their rules apply moregenerally. They summarized their results inthe book Conservation of Orbital Symme-try (1970). Hoffmann shared the 1979Nobel Prize for chemistry with KenichiFUKUI for this work. Hoffmann has alsodone a great deal to popularize chemistryin books and television programmes.

Hofmann, August Wilhelm von(1818–92) German organic chemist. Hof-mann was one of the most influential or-

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Hofmann, August Wilhelm von

CHCHCH222

NNN

CHCHCH222

HHH222CCC CHCHCH222

NNNNNN

NNN CH2CH2

Hexamethylenetetramine

ganic chemists of the 19th century. He wasparticularly influential in Britain and Ger-many. Much of his work was concernedwith the constituents of coal tar, particu-larly aniline and phenol. He discovered orinvestigated a number of compounds, in-cluding quaternary ammonium salts. Healso discovered the reaction known as Hof-mann degradation, which consists of treat-ing an amide with bromine and alkali togive an amine with one fewer carbon atom.He was one of the first people to investi-gate formaldehyde. In 1858 he obtainedthe dye magenta by reacting carbon tetra-chloride with aniline.

Hofmann degradation A method ofpreparing primary amines from acidamides. The amide is refluxed with aque-ous sodium hydroxide and bromine:

RCONH2 + NaOH + Br2 →RCONHBr + NaBr + H2O

RCONHBr + OH– → RCON–Br + H2ORCON–Br → R–N = C = O + Br–

RNCO + 2OH– → RNH2 + CO22–

The reaction is a ‘degradation’ in thesense that a carbon atom is removed fromthe amide chain. It is named for the Ger-man chemist August Wilhelm von Hof-mann.

Hofmann’s method A method for-merly used for determining the vapor den-sity of volatile liquids. A known weight ofsample is introduced into a mercurybarometer tube, which is surrounded by aheating jacket. The volume of vapor canthus be read off directly, the temperature isknown, and the pressure is obtained bytaking the atmospheric pressure minus themercury height in the barometer (with cor-rections for the density of mercury athigher temperatures). The method’s onlyadvantage is that it may be used for sam-ples that decompose at their normal boil-ing point.

holoenzyme A catalytically active com-plex made up of an apoenzyme and a coen-zyme. The former is responsible for thespecificity of the holoenzyme whilst the lat-ter determines the nature of the reaction.

HOMO See frontier orbital.

homocyclic compound A compoundcontaining a ring made up of the sameatoms. Benzene is an example of a homo-cyclic compound. Compare heterocycliccompound.

homogeneous Relating to a singlephase. A homogeneous mixture, for in-stance, consists of only one phase. In ho-mogeneous catalysis, the catalyst has thesame phase as the reactants.

homologous series A group of organiccompounds possessing the same functionalgroup and having a regular structural pat-tern so that each member of the series dif-fers from the next one by a fixed number ofatoms. The members of a homologous se-ries can be represented by a general for-mula. For example, the homologous seriesof alkane alcohols CH3OH, C2H5OH,C3H7OH, …, has a general formulaCnH2n+1OH. Each member differs by CH2

from the next. Any two successive mem-bers of a series are called homologs.

homologs See homologous series.

homolysis See homolytic fission.

homolytic fission (homolysis) Thebreaking of a covalent bond so that oneelectron from the bond is left on each frag-ment. Two free radicals result:

RR′ → R• + R′•Compare heterolytic fission.

homopolymer See polymerization.

host–guest chemistry A branch ofsupramolecular chemistry in which a mo-lecular structure acts as a ‘host’ to hold anion or molecule (the ‘guest’). The guestmay be coordinated to the host or may betrapped by its structure. For example, cal-ixarenes are compounds with cup-shapedmolecules that may accept guest molecules.See also crown ether; supramolecularchemistry.

Hofmann degradation

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HPLC High-performance liquid chro-matography; a sensitive analytical tech-nique, similar to gas-liquid chromatog-raphy but using a liquid carrier. The carrieris specifically choosen for the particularsubstance to be detected.

Hückel, Erich Armand ArthurJoseph (1896–1980) German physicaland theoretical chemist. Hückel has twomain claims to fame. His first is the theoryof electrolytes which Peter Debye and heproduced in 1923 and which gives a gooddescription of the electrical and thermody-namic properties of dilue electrolytes. Hissecond major work was the application ofquantum mechanics to aromatic moleculessuch as benzene. He used molecular orbitaltheory to show that in the benzene mol-ecule the electrons in the pi orbitals arespread out directly above and below thering of carbon atoms, thus making the mol-ecule more stable than it would be if onehad alternating double and single bonds.Hückel started this work in 1930 and soonextended it to predict the Hückel rulewhich states that a molecule is aromatic ifit has (4n + 2) pi electrons.

Hückel rule See aromatic compound.

humectant A hygroscopic substanceused to maintain moisture levels. Glycerol,mannitol, and sorbitol are commonly usedin foodstuffs, tobacco, etc.

Hund’s rule A rule that states that theelectronic configuration in degenerate or-bitals will have the minimum number ofpaired electrons.

hybrid orbital See orbital.

hydrate A compound coordinated withwater molecules. When water is bound upin a compound it is known as water ofcrystallization.

hydration The solvation of such speciesas ions in water.

hydrazine (N2H4) A colorless liquidthat can be prepared by the oxidation of

ammonia with sodium chlorate(I) or by thegas phase reaction of ammonia with chlo-rine. Hydrazine is a weak base, formingsalts (e.g. N2H4.HCl) with strong acids andis also a powerful reducing agent. Withaldehydes and ketones it forms HYDRA-ZONES.

hydrazone A type of organic compoundcontaining the C:NNH2 group, formed bythe reaction between an aldehyde or ke-tone and hydrazine (N2H4). Derivatives ofhydrazine were formerly used to producecrystalline products, which have sharpmelting points that can be used to charac-terize the original aldehyde or ketone.Phenylhydrazine (C6H5NH.NH2), for in-stance, produces phenylhydrazones.

hydride A compound of hydrogen. Ionichydrides are formed with highly elec-tropositive elements and contain the H– ion(hydride ion). Non-metals form covalenthydrides, as in methane (CH4) or silane(SiH4). The boron hydrides are electron-deficient covalent compounds. Many tran-sition metals absorb hydrogen to forminterstitial hydrides.

hydrobromic acid (HBr) A colorlessliquid produced by adding hydrogen bro-mide to water. It shows the typical proper-ties of a strong acid and it is a strongreducing agent.

hydrocarbon Any compound contain-ing only the elements carbon and hydro-gen. Examples are the alkanes, alkenes,alkynes, and aromatics such as benzeneand naphthalene.

hydrochloric acid (HCl) A colorlessfuming liquid made by adding hydrogenchloride to water. Dissociation into ions isextensive and hydrochloric acid shows thetypical properties of a strong acid. Hy-drochloric acid is used in making dyes,drugs, and photographic materials.

hydrochlorofluorocarbon See halo-carbon.

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hydrochlorofluorocarbon

hydrocyanic acid (prussic acid; HCN) Ahighly poisonous weak acid formed whenhydrogen cyanide gas dissolves in water.Its salts are cyanides. Hydrogen cyanide isused in making acrylic plastics.

hydrofluoric acid (HF) A colorless liq-uid produced by dissolving hydrogen fluo-ride in water. It is a weak acid, but willdissolve most silicates and hence can beused to etch glass. As the interatomic dis-tance in HF is relatively small, the H–Fbond energy is very high and hydrogen flu-oride is not a good proton donor. It does,however, form hydrogen bonds.

hydrofluorocarbon See halocarbon.

hydrogen A colorless gaseous element.Hydrogen has some similarities to both thealkali metals (group 1) and the halogens(group 17), but is not normally classified inany particular group of the periodic table.It is the most abundant element in the Uni-verse and the ninth most abundant elementin the Earth’s crust and atmosphere (bymass). It occurs principally in the form ofwater and petroleum products; traces ofmolecular hydrogen are found in some nat-ural gases and in the upper atmosphere.

Symbol: H; m.p. 14.01 K; b.p. 20.28 K;d. 0.089 88 kg m–3 (0°C); p.n. 1; r.a.m.1.0079.

hydrogenation The reaction of a com-pound with hydrogen. In organic chem-istry, hydrogenation usually refers to theaddition of hydrogen to multiple bonds,often with the aid of a catalyst. Unsatu-rated natural liquid vegetable oils can behydrogenated to form saturated semisolidfats – a reaction used in making types ofmargarine. See Bergius process.

hydrogen bond An intermolecularbond between molecules in which hydro-gen is bound to a strongly electronegativeelement. Bond polarization by the elec-tronegative element X leads to a positivecharge on hydrogen Xδ––Hδ+; this hy-drogen can then interact directly withelectronegative elements of adjacent mol-

ecules. The hydrogen bond is representedas a dotted line:

Xδ– – Hδ+ ....... Xδ– – Hδ+ …The length of a hydrogen bond is charac-teristically 0.15–0.2 nm. Hydrogen bond-ing may lead to the formation of dimers(for example, in carboxylic acids) and isused to explain the anomalously high boil-ing points of H2O and HF. Hydrogenbonding is important in many biochemicalsystems. It occurs between bases in thechains of DNA. It also occurs betweenC=O and N–H groups in PROTEINS, whereit is responsible for maintaining the sec-ondary structure.

hydrogen bromide (HBr) A colorlesssharp-smelling gas that is very soluble inwater. It is produced by direct combinationof hydrogen and bromine in the presence ofa platinum catalyst or by the reaction ofphosphorus tribromide with water. It dis-solves in water to give HYDROBROMIC ACID.

hydrogencarbonate (bicarbonate) Asalt containing the ion –HCO3.

hydrogen chloride (HCl) A colorlessgas that has a strong irritating odor andfumes strongly in moist air. It is preparedby the action of concentrated sulfuric acidon sodium chloride. The gas is made in-dustrially by burning a stream of hydrogenin chlorine. It is not particularly reactivebut will form dense white clouds of ammo-nium chloride when mixed with ammonia.It is very soluble in water and ionizes al-most completely to give HYDROCHLORIC

ACID. Hydrogen chloride is used in themanufacture of organic chlorine com-pounds, such as polyvinyl chloride (PVC).

hydrogen cyanide See hydrocyanicacid.

hydrogen fluoride (HF) A colorless liq-uid produced by the reaction of concen-trated sulfuric acid on calcium fluoride. Itproduces toxic corrosive fumes and dis-solves readily in water to give HYDROFLUO-RIC ACID. Hydrogen fluoride is atypical ofthe hydrogen halides as the individual H–Funits are associated into much larger units,

hydrocyanic acid

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forming zigzag chains and rings. This iscaused by hydrogen bonds that form be-tween the hydrogen and the highly elec-tronegative fluoride ions. Hydrogenfluoride is used extensively as a catalyst inthe petroleum industry.

hydrogen ion A positively charged hy-drogen atom, H+, i.e. a proton. Hydrogenions are produced by all acids in water, inwhich they are hydrated to hydroxonium(hydronium) ions, H3O+. See acid; pH.

hydrogen peroxide (H2O2) A colorlesssyrupy liquid, usually used in solution inwater. Although it is stable when pure, oncontact with bases such as manganese(IV)oxide it gives off oxygen, the manga-nese(IV) oxide acting as a catalyst:

2H2O2 → 2H2O + O2Hydrogen peroxide can act as an oxi-

dizing agent, converting iron(II) ions toiron(III) ions, or as a reducing agent withpotassium manganate(VII). It is used as ableach and in rocket fuel. The strength ofsolutions is usually given as volumestrength – the volume of oxygen (dm3) atSTP given by decomposition of 1 dm3 ofthe solution.

hydrogensulfate (bisulfate, HSO4–) An

acidic salt or ester of sulfuric acid (H2SO4),in which only one of the acid’s hydrogenatoms has been replaced by a metal or or-ganic radical. An example is sodium hy-drogensulfate, NaHSO4.

hydrogen sulfide (sulfuretted hydrogen,H2S) A colorless very poisonous gas withan odor of bad eggs. Hydrogen sulfide isprepared by reacting hydrochloric acidwith iron(II) sulfide. It is tested for by mix-ing with lead nitrate, with which it gives ablack precipitate. Its aqueous solution isweakly acidic. Hydrogen sulfide reducesiron(III) chloride to iron(II) chloride, form-ing hydrochloric acid and a yellow precip-itate of sulfur. Hydrogen sulfideprecipitates insoluble sulfides, and is usedin qualitative analysis. It burns with a blueflame in oxygen to form sulfur(IV) oxideand water. Natural gas contains some hy-

drogen sulfide, which is removed beforesupply to the consumer.

hydrogensulfite (bisulfite, HSO3–) An

acidic salt or ester of sulfurous acid(H2SO3), in which only one of the acid’shydrogen atoms has been replaced by ametal or organic radical. An example issodium hydrogensulfite, NaHSO3.

hydrolysis A reaction between a com-pound and water. An example is the hy-drolysis of an ESTER to give a carboxylicacid and an alcohol:

CH3COOC2H5 + H2O ˆ CH3COOH+ C2H5OH

hydron The positive ion H+. The nameis used when the isotope is not relevant, i.e.a hydron could be a proton, deuteron, ortriton.

hydrophilic Water attracting. Seelyophilic.

hydrophobic Water repelling. Seelyophobic.

hydroquinone See benzene-1,4-diol.

hydrosol A colloid in aqueous solution.

hydroxide A compound containing theion OH– or the group –OH.

hydroxonium ion See hydrogen ion.

hydroxybenzene See phenol.

hydroxybenzoate See salicylate.

hydroxybenzoic acid See salicylic acid.

2-hydroxypropanoic acid See lacticacid.

hydroxyl group A group (–OH) con-taining hydrogen and oxygen, characteris-tic of alcohols and phenols, and somehydroxides. It should not be confused withthe hydroxide ion (OH–).

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hydroxyl group

hygroscopic Describing a substancethat absorbs moisture from the atmos-phere. See also deliquescent.

hyperconjugation The interaction ofsigma bonds with pi bonds. It is sometimesdescribed in terms of resonance structuresof the type:

C6H5CH3 → C6H5CH2–H+

to explain the interaction of the methylgroup with the pi electrons of the benzenering in methylbenzene (toluene).

hyperfine structure See fine structure.

hypertonic solution A solution that

has a higher osmotic pressure than some

other solution. Compare hypotonic solu-

tion.

hypotonic solution A solution that has

a lower osmotic pressure than some other

solution. Compare hypertonic solution.

hygroscopic

116

117

IAA (indole acetic acid) A naturally oc-curring auxin. See auxin.

ideal gas (perfect gas) See gas laws.

ideal solution A hypothetical solutionthat obeys RAOULT’S LAW.

ignis fatuus (will-o’-the-wisp) A lightsometimes seen over marshy ground. It iscaused by methane produced by rottingvegetation, which is ignited by the presenceof small amounts of spontaneously flam-mable phosphine (PH3).

ignition temperature The lowest tem-perature to which a given substance can beheated before it ignites (without the appli-cation of a spark or flame). It is often calledthe autoignition temperature. Compareflash point.

imide An organic compound containingthe group –CO.NH.CO–, i.e. a –NH groupattached to two carbonyl groups. Simpleimides have the general formulaR1.CO.NH.CO.R2, where R1 and R2 arealkyl or aryl groups. The group is known

as the imido group, and it can form part ofa ring in cyclic imides.

imido group See imide.

imine An organic compound containingthe group C=N–, in which there is a doublebond between the carbon and the nitrogen.A general formula for imines isR1R2C=N–R3, where R1, R2, and R3 arehydrocarbon groups or hydrogen. Theycan be made by the reaction of aldehydesand ketones with primary amines. For ex-ample, propanone (acetone; CH3COCH3)with ethylamine (C2H6NH2):

CH3COCH3 + C2H6NH2 →(CH3)2C=N–C2H6 + H2O

The reaction is acid-catalyzed. Whenammonia is used the imine contains theC=N–H group:

CH3COCH3 + NH3 → (CH3)2C=N–H + H2O

Most imines are unstable and can be de-tected only in solution unless R1, R2, or R3

are aryl groups. Related compounds suchas OXIMES, HYDRAZONES, and SEMICAR-BAZONES, in which the nitrogen is attachedto an electronegative group, are more sta-ble and can be isolated. Intermediates in

I

C

N

R1 R2

imine

R3

C

N

R1 R2

iminium ion

R3H +

Imine

which an imine adds a proton to form apositive ion (e.g. R1R2C=NR3H+) areknown as iminium ions.

iminium ion See imine.

imino group See imine.

iminourea See guanidine.

immiscible Describing two or more liq-uids that will not mix, such as oil andwater. After being shaken together and leftto stand immiscible liquids form separatelayers.

indene (C9H8) A colorless flammablehydrocarbon. It has a benzene ring fused toa five-membered ring.

indicator A compound that reversiblychanges color depending on the presenceor absence of a chemical substance. Inacid–base indicators the color depends onthe pH of the solution in which it is dis-solved. Methyl orange and phenolph-thalein are examples of acid–baseindicators. Redox titrations require eitherspecific indicators, which detect one of thecomponents of the reaction (e.g. starch foriodine, potassium thiocyanate for Fe3+) ortrue redox indicators in which the transi-tion potential of the indicator between ox-idized and reduced forms is important. Thetransition potential of a redox indicator isanalogous to the transition pH in acid–base systems. Complexometric titrationsrequire indicators that complex with metalions and change color between the freestate and the complex state.

indigo (C16H10N2O2) A blue organicdye that occurs (as a glucoside) in plants ofthe genus Indigofera. It is a derivative of in-dole, and is now made synthetically.

indole (benzpyrrole; C8H7N) A color-less solid organic compound that occurs incoal tar and various plants, and is the basisof indigo and of several plant hormones.The indole molecule has a benzene ringfused to a pyrrole ring.

indole acetic acid (IAA) A naturally oc-curring auxin. See auxin.

inductive effect The effect in whichsubstituent atoms or groups in an organiccompound can attract (–I) or push awayelectrons (+I), forming polar bonds. Elec-tron-attracting groups include –NO2,–CN, –COOH, and the halogens. Electron-releasing groups include –OH, –NH2,–OR, and R, where R is an alkyl group. In-ductive effects can influence the reactivityof other parts of a molecule. For example,an electron-attracting group substituted ona benzene ring withdraws electrons andmakes the ring less susceptible to elec-trophilic substitution. An electron-releas-ing group makes the ring more susceptible.

infrared (IR) Electromagnetic radiationwith longer wavelengths than visible radia-tion. The wavelength range is approxi-mately 0.7 µm to 1 mm. Many materialstransparent to visible light are opaque toinfrared, including glass. Rock salt, quartz,germanium, or polyethene prisms andlenses are suitable for use with infrared. In-frared radiation is produced by movementof charges on the molecular scale; i.e. byvibrational or rotational motion of mol-ecules. Infrared spectroscopy is of particu-lar importance in organic chemistry andabsorption spectra are used extensively inidentifying compounds. Certain bonds be-tween pairs of atoms (C–C, C=C, C=O,etc.) have characteristic vibrational fre-quencies, which correspond to bands in theinfrared spectrum. Infrared spectra arethus used in finding the structures of neworganic compounds by indicating the pres-ence of certain groups. They are also usedto ‘fingerprint’ and thus identify knowncompounds. At shorter wavelengths, in-frared absorption corresponds to transi-tions between rotational energy levels, and

iminium ion

118

NHNHNH

Indole

119

iodine

can be used to find the dimensions of mol-ecules (by their moment of inertia).

Ingold, Sir Christopher Kelk (1893–1970) British organic chemist. Ingold de-voted his career to understanding themechanisms of organic reactions in termsof the electrons of the molecules con-cerned. For example, he postulated theconcept of mesomerism in 1926 to explainhow a molecule could exist as a hybrid oftwo possible structures. Ingold was partic-ularly concerned with the mechanisms ofelimination and substitution reactions. In-gold summarized his work in the classicbook Structure and Mechanisms in Or-ganic Chemistry, the second edition ofwhich was published in 1969.

inhibitor A substance that slows downthe rate of a chemical reaction.

inner Describing a ring compound thatis formed, or regarded as formed, by onepart of a molecule reacting with another.For example, a LACTAM is an inner amideand a LACTONE is an inner ester.

inorganic chemistry The branch ofchemistry concerned with elements otherthan carbon and with the preparation,properties, and reactions of their com-pounds. Certain simple carbon compoundsare treated in inorganic chemistry, includ-ing carbon oxides, carbon disulfide, car-bon halides, hydrogen cyanide, and certainsimple salts, such as the cyanides, cyanates,carbonates, and hydrogencarbonates.

insertion reaction A reaction in whichan atom or group is inserted between twoother groups. See carbene.

insoluble Describing a compound thathas a very low solubility (in a specified sol-vent).

intermediate 1. A compound that re-quires further chemical treatment to pro-duce a finished industrial product.2. A transient chemical entity in a complexreaction.

See also precursor.

intermediate bond A form of covalentbond that also has an ionic or electrovalentcharacter. See polar bond.

intermolecular force A force of attrac-tion between molecules, as distinguishedfrom a force within the molecule (a chemi-cal bond). Forces of attraction betweenmolecules are the result of interactions be-tween dipoles. See hydrogen bond; Van derWaals force.

internal energy Symbol: U The energyof a system that is the total of the kineticand potential energies of its constituentparticles (e.g. atoms and molecules). If thetemperature of a substance is raised, bytransferring energy to it, the internal en-ergy increases (the particles move faster).Similarly, work done on or by a system re-sults in an increase or decrease in the inter-nal energy. The relationship between heat,work, and internal energy is given by thefirst law of thermodynamics. Sometimesthe internal energy of a system is looselyspoken of as ‘heat’ or ‘heat energy’.Strictly, this is incorrect; heat is the trans-fer of energy as a result of a temperaturedifference.

inversion A change in which a com-pound is converted from one optical iso-mer to the other. See Walden inversion.

invert sugar See sucrose.

in vitro Literally ‘in glass’; describingexperiments or techniques performed inlaboratory apparatus rather than in the liv-ing organism. Cell tissue cultures and invitro fertilization (to produce ‘test-tube ba-bies’) are examples. Compare in vivo.

in vivo Literally ‘in life’; describingprocesses that occur within the living or-ganism. Compare in vitro.

iodide See halide.

iodine A dark-violet volatile solid el-ement belonging to the HALOGENS (group17, formerly VIIA, of the periodic table). Itoccurs in seawater and is concentrated by

iodoethane

120

various marine organisms in the form of io-dides. Significant deposits also occur in theform of iodates. A large number of organiciodine compounds are known.

iodoethane (ethyl iodide; C2H5I) A col-orless liquid haloalkane made by reactionof ethanol with iodine in the presence ofred phosphorus.

iodoform See triiodomethane.

iodoform reaction See triiodomethane.

iodomethane (methyl iodide; CH3I) Aliquid haloalkane made by reaction ofmethanol with iodine in the presence of redphosphorus.

ion An atom or molecule that has a neg-ative or positive charge as a result of losingor gaining one or more electrons. See alsoionization.

ion exchange A process that takes placein certain insoluble materials that containions capable of exchanging with ions in thesurrounding medium. Zeolites, the firstion-exchange materials, were used forwater softening. These have largely beenreplaced by synthetic resins made of aninert backbone material, such aspolyphenylethene, to which ionic groupsare weakly attached. If the ions exchangedare positive, the resin is a cationic resin. Ananionic resin exchanges negative ions. Whenall available ions have been exchanged (e.g.sodium ions replacing calcium ions) thematerial can be regenerated by passingconcentrated solutions (e.g. sodium chlo-ride) through it. The calcium ions are thenreplaced by sodium ions. Ion-exchangetechniques are used for a vast range of pu-rification and analytical purposes.

ionic bond See electrovalent bond.

ionic crystal A crystal composed of ionsof two or more elements. The positive andnegative ions are arranged in definite pat-terns and are held together by electrostaticattraction. Sodium chloride is a typical ex-ample.

ionic product The product of concen-trations:

KW = [H+][OH–]in water as a result of a small amount ofself-ionization:

H2O ˆ H+ + OH–

ionic radius A measure of the effectiveradius of an ion in a compound. For an iso-lated ion, the concept is not very meaning-ful, since the ion is a nucleus surrounded byan ‘electron cloud’. Values of ionic radiican be assigned, however, based on the dis-tances between ions in crystals.

ionic strength For an ionic solution aquantity can be introduced that empha-sizes the charges of the ions present:

I = ½Σimiz2i

where m is the molality and z the ioniccharge. The summation is continued overall the different ions in the solution, i.

ionization The process of producingions. There are several ways in which ionsmay be formed from atoms or molecules.In certain chemical reactions ionization oc-curs by transfer of electrons; for example,sodium atoms and chlorine atoms react toform sodium chloride, which consists ofsodium ions (Na+) and chloride ions (Cl–).Certain molecules can ionize in solution;acids, for example, form hydrogen ions asin the reaction

H2SO4 → 2H+ + SO42–

The ‘driving force’ for ionization in asolution is solvation of the ions by mol-ecules of the solvent. H+, for example, issolvated as a hydroxonium (hydronium)ion, H3O+.

Ions can also be produced by ionizingradiation; i.e. by the impact of particles orphotons with sufficient energy to break upmolecules or detach electrons from atoms:A → A+ + e–. Negative ions can be formedby capture of electrons by atoms or mol-ecules: A + e– → A–.

ionization energy See ionization poten-tial.

ionization potential (IP; Symbol: I) Theenergy required to remove an electron

from an atom (or molecule or group) in thegas phase, i.e. the energy required for theprocess:

M → M+ + e–

It gives a measure of the ability of metals toform positive ions. The second ionizationpotential is the energy required to removetwo electrons and form a doubly chargedion:

M → M2+ + 2e–

Ionization potentials stated in this wayare positive; often they are given in elec-tronvolts. Ionization energy is the energyrequired to ionize one mole of the sub-stance, and is usually stated in kilojoulesper mole (kJ mol–1).

In chemistry, the terms ‘second’, ‘third’,etc., ionization potentials are usually usedfor the formation of doubly, triply, etc.,charged ions. However, in spectroscopyand physics, they are often used with a dif-ferent meaning. The second ionization po-tential is the energy to remove the secondleast strongly bound electron in forming asingly charge ion. For lithium (ls22s1) itwould refer to removal of a 1s electron toproduce an excited ion with the configura-tion 1s12s1.

ionizing radiation Radiation of suffi-ciently high energy to cause IONIZATION. Itmay be short-wavelength electromagneticradiation (ultraviolet, x-rays, or gammarays) or streams of particles.

ion pair A positive ion and a negativeion in close proximity in solution, held by the attractive force between theircharges.

IP See ionization potential.

IR See infrared.

iron A transition element occurring inmany ores, especially the oxides (hematiteand magnetite) and carbonate. Iron is pre-sent in a number of bioinorganic com-pounds, notably HEMOGLOBIN.

irreversible change See reversible change.

irreversible reaction A reaction in

which conversion to products is com-plete; i.e. there is little or no back re-action.

isoenzyme (isozyme) An ENZYME thatoccurs in different structural forms withina single species. The isomeric forms allhave the same molecular weight but differ-ing structural configurations and proper-ties. Large numbers of different enzymesare known to have isomeric forms; for ex-ample, lactate dehydrogenase has fiveforms. Variations in the isoenzyme consti-tution of individuals can be distinguishedby electrophoresis.

isocyanide See isonitrile.

isocyanide test (carbylamine reaction)A test for the primary amine group in or-ganic compounds. The sample is warmedwith trichloromethane in an alcoholic solu-tion of potassium hydroxide. If a primaryamine is present the resulting isocyanide(RNC) has a characteristic smell of badonions (and is very toxic):

CHCl3 + 3KOH + RNH2 → RNC +3KCl + 3H2O

isoelectronic Describing compoundsthat have the same number of electrons.For example, carbon monoxide (CO) andnitrogen (N2) are isoelectronic.

isoleucine See amino acid.

isomer See isomerism.

isomerism The existence of two or morechemical compounds with the same mo-lecular formulae but different structuralformulae or different spatial arrangementsof atoms. The different forms are known asisomers. For example, the compoundC4H10 may be butane (with a straight chainof carbon atoms) or 2-methyl propane(CH3CH(CH3)CH3, with a branchedchain).

Structural isomerism is the type of iso-merism in which the structural formulae ofthe compounds differ. There are two maintypes. In one the isomers are different typesof compound. An example is the com-

121

isomerism

isonitrile

122

pounds ethanol (C2H5OH) and methoxy-methane (CH3OCH3), both having the for-mula C2H6O but different functionalgroups. In the other type of structural iso-merism, the isomers differ because of theposition of a functional group in the mol-ecule. For example, the primary alcoholpropan-1-ol (CH3CH2CH2OH) and thesecondary alcohol propan-2-ol (CH3-CH(OH)CH3) are isomers; both have themolecular formula C3H7OH.

Stereoisomerism occurs when two com-pounds with the same molecular formulaeand the same groups differ only in the

arrangement of the groups in space. Thereare two types of stereoisomerism.

Cis-trans isomerism (or geometrical iso-merism) occurs when there is restricted ro-tation about a bond between two atoms.Groups attached to each atom may be onthe same side of the bond (the cis isomer)or opposite sides (the trans isomer). Seealso E–Z convention.

Optical isomerism occurs when thecompound has no plane of symmetry andcan exist in left- and right-handed formsthat are mirror images of each other. Suchmolecules have an asymmetric atom – i.e.

H2CH3C

H2

CCH3

C

CH3

CH3 H3

CH

butane(n-butane)

methylpropane(isobutane)

Isomer: isomers differing in carbon skeleton

O

H3CCH3

C

CH3 OH

dimethyl ether ethanol

H2

Isomer: isomers differing in the nature of the functional group

H2OHC

H2

CCH3

C

OH

CH3 H3

CH

propan-1-ol(n-propanol)

propan-2-ol(isopropanol)

Isomer: isomers differing in the position of a functional group

propan-1-yne propan-2-yne

CCH3C CH3CH2CCH

CH3

Isomer: isomers differing in the position of a multiple bond. See also illustration at alkene.

123

isotope

one attached to four different groups –called a chiral center. Molecules that aremirror images of each other are more prop-erly called enantiomers. Stereoisomers thatare not mirror images are called diastereo-isomers. ANOMERS are examples of dia-stereoisomers. See also optical activity.

isonitrile (isocyanide) An organic com-pound of the formula R–NC.

isoprene See methylbuta-1,3-diene.

isopropanol Propan-2-ol. See propanol.

isotactic polymer See polymerization.

isotherm A line on a chart or graph join-ing points of equal temperature. See alsoisothermal change.

isothermal change A process that takesplace at a constant temperature. Through-out an isothermal process, the system is in

thermal equilibrium with its surroundings.For example, a cylinder of gas in contactwith a constant-temperature box may becompressed slowly by a piston. The workdone appears as energy, which flows intothe reservoir to keep the gas at the sametemperature. Isothermal changes are con-trasted with adiabatic changes, in which noenergy enters or leaves the system, and thetemperature of the system changes. In prac-tice no process is perfectly isothermal andnone is perfectly adiabatic, although somecan approximate in behavior to one ofthese ideals.

isotones Two or more nuclides thathave the same neutron numbers but differ-ent proton numbers.

isotonic Describing solutions that havethe same osmotic pressure.

isotope One of two or more species ofthe same element differing in their mass

cis-dichloroethene

CC

Cl Cl

HH

trans-dichloroethene

CC

Cl H

ClH

Isomer: cis–trans isomerism in an alkene

E-methylethylketone oxime

NC

CH3 OH

C2H6

Z-methylethylketone oxime

NC

CH3

OHC2H6

Isomer: E–Z isomerism in an oxime

cis-diethyl epoxide trans-diethyl epoxide

CC

H H

C2H5 C2H5

O

C2H5

CC

H C2H5

H

O

Isomer: cis–trans isomerism in a ring compound

numbers because of differing numbers ofneutrons in their nuclei. The nuclei musthave the same number of protons (an el-ement is characterized by its proton num-ber). Isotopes of the same element havevery similar properties because they havethe same electron configuration, but differslightly in their physical properties. An un-stable isotope is termed a radioactive iso-tope or radioisotope

Isotopes of elements are useful in chem-istry for studies of the mechanisms of chem-ical reactions. A standard technique is tolabel one of the atoms in a molecule by usingan isotope of the element. It is then possibleto trace the way in which this atom behavesthroughout the course of the reaction. Forexample, in the esterification reaction:

ROH + R′COOH ˆ H2O + R′COORit is possible to find which bonds are bro-ken by using a labeled oxygen atom. If thereaction is performed using 18O in the al-cohol it is found that this nuclide appearsin the ester, showing that the C–OH bondof the acid is broken in the reaction. In la-beling, radioisotopes are detected by coun-

ters; stable isotopes can also be used, anddetected by a mass spectrum.

Isotopes are also used in kinetic studiesto investigate the mechanism of a particu-lar reaction. For example, if the bond be-tween two atoms X–Y is broken in therate-determining step, and Y is replaced bya heavier isotope of the element, Y*, thenthe reaction rate will be slightly lower withthe Y* present. This difference in rate,known as a kinetic isotope effect, is signif-icant only for reactions in which the rate-determining step involves breaking a bondto hydrogen (or deuterium) because thedeuterium atom has twice the mass of thehydrogen atom.

isotopic mass (isotopic weight) The massnumber of a given isotope of an element.

isotopic number The difference be-tween the number of neutrons in an atomand the number of protons.

isotopic weight See isotopic mass.

isozyme See isoenzyme.

isotopic mass

124

Jones oxidation The reaction in whicha secondary alcohol is oxidized to a ketoneusing sodium chromate(VII) (Na2Cr2O7) indilute sulfuric acid. The reaction involvesthe formation of an intermediate chromateester of the type R2HC–O–CrO3H.

joule Symbol: J The SI unit of energyand work, equal to the work done whenthe point of application of a force of onenewton moves one meter in the direction ofaction of the force. 1 J = 1 N m. The jouleis the unit of all forms of energy.

125

J

kairomone A chemical messenger emit-ted by an individual of a species and caus-ing a response in an individual of anotherspecies. This may be detrimental to theproducer of the kairomone, for examplemany parasites are attracted to their hostsby an excreted kairomone. See alsopheromone.

Karrer, Paul (1889–1971) Swiss or-ganic chemist. Karrer was famous for hiswork on vitamins and vegetable pigments.In 1930 he determined the structure ofcarotene. In 1931 he synthesized vitaminA, having worked out the structure. Karrerwas aware of the similarity between thetwo molecules. He subsequently deter-mined the structures of each and synthe-sized riboflavin (vitamin B2) in 1937 andtocopherol (vitamin E) in 1938. Karrershared the 1937 Nobel Prize for chemistrywith Norman HAWORTH for ‘the constitu-tion of carotenoids, flavins, and vitamins Aand B’. Karrer wrote an influential text-book entitled Textbook of Organic Chem-istry (1927).

katharometer See gas chromatography.

Kekulé, Friedrich August vonStradonitz (1829–96) German organicchemist. Kekulé was the founder of theconcept of structure in organic chemistrybut is probably best remembered for thering structure of benzene. In 1858, inde-pendently of Archibald Couper, Kekuléput forward the idea that carbon is aquadrivalent element which can combinewith other carbon atoms. In 1865 he pro-posed the idea that the structure of benzeneis a hexagonal ring of carbon atoms. In re-sponse to objections that benzene does not

behave like a substance with double bondsKekulé put forward the idea in 1872 thatthere is oscillation between two structuresthat are isomers. This view was justifiedabout 60 years later by Linus PAULING

using quantum mechanics.

kelvin Symbol: K The SI base unit ofthermodynamic temperature. It is definedas the fraction 1/273.16 of the thermody-namic temperature of the triple point ofwater. Zero kelvin (0 K) is absolute zero.One kelvin is the same as one degree on theCelsius scale of temperature. The unit isnamed for the British physicist Lord Kelvin(William Thomson; 1824–1907).

Kendrew, Sir John Cowdery (1917–97)British biochemist. Kendrew is renownedfor having determined the structure of theprotein molecule myoglobin. He did sousing x-ray crystallography. He was one ofthe first people to use electronic computersas an aid in analyzing the data produced byx-ray diffraction. He was able to determinethe structure of myoglobin by 1960. Heshared the 1962 Nobel Prize for chemistrywith Max PERUTZ, his colleague at the Lab-oratory for Molecular Biology, Cam-bridge.

keratin One of a group of fibrous insol-uble sulfur-containing proteins (scleropro-teins) found in ectodermal cells of animals,as in hair, horns, and nails. Leather is al-most pure keratin. There are two types: αkeratins and β keratins. The former have acoiled structure, whereas the latter have abeta pleated sheet structure.

kerosene See petroleum.

126

K

ketal See acetal.

keten See ketene.

ketene (keten) A member of a group oforganic compounds, general formulaR2C:CO, where R is hydrogen or an or-ganic radical. The simplest member is thecolorless gas ketene, CH2:CO; the otherketenes are colored because of the presenceof the double bonds. Ketenes are unstableand react with other unsaturated com-pounds to give cyclic compounds.

keto–enol tautomerism A type of tau-tomerism in which a compound containingthe –CH2–CO– group (the keto form)interconverts with one containing–CH=CH(OH)– (the enol) by hydrogenmigration.

keto form See keto–enol tautomerism.

ketohexose A ketose sugar with six car-bon atoms. See sugar.

ketone A type of organic compoundwith the general formula RCOR, havingtwo alkyl or aryl groups bound to a car-bonyl group. They are made by oxidizingsecondary alcohols (just as aldehydes aremade from primary alcohols). Simple ex-amples are propanone (acetone,CH3COCH3) and butanone (methyl ethylketone, CH3COC2H5).

The chemical reactions of ketones aresimilar in many ways to those of ALDEHY-DES. The carbonyl group is polarized, withpositive charge on the carbon and negativecharge on the oxygen. Thus nucleophilicaddition can occur at the carbonyl group.Ketones thus:

1. Undergo addition reactions with hydro-gen cyanide and hydrogensulfite (bisul-fite) ions.

2. Undergo condensation reactions withhydroxylamine, hydrazine, and their de-rivatives.

3. Are reduced to (secondary) alcohols.They are not, however, easily oxidized.Strong oxidizing agents give a mixture ofcarboxylic acids. They do not react withFehling’s solution or Tollen’s reagent, anddo not easily polymerize.

ketone body One of a group of organicsubstances formed in fat metabolism,mainly in the liver. Examples are aceto-acetic acid and acetone. Ketone bodies arethe major fuel source for resting skeletalmuscle. If the body has little or no carbo-hydrate as a respiratory substrate, ketosisoccurs, in which more ketone bodies areproduced than the body can use.

ketopentose A ketose SUGAR with fivecarbon atoms.

ketose A SUGAR containing a keto-(=CO) or potential keto- group.

kieselguhr A siliceous deposit formedby diatoms, used as an absorbent, filter,and filler.

kilo- Symbol: k A prefix denoting 103.For example, 1 kilometer (km) = 103

meters (m).

kilogram (kilogramme; symbol: kg) TheSI base unit of mass, equal to the mass ofthe international prototype of the kilo-gram, which is a piece of platinum–iridiumkept at Sèvres in France.

127

kilogram

C C

H

OH

C C

H

O

H

ˆenol form keto form

Keto–enol tautomerization

kilogramme An alternative spelling ofkilogram.

kilowatt-hour Symbol: kWh A unit ofenergy, usually electrical, equal to the en-ergy transferred by one kilowatt of powerin one hour. It has a value of 3.6 × 106

joules.

kinetic energy See energy.

kinetic isotope effect See isotope.

kinetics A branch of physical chemistryconcerned with the study of rates of chem-ical reactions and the effect of physicalconditions that influence the rate of reac-tion, e.g. temperature, light, concentration,etc. The measurement of these rates underdifferent conditions gives information onthe mechanism of the reaction, i.e. on thesequence of processes by which reactantsare converted into products.

Kipp’s apparatus An apparatus for theproduction of a gas from the reaction of aliquid on a solid. It consists of three globes,the upper globe being connected via a widetube to the lower globe. The upper globe isthe liquid reservoir. The middle globe con-tains the solid and also has a tap at whichthe gas may be drawn off. When the gas isdrawn off the liquid rises from the lowerglobe to enter the middle globe and reactswith the solid, thereby releasing more gas.When turned off the gas released forces theliquid back down into the lower globe andup into the reservoir, thus stopping the re-action.

Kjeldahl’s method A method used forthe determination of nitrogen in organiccompounds. The nitrogenous substance isconverted to ammonium sulfate by boilingwith concentrated sulfuric acid (often witha catalyst such as CuSO4) in a specially de-signed long-necked Kjeldahl flask. Themixture is then made alkaline and the am-monia distilled off into standard acid formeasurement by titration. It is named forthe Danish chemist Johan Kjeldahl(1849–1900).

Klug, (Sir) Aaron (1926– ) Lithuan-ian-born British molecular biologist distin-guished for his determination of thestructure of transfer RNA and for his workon three-dimensional structure of com-plexes of proteins and nucleic acids. Hewas awarded the Nobel Prize for chemistryin 1982 for his development of crystallo-graphic electron microscopy and his workon the structures of biologically importantnucleic acid–protein complexes.

Kolbe, Adolph Wilhelm Hermann(1818–84) German organic chemist. Kolbemade many important contributions to thedevelopment of organic chemistry. Theseincluded the synthesis of ethanoic acidfrom completely inorganic materials, theformation and hydrolysis of nitriles, theelectrolysis of solutions of fatty acid saltsand the synthesis of salicylic acid from phe-nol and carbon dioxide. In his later yearshis influence had a negative effect on thedevelopment of chemistry because he op-posed concepts such as structure.

Kolbe electrolysis The electrolysis ofsodium salts of carboxylic acids to preparealkanes. The alkane is produced at theanode after discharge of the carboxylateanion and decomposition of the radical:

RCOO– → RCOO• + e–

RCOO• → R• + CO2R• + RCOO• → R – R + CO2

and2R• → R – R

As the reaction is a coupling reaction,only alkanes with an even number of car-bon atoms in the chain can be prepared inthis way. It is named for the Germanchemist Adolph Kolbe.

Kornberg, Arthur (1918– ) Ameri-can biochemist. Kornberg conducted re-search on enzymes at the start of his career.In this period he clarified the chemical re-actions leading to certain enzymes. In 1956he discovered an enzyme, which he calledDNA polymerase, that catalyses theprocess of forming polynucleotides fromnucleoside triphosphates. This enabled himto synthesize short DNA molecules, start-ing from a DNA template and triphosphate

kilogramme

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bases. He shared the 1959 Nobel Prize formedicine for this work with Severo Ochoa,who found the enzyme that catalyses theformation of RNA.

Kossel, Karl Martin Leonhard Al-brecht (1853–1927) German chemistnoted for his discovery of adenine, cyto-sine, thymine, and uracil as breakdownproducts of nucleic acids. He also dis-covered histone and histidine. He wasawarded the Nobel Prize for physiology ormedicine in 1910 in recognition of hiswork on cell chemistry.

Krebs, (Sir) Hans Adolf (1900–81) German-born British biochemist renownedfor his work on metabolism and, in partic-ular, for the discovery of the cyclic meta-bolic pathways named after him. He wasawarded the Nobel Prize for physiology ormedicine in 1953 for his discovery of theTCA cycle. The prize was shared with F. A.Lipmann.

Krebs cycle (tricarboxylic acid cycle; TCAcycle; citric acid cycle) A complex andalmost universal cycle of reactions inwhich the acetyl group of acetyl CoA is ox-idized to carbon dioxide and water, withthe production of large amounts of energy.It is the final common pathway for the ox-idation of carbohydrates, fatty acids, andamino acids. It requires oxygen, and in eu-karyotes occurs in the mitochondrial ma-trix.

2-carbon acetate reacts with 4-carbonoxaloacetate to form 6-carbon citrate,which is then decarboxylated to reconsti-tute oxaloacetate. Some ATP is producedby direct coupling with cycle reactions, butmost production is coupled to the electron-transport chain via the generation of re-duced coenzymes, NADH and FADH2. Seeelectron-transport chain.

Kroto, Sir Harold Walter (1939– )British chemist. In the mid-1980s HaroldKroto heard that the American chemist

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H2O

In the Krebs cycle acetyl (frompyruvate) is broken down to CO2and H. The H is held in NADH (fromreduction of NAD+). The NADHthen is reoxidized in an electron-transport chain, with production ofATP.

glycolysis

pyruvate

acetyl CoA

CoA

citrate

isocitrate

CO2 + 2H

α-oxoglutarate

CO2 + 2HsuccinylCoA

ADPATP

succinate

2H

fumarate

malate

oxaloacetate2H

Krebs cycle

Richard Smalley was using laser bombard-ment as a new technique to produce clus-ters of atoms. When Kroto visited Smalleyhe persuaded him to bombard graphitewith a laser beam. When Smalley did so hefound not only the chains that Kroto hadbeen expecting but a molecule with 60 car-bon atoms. Kroto and Smalley correctlypostulated that this molecule is a poly-hedron in which the faces are pentagons or

hexagons. Kroto called this molecule buck-minsterfullerene after its resemblance tothe designs of the architect BuckminsterFuller. The name of the molecule is usuallyabbreviated to fullerene. Such molecularstructures are frequently called ‘buckyballs’. Kroto and Smalley shared the 1996Nobel Prize for chemistry with RobertCurl for their parts in the discovery of C60molecules.

Kroto, Sir Harold Walter

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label A stable or radioactive ISOTOPE

used to investigate a chemical reaction. La-beling is a common method of investigat-ing chemical reaction mechanisms. Aclassic example is the hydrolysis of an esterto give a carboxylic acid and an alcohol, asin:

H2O + CH3COOCH3 → CH3COOH +CH3OH

It is possible to investigate the mechanismby using an ester enriched with the isotope18O. If the 18O is on the oxygen attached tothe carbonyl, it is found that it ends up inthe alcohol rather than the acid:

H2O + CH3CO18OH → CH3COOH +CH3

18OHThis, and similar experiments using labeledwater and labeling of the carbonyl oxygen,help to establish the mechanism.

Certain radioisotopes such as tritium(3H) and 14C have been used as labels butit is now more usual to use deuterium (2H),13C, and 18O, which can be detected bymass spectroscopy.

lactam A type of organic compoundcontaining the –NH.CO– group as part ofa ring in the molecule. Lactams can be re-garded as formed from a straight-chaincompound that has an amino group(–NH2) at one end of the molecule and acarboxylic acid group (–COOH) at theother; i.e. from an amino acid. The reac-tion of the amine group with the carboxylicacid group, with elimination of water,leads to the cyclic lactam, which is thus aninternal (or inner) amide. Lactams canexist in an alternative tautomeric form inwhich the hydrogen atom has migratedfrom the nitrogen onto the O of the car-bonyl. This, the lactim form, contains thegroup –N=C(OH)–.

lactate A salt or ester of lactic acid.

lactic acid (2-hydroxypropanoic acid) Acolorless liquid carboxylic acid:

CH3CH(OH)COOH See optical activity.

lactim See lactam.

lactone A type of organic compoundcontaining the group –O.CO– as part of aring in the molecule. A lactone can be re-garded as formed from a compound withan alcohol (–OH) group on one end of thechain and a carboxylic acid (–COOH)group on the other. The lactone then re-sults from reaction of the –OH group withthe –COOH group; i.e. it is an internalester.

lactose (milk sugar; C12H22O11) ASUGAR found in milk. It is a disaccharidecomposed of glucose and galactose units.

Ladenburg benzene An incorrectstructure for BENZENE in which the six car-bon atoms are at the corners of a triangu-lar prism. It is named for Albert Ladenburg(1842–1911). The actual compound,known as prismane, was synthesized in1973.

lake A pigment formed by absorbing anorganic dyestuff on an inorganic oxide, hy-droxide, or salt.

lamellar compound A compound witha crystal structure composed of thin platesor layers. Silicates form many compoundswith distinct layers. Typical examples aretalc (Mg3(OH)2Si4O10) and pyrophyllite(Al2(OH)2Si4O10).

L

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lanolin A yellowish viscous substanceobtained from wool fat. It contains choles-terol and terpene compounds, and is usedin cosmetics, in ointments, and in treatingleather.

Lapworth, Arthur (1872–1941) Britishorganic chemist. In the early part of his ca-reer Lapworth investigated the structure ofcamphor and related compounds. How-ever, his most significant contribution tochemistry was his work, starting in 1920,on the mechanisms of organic molecules interms of the electrons in the molecules. Forexample, he was one of the first people toemphasize that ionization can occur in or-ganic molecules and that parts of thesemolecules can have electrical charges, ei-ther on a permanent basis or when the re-action is occurring. Similar views ondescribing organic reactions in terms ofelectrons were developed by Sir Christo-pher INGOLD and Sir Robert ROBINSON.

laser An acronym for Light Amplifica-tion by Stimulated Emission of Radiation.A laser device produces high-intensity,monochromatic, coherent beams of light.In the laser process the molecules of a sam-ple (such as ruby doped with Cr3+ ions) arepromoted to an excited state. As the sam-ple is in a cavity between two reflectivesurfaces, when a molecule emits sponta-neously, the photon so generated ricochetsbackward and forward. In this way othermolecules are stimulated to emit photonsof the same energy. If one of the reflectivesurfaces is partially transmitting this radia-tion can be tapped.

latent heat The heat evolved or ab-sorbed when a substance changes its phys-ical state, e.g. the latent heat of fusion is theheat absorbed when a substance changesfrom a solid to a liquid.

latex A liquid found in some floweringplants contained in special cells or vesselscalled laticifers (or laticiferous vessels). It isa complex variable substance that maycontain terpenes (e.g. rubber), resins, tan-nins, waxes, alkaloids, sugar, starch, en-zymes, crystals, etc. It is often milky in

appearance but may be colorless, orange,or brown. Its function is obscure, but maybe involved in wound healing as well as arepository for excretory substances. Com-mercial rubber comes from the latex of therubber plants Ficus elastica and Heveabrasiliensis.Opium comes from alkaloidsfound in the latex of the opium poppy.

lattice A regular three-dimensionalarrangement of points. A lattice is used todescribe the positions of the particles(atoms, ions, or molecules) in a crystallinesolid. The lattice structure can be exam-ined by x-ray diffraction techniques.

lauric acid See dodecanoic acid.

law of conservation of energy Seeconservation of energy; law of.

law of conservation of mass See con-servation of mass; law of.

law of constant composition See con-stant proportions; law of.

law of constant proportions See con-stant proportions; law of.

law of definite proportions See defi-nite proportions; law of.

law of equivalent proportions Seeequivalent proportions, law of.

law of mass action See mass action;law of.

law of reciprocal proportions Seeequivalent proportions, law of.

laws of chemical combination Seechemical combination; laws of.

LDL See low-density lipoprotein.

leaching The washing out of a solublematerial from an insoluble solid using asolvent. This is often carried out in batchtanks or by dispersing the crushed solid ina liquid.

lead-free fuel Vehicle fuel that containsnone of the anti-knock agent LEAD TETRA-ETHYL.

lead tetraethyl (tetraethyl lead;Pb(CH2CH3)4) A poisonous liquid that isinsoluble in water but soluble in organicsolvents. It is manufactured by the reactionof an alloy of sodium and lead with 1-chloroethane. The product is obtainedby steam distillation. Lead tetraethyl wasformerly extensively used as an additive ininternal-combustion engine fuel to increaseits octane number and thus prevent pre-ignition (knocking).

leaving group The group leaving a mol-ecule in a substitution or elimination reac-tion. The nature of the leaving group is animportant factor in the progress of the re-action.

Le Chatelier’s principle If a system isat equilibrium and a change is made in theconditions, the equilibrium adjusts so as tooppose the change. The principle can beapplied to the effect of temperature andpressure on chemical reactions. A good ex-ample is the Haber process for synthesis ofammonia:

N2 + 3H2 ˆ 2NH3The ‘forward’ reaction

N2 + 3H2 → 2NH3is exothermic. Thus, reducing the tempera-ture displaces the equilibrium towards pro-duction of NH3 (as this tends to increasetemperature). Increasing the pressure alsofavors formation of NH3, because thisleads to a reduction in the total number ofmolecules (and hence pressure). The princi-ple is named for the French chemist HenriLouis Le Chatalier (1850–1936), who dis-covered it in 1884.

leucine See amino acid.

levo-form See optical activity.

levorotatory See optical activity.

Lewis acid See acid.

Lewis base See acid.

L-form See optical activity.

Liebig, Justus von (1803–73) Germanorganic chemist. Lieberg was one of themost influential chemists of the 19th cen-tury. The early part of his career was de-voted to classical organic chemistry. After1840 he mostly devoted himself to bio-chemistry and the application of chemistryto agriculture. In 1826 he made his firstsignificant discovery when, together withhis long-standing colleague Friedrich WÖH-LER, he discovered that silver fulminate andsilver cyanate have the same chemical for-mula. In this way, Liebig and Wöhler dis-covered isomerism. Liebig and Wöhlerdiscovered the benzoyl radical in 1832.When Liebig moved into biochemistry histendency towards dogmatism became evenmore pronounced than it had been previ-ously. As a result, he became involved inmany acrimonious controversies. He was aprolific writer of books and papers.

Liebig condenser A simple type of lab-oratory condenser. It consists of a straightglass tube, in which the vapor is con-densed, with a surrounding glass jacketthrough which cooling water flows. It isnamed for the German chemist Justus vonLiebig.

ligand A molecule or ion that forms acoordinate bond to a metal atom or ion ina complex.

light (visible radiation) A form of elec-tromagnetic radiation able to be detectedby the human eye. Its wavelength range isbetween about 400 nm (far red) and about700 nm (far violet). The boundaries are notprecise because individuals vary in theirability to detect extreme wavelengths; thisability also declines with age.

lignin One of the main structural ma-terials of vascular plants. With cellulose it is one of the main constituents of wood.Lignified tissues include sclerenchyma andxylem. Lignin is deposited during sec-ondary thickening of cell walls. The degreeof lignification varies but values of 25–30% lignin and 50% cellulose are average.

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lignite

134

It is a complex variable polymer, derivedfrom sugars via aromatic alcohols. Phenyl-propane (C6–C3) units are linked in variousways by oxidation reactions during poly-merization.

lignite (brown coal) The poorest gradeof coal, containing up to 45% carbon andwith a high moisture content.

ligroin A mixture of hydrocarbons ob-tained from petroleum, used as a generalsolvent. It has a boiling range of 80 to120°C. See petroleum.

limiting step See rate-determining step.

line spectrum A SPECTRUM composed ofa number of discrete lines corresponding tosingle wavelengths of emitted or absorbedradiation. Line spectra are produced byatoms or simple (monatomic) ions in gases.Each line corresponds to a change in elec-tron orbit, with emission or absorption ofradiation.

linoleic acid (C17H31COOH) An unsat-urated carboxylic acid that occurs in LIN-SEED OIL and other plant oils. It containstwo double bonds.

linolenic acid (C17H29COOH) A liquidunsaturated carboxylic acid that occurs inLINSEED OIL and other plant oils. It containsthree double bonds.

linseed oil An oil extracted from theseeds of flax (linseed). It hardens on expo-sure to air (it is a drying oil) because it con-tains linoleic acid, and is used in enamels,paints, putty, and varnishes. See linoleicacid.

lipid A collective term used to describe agroup of substances in cells characterizedby their solubility in organic solvents suchas ether and benzene, and their absence ofsolubility in water. The group is rather het-erogeneous in terms of both function andstructure. They encompass the followingbroad bands of biological roles: (1) basicstructural units of cellular membranes andcytologically distinct subcellular bodies

such as chloroplasts and mitochondria; (2)compartmentalizing units for metaboli-cally active proteins localized in mem-branes; (3) a store of chemical energy andcarbon skeletons; and (4) primary trans-port systems of nonpolar material throughbiological fluids. There are also the morephysiologically specific lipid hormones,e.g. the steroid hormones and lipid vitamins.

The simple lipids include neutral lipidsor glycerides, which are esters of glyceroland fatty acids, and the waxes, which areesters of long-chain monohydric alcoholsand fatty acids.

Compound lipids have one of the fattyacid parts replaced, such that complete hy-drolysis gives only two fatty acids; thephospholipids, which are particularly im-portant examples.

lipoic acid A sulfur-containing fattyacid found in a wide variety of natural ma-terials. It is an essential as a coenzyme forcertain dehydrogenase enzymes, notablypyruvate dehydrogenase, which catalyzesthe dehydrogenation of pyruvic acid toform acetyl-CoA. Lipoic acid is classifiedwith the water-soluble B vitamins and hasnot yet been demonstrated to be requiredin the diet of higher animals.

lipopolysaccharide A conjugated poly-saccharide in which the noncarbohydratepart is a lipid. Lipopolysaccharides are aconstituent of the cell walls of certain bac-teria.

lipoprotein Any conjugated proteinformed by the combination of a proteinwith a lipid. In the blood of humans andother mammals, cholesterol, triglycerides,and phospholipids associate with variousplasma proteins to form lipoproteins.These are particles with diameters in the re-gion of 7.5–70 nm, and are placed in sev-eral classes. The largest lipoproteins in thisrange are the very low-density lipoproteins(VLDLs), which are formed in the liverand contain up to about 20% choles-terol. Low-density lipoproteins (LDLs) areformed in plasma from VLDLs and containover 50% cholesterol. LDLs transport cho-lesterol from the liver to peripheral tissues,

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and an excess of LDLs in the blood is afactor in the development of fatty arterialdeposits and cardiovascular disease. High-density lipoproteins (HDLs), with about20% cholesterol, are the smallest of theplasma lipoproteins, and apparently func-tion in transporting cholesterol from tis-sues to the liver.

liquefied natural gas (LNG) Liquidmethane, obtained from natural gas andused as a fuel. See methane; natural gas.

liquefied petroleum gas (LPG) A mix-ture of liquefied hydrocarbon gases(mainly propane) extracted from petro-leum, used as a fuel for internal com-bustion engines and for heating. Seepetroleum; propane.

liquid The state of matter in which theparticles of a substance are loosely boundby intermolecular forces. The weakness ofthese forces permits movement of the par-ticles and consequently liquids can changetheir shape within a fixed volume. The liq-uid state lacks the order of the solid state.Thus, amorphous materials, such as glass,in which the particles are disordered andcan move relative to each other, can beclassed as liquids.

liquid–liquid extraction See solventextraction.

liter Symbol: l A unit of volume now de-fined as 10–3 meter3; i.e. 1000 cm3. Themilliliter (ml) is thus the same as the cubiccentimeter (cm3). However, the name is notrecommended for precise measurementsbecause the liter was formerly defined asthe volume of one kilogram of pure waterat 4°C and standard pressure. On this def-inition, one liter is the same as 1000.028cubic centimeters.

lithium aluminum hydride See lithiumtetrahydridoaluminate(III).

lithium tetrahydridoaluminate(III)(lithium aluminum hydride; LiAlH4) Awhite solid produced by action of lithiumhydride on aluminum chloride, the hydride

being in excess. Lithium tetrahydridoalu-minate reacts violently with water. It is apowerful reducing agent, reducing ketonesand carboxylic acids to their correspond-ing alcohols. In inorganic chemistry it isused in the preparation of hydrides.

litmus A natural pigment that changescolor when in contact with acids and alka-lis; above a pH of 8.3 it is blue and belowa pH of 4.5 it is red. Thus it gives a roughindication of the acidity or basicity of a so-lution; because of its rather broad range ofcolor change it is not used for precise work.Litmus is used both in solution and as lit-mus paper.

lixiviation The process of separatingsoluble components from a mixture bywashing them out with water.

LNG See liquefied natural gas.

localized bond A bond in which theelectrons contributing to the bond remainbetween the two atoms concerned, i.e. thebonding orbital is localized. The majorityof bonds are of this type. Compare delo-calized bond.

lone pair A pair of valence electronshaving opposite spin that are located to-gether on one atom, i.e. are not shared asin a covalent bond. Lone pairs occupy sim-ilar positions in space to bond pairs and ac-count for the shapes of molecules. Amolecule with a lone pair can donate thepair of electrons to an electron acceptor,such as H+ or a metal ion, to form coordi-nate bonds.

long period See period.

Lonsdale, Dame Kathleen (1903–71)British x-ray crystallographer. Dame Kath-leen Lonsdale was an early pioneer of x-raycrystallography. In 1929 she showed thehexagonal ring nature of the hexamethyl-benzene molecule. In 1931 she investigatedthe structure of hexachlorobenzene. Thiswas the first investigation in which Fourieranalysis had been used for an organic mol-ecule. She subsequently investigated many

problems including thermal motion incrystals, the magnetic susceptibility of crys-tals and the composition of bladder stones.She reviewed her research in her bookCrystals and X-Rays (1948). She was alsothe editor of the first three volumes of theInternational Tables for X-Ray Crystallog-raphy (1952, 1959, 1962).

low-density lipoprotein (LDL) Aspherical particle, typically about 20–25nm in diameter, that is found in bloodplasma and transports cholesterol to tissuecells. It is bounded by a single layer ofphospholipid and free cholesterol, whichencloses a core of cholesterol esterified to along-chain fatty acid. Embedded in the sur-face layer is a single large protein, calledapo-B, which assists in binding of the LDLto cell-surface receptors. LDLs are takeninto cells by receptor-mediated endocyto-sis. The cholesterol is incorporated into cellmembranes or stored as lipid droplets.High concentrations of LDLs in the bloodhave been associated with an increased riskof atherosclerosis (‘hardening of the arter-ies’).

lowering of vapor pressure A colliga-tive property of solutions in which thevapor pressure of a solvent is lowered as asolute is introduced. When both solventand solute are volatile the effect of increas-ing the solute concentration is to lower thepartial vapor pressure of each component.When the solute is a solid of negligiblevapor pressure the lowering of the vaporpressure of the solution is directly propor-tional to the number of species introducedrather than to their nature and the propor-tionality constant is regarded as a generalsolvent property. Thus the introduction ofthe same number of moles of any solutecauses the same lowering of vapor pres-sure, if dissociation does not occur. If thesolute dissociates into two species on dis-solution the effect is doubled. The kineticmodel for the lowering of vapor pressuretreats the solute molecules as occupyingpart of the surface of the liquid phase andthereby restricting the escape of solventmolecules. The effect can be used in themeasurement of relative molecular masses,

particularly for large molecules, such aspolymers. See also Raoult’s law.

Lowry–Brønsted theory See acid.

LPG See liquefied petroleum gas.

LSD See lysergic acid diethylamide.

lumen Symbol: lm The SI unit of lumi-nous flux, equal to the luminous flux emit-ted by a point source of one candela in asolid angle of one steradian. 1 lm = 1 cd sr.

luminescence The emission of radiationfrom a substance in which the particleshave absorbed energy and gone into ex-cited states. They then return to lower en-ergy states with the emission ofelectromagnetic radiation. If the lumines-cence persists after the source of excitationis removed it is called phosphorescence: ifnot, it is called fluorescence.

lutein The commonest of the xantho-phyll pigments. It is found in green leavesand certain algae, e.g. the Rhodophyceae.See photosynthetic pigments.

lux Symbol: lx The SI unit of illumina-tion, equal to the illumination produced bya luminous flux of one lumen falling on asurface of one square meter. 1 lx = 1 lmm–2.

lyophilic Solvent attracting. When thesolvent is water, the word hydrophilic isoften used. The terms are applied to: 1. Ions or groups on a molecule. In aque-

ous or other polar solutions ions orpolar groups are lyophilic. For example,the –COO– group on a soap is thelyophilic (hydrophilic) part of the mol-ecule.

2. The disperse phase in colloids. Inlyophilic colloids the dispersed particleshave an affinity for the solvent, and thecolloids are generally stable. Comparelyophobic.

lyophobic Solvent repelling. When thesolvent is water, the word hydrophobic isused. The terms are applied to:

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1. Ions or groups on a molecule. In aque-ous or other polar solvents, the lyopho-bic group will be nonpolar. Forexample, the hydrocarbon group on asoap molecule is the lyophobic (hy-drophobic) part.

2. The disperse phase in colloids. Inlyophobic colloids the dispersed parti-cles are not solvated and the colloid iseasily solvated. Gold and sulfur sols areexamples. Compare lyophilic.

lysergic acid diethylamide (LSD) Asynthetic organic compound that has phys-iological effects similar to those producedby alkaloids in certain fungi. Even smallquantities, if ingested, produce hallucina-tions and extreme mental disturbances.The initials LSD come from the Germanform of the chemical’s name, Lysergic-Saure-Diathylamide.

lysine See amino acid.

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138

macromolecular crystal A crystalcomposed of atoms joined together by co-valent bonds that form giant three-dimen-sional or two-dimensional networks.Diamond is an example of a macromolecu-lar crystal.

macromolecule A large molecule; e.g. anatural or synthetic polymer.

magic acid See superacid.

magnetic quantum number See atom.

magnetism The study of the nature andcause of magnetic force fields, and how dif-ferent substances are affected by them.Magnetic fields are produced by movingcharge – on a large scale (as with a currentin a coil, forming an electromagnet), or onthe small scale of the moving charges in theatoms. It is generally assumed that theEarth’s magnetism and that of other plan-ets, stars, and galaxies have the same cause.

Substances may be classified on thebasis of how samples interact with fields.Different types of magnetic behavior resultfrom the type of atom. Diamagnetism,which is common to all substances, is dueto the orbital motion of electrons. Para-magnetism is due to electron spin, and aproperty of materials containing unpairedelectrons. It is particularly important intransition-metal chemistry, in which thecomplexes often contain unpaired elec-trons. Magnetic measurements can give in-formation about the bonding in thesecomplexes. Ferromagnetism, the strongesteffect, also involves electron spin and thealignment of magnetic moments in do-mains.

maleic acid See butenedioic acid.

malic acid (2-hydroxybutanedioic acid;HCOOCH2CH(OH)COOH) A color-less crystalline CARBOXYLIC ACID found inunripe fruits. It tastes of apples and is usedin food flavorings.

malonic acid See propanedioic acid.

maltose (C12H22O11) A SUGAR found ingerminating cereal seeds. It is a disaccha-ride composed of two glucose units. Mal-tose is an important intermediate in theenzyme hydrolysis of starch. It is furtherhydrolyzed to glucose.

mannitol (HOCH2(CHOH)4CH2OH) Asoluble hexahydric alcohol that occurs inmany plants and fungi. It is used in medi-cines and as a sweetener (particularly infoods for diabetics). It is an isomer of sor-bitol.

mannose (C6H12O6) A simple SUGAR

found in many polysaccharides. It is analdohexose, isomeric with glucose.

manometer A device for measuringpressure. A simple type is a U-shaped glasstube containing mercury or other liquid.The pressure difference between the armsof the tube is indicated by the difference inheights of the liquid.

Markovnikoff’s rule A rule that pre-dicts the quantities of the products formedwhen an acid (HA) adds to the doublebond in an alkene. If the alkene is not sym-metrical two products may result; for in-stance (CH3)2C:CH2 can yield either(CH3)2HCCH2A or (CH3)2ACCH3. The

M

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mass spectrometer

rule states that the major product will bethe one in which the hydrogen atom at-taches itself to the carbon atom with thelarger number of hydrogen atoms. In theexample above, therefore, the major prod-uct is (CH3)2ACCH3.

The Markovnikoff rule is explainable ifthe mechanism is ionic. The first step is ad-dition of H+ to one side of the double bond,forming a carbonium ion. The more stableform of carbonium ion will be the form inwhich the positive charge appears on thecarbon atom with the largest number ofalkyl groups – thus the hydrogen tends toattach itself to the other carbon. The posi-tive charge is partially stabilized by theelectron-releasing (inductive) effect of thealkyl groups.

Additions of this type do not alwaysfollow the Markovnikoff rule. Under cer-tain conditions the reaction may involvethe free radicals H• and A•, in which casethe opposite (anti-Markovnikoff) effectoccurs.

marsh gas Methane produced inmarshes by decomposing vegetation.

Martin, Archer John Porter (1910– )British biochemist. Martin is best knownfor the development of paper chromatog-raphy with Richard SYNGE, starting in1941. Martin and Synge won the 1952Nobel Prize for chemistry for this work. Inthe method they used a drop of the mixturewhich is being analyzed is placed at one ofthe ends of a strip of filter paper. This isthen soaked in a solvent which carries thecomponents of the mixture at differentrates as it moves along the strip. The posi-tions of the components are revealed byspraying the strip with a reagent whichcauses the components to change color.Martin and Synge were able to identifyamino acids in a protein this way, usingninhydrin to record the positions of theamino acid on the strip.

mass Symbol: m A measure of the quan-tity of matter in an object. Mass is deter-mined in two ways: the inertial mass of abody determines its tendency to resistchange in motion; the gravitational mass

determines its gravitational attraction forother masses. The SI unit of mass is thekilogram.

mass action, law of The principle that,at constant temperature, the rate of achemical reaction is directly proportionalto the active mass of the reactants, the ac-tive mass being taken as the concentrationfor a reaction in solution or the partialpressure for a gas reaction. For the reactionA + B → products, the law of mass actionstates that:

rate = k[A][B]where [A] represents the concentration ofA, [B] the concentration of B, and k is aconstant dependent on the particular reac-tion. The interpretation of active mass asconcentration or partial pressure is onlyvalid if there is no interaction or interfer-ence between the reacting molecules. Ingeneral the concentration has to be multi-plied by an ACTIVITY COEFFICIENT in orderto obtain the actual active mass. See activ-ity coefficient.

mass number See nucleon number.

mass spectrometer An instrument forproducing ions and analyzing them accord-ing to their charge/mass ratio. The earliestexperiments by the British physicist J. J.Thomson (1856–1940) used a stream ofpositive ions from a discharge tube, whichwere deflected by parallel electric and mag-netic fields at right angles to the beam.Each type of ion formed a parabolic traceon a photographic plate (a mass spectro-graph). The design was improved by theBritish chemist Francis William Aston(1877–1945). In modern instruments, theions are usually produced by ionizing a gaswith electrons. The positive ions are accel-erated out of this ion source into a high-vacuum region. Here, the stream of ions isdeflected and focused by a combination ofelectric and magnetic fields, which can bevaried so that different types of ion fall ona detector. In this way, the ions can be an-alyzed according to their mass, giving amass spectrum of the material. Mass spec-trometers are used for accurate measure-ments of relative atomic mass and for

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analysis of isotope abundance. They canalso be used to identify compounds and an-alyze mixtures. An organic compoundbombarded with electrons forms a numberof fragment ions. (Ethane (C2H6), for in-stance, might form CH+, C2H5

+, CH2+,

etc.) The relative proportions of differenttypes of ions may be used to find the struc-ture of new compounds. The characteristicspectrum can also identify compounds bycomparison with standard spectra.

mass spectrum See mass spectrometer.

matrix A continuous solid phase inwhich particles of a different solid phaseare embedded.

mechanism A step-by-step descriptionof the events taking place in a chemical re-action. It is a theoretical framework ac-counting for the fate of bonding electronsand illustrates which bonds are broken andwhich are formed. For example, in thechlorination of methane to give chloro-methane:step 1

Cl:Cl → 2Cl•step 2

Cl• + CH4 → HCl + CH3•step 3

CH3• + Cl:Cl → CH3Cl + Cl•See also nucleophilic substitution.

mega- Symbol: M A prefix denoting106. For example, 1 megahertz (MHz) =106 hertz (Hz).

melamine (triaminotriazine; C3N3(NH2)3)A white solid organic compound whosemolecules consist of a six-membered hete-rocyclic ring of alternate carbon and nitro-gen atoms with three amino groupsattached to the carbons. Condensationpolymerization with methanal or otheraldehydes produces melamine resins,which are important thermosetting plas-tics.

melting (fusion) The process by which asolid is converted into a liquid by heat orpressure.

melting point The temperature atwhich a solid is in equilibrium with its liq-uid phase at standard pressure and abovewhich the solid melts. This temperature isalways the same for a particular solid. Ion-ically bonded solids generally have muchhigher melting points than those in whichthe forces are covalent or intermolecular.

membrane A thin pliable sheet of tissueor other material acting as a boundary. Themembrane may be either natural (as incells, skin, etc.) or synthetic modificationsof natural materials (cellulose derivativesor rubbers). In many physicochemicalstudies membranes are supported onporous materials, such as porcelain, to pro-vide mechanical strength. Membranes aregenerally permeable to some degree.

Membranes can be prepared to permitthe passage of other molecules and micro-molecular material. Because of permeabil-ity effects, concentration differences at amembrane give rise to a whole range ofmembrane-equilibrium studies, of whichosmosis, dialysis, and ultrafiltration are ex-amples. See also semipermeable mem-brane.

menaquinone See vitamin K.

Mendius reaction The reduction of thecyanide group to a primary amine groupusing sodium in alcohol:

RCN + 2H2 → RCH2NH2It is a method of increasing the chain lengthof compounds in ascending a homologousseries of compounds.

mer See polymer.

mercaptan See thiol.

meso-form See optical activity.

mesomerism See resonance.

meta- 1. Designating a benzene com-pound with substituents in the 1,3 posi-tions. The position on a benzene ring thatis two carbon atoms away from a sub-stituent is the meta position. This was usedin the systematic naming of benzene deriv-

atives. For example, meta-dinitrobenzene(or m-dinitrobenzene) is 1,3-dinitroben-zene.2. Certain inorganic acids regarded asformed from an anhydride and water arenamed meta acids to distinguish them fromthe more hydrated ortho acids. For exam-ple, H2SiO3 (SiO2 + H2O) is metasilicicacid; H4SiO4 (SiO2 + 2H2O) is orthosilicicacid.

See also ortho-; para-.

metabolic pathway See metabolism.

metabolism The biochemical reactionsthat take place in cells. The molecules tak-ing part in these reactions, either as a reac-tant or a product, are termed metabolites.Some are synthesized within the organismitself, whereas others have to be taken in asfood. Metabolic reactions characteristi-cally occur in small steps that togethermake up a metabolic pathway. They in-volve the breaking down of molecules toprovide energy (catabolism) and the build-ing up of more complex molecules andstructures from simpler molecules (an-abolism).

metabolite See metabolism.

metaldehyde See ethanal.

metallic bond A bond formed betweenatoms of a metallic element in its zero oxi-dation state and in an array of similaratoms. The outer electrons of each atomare regarded as contributing to an ‘electrongas’, which occupies the whole crystal ofthe metal. It is the attraction of the positiveatomic cores for the negative electron gasthat provides the strength of the metallicbond.

metallocene A SANDWICH COMPOUND inwhich a metal atom or ion is coordinatedto two cyclopentadienyl ions. Ferrocene(Fe(C5H5)2) is the commonest example.

metalloid Any of a class of chemical el-ements that are intermediate in propertiesbetween metals and nonmetals. Examplesare germanium, arsenic, and tellurium.

metastable species An excited state ofan atom, ion, or molecule, that has a rela-tively long lifetime before reverting to theground state. Metastable species are inter-mediates in some chemical reactions.

metastable state A condition of a sys-tem or body in which it appears to be instable equilibrium but, if disturbed, cansettle into a lower energy state. For exam-ple, supercooled water is liquid below 0°C(at standard pressure). When a small crys-tal of ice or dust (for example) is intro-duced, rapid freezing occurs.

meter Symbol: m The SI base unit oflength, defined as the distance traveled bylight in vacuum in 1/(2.99 792 458 × 108)second. This definition was adopted in1983 to replace the 1967 definition of alength equal to 1 650 763.73 wavelengthsin vacuum corresponding to the transitionbetween the levels 2p10 and 5d5 of the 86Kratom.

methanal (formaldehyde; HCOH) Acolorless gaseous aldehyde. It is manufac-tured by the oxidation of methanol (500°Cand a silver catalyst):

2CH3OH + O2 → 2HCOH + 2H2OThe compound is used in the manufactureof UREA–FORMALDEHYDE RESINS. A solutionof methanal (40%) in water is called for-malin. It is extensively used as a preserva-tive for biological specimens.

If an aqueous solution of methanal isevaporated a polymer – polymethanal – isformed:

–O–CH2–O–CH2–O–CH2–This was formerly called paraformalde-hyde. If methanal is distilled from acidic

141

methanal

CH2

O

CH2

O

H2C

O

Methanal: methanal trimer

solutions a cyclic methanal trimer(C3O3H6) is produced.

methanal trimer See methanal.

methane (CH4) A gaseous alkane. Nat-ural gas is about 99% methane and thisprovides an important starting material forthe organic-chemicals industry. Methanecan be chlorinated directly to produce themore reactive chloromethanes, or it can be‘reformed’ by partial oxidation or usingsteam to give mixtures of carbon oxidesand hydrogen. Methane is the first memberof the homologous series of alkanes andthe simplest organic compound.

methanoate (formate) A salt or ester ofmethanoic acid.

methanoic acid (formic acid;HCOOH) A liquid carboxylic acid madeby the action of sulfuric acid on sodiummethanoate (NaOOCH). It is a strong re-ducing agent. Methanoic acid is the sub-stance responsible for the stings of ants andnettles.

methanol (methyl alcohol; wood alcohol;CH3OH) A colorless liquid alcohol,which is used as a solvent and in the man-ufacture of methanal (formaldehyde) forthe plastics and drugs industries. Methanolwas originally produced from the distilla-tion of wood. Now it is manufactured bythe catalytic oxidation of methane fromnatural gas.

methionine See amino acids.

methoxy group The group CH3O–.

methyl acetate See methyl ethanoate.

methyl alcohol See methanol.

methylamine (CH3NH2) A colorlessflammable gas that smells like ammonia. Itis the simplest primary amine, used formaking herbicides and other organic chem-icals.

methylaniline See toluidine.

methylated spirits Ethanol to which isadded methanol (about 9.5%), pyridine(about 0.5%), and a blue dye. The ethanolis denatured in this way so that it can besold without excise duty for use as a fueland solvent.

methylation A reaction in which amethyl group (CH3–) is introduced into acompound.

methylbenzene (toluene; C6H5CH3) Acolorless liquid hydrocarbon, similar tobenzene both in structure and properties.As methylbenzene is much less toxic thanbenzene it is more widely used, especiallyas a solvent. Large quantities are requiredfor the manufacture of TNT (trinitro-toluene).

Methylbenzene can be obtained by thefractional distillation of coal tar or synthe-sized from methylcyclohexane (a con-stituent of some crude oils):

C6H11CH3 → C6H5CH3 + 3H2A catalyst of aluminum and molybdenumoxides is employed at high temperaturesand pressures.

methyl bromide See bromomethane.

2-methylbuta-1,3-diene (isoprene; CH2-CH:CH2) A colorless unsaturated liquidhydrocarbon, which occurs in terpenes andnatural rubber. Is is used to make syntheticrubber.

methyl chloride See chloromethane.

methyl cyanide (acetonitrile; CH3CN)A pleasant-smelling poisonous colorlessliquid organic NITRILE. It is a polar solvent,widely used for dissolving both inorganicand organic compounds.

methylene See carbene.

methylene group The group :CH2.

methyl ethanoate (methyl acetate;CH3COOCH3) A colorless liquid esterwith a fragrant odor, used as a solvent.

methanal trimer

142

methyl ethyl ketone See butanone.

methyl group The group CH3–.

methyl 2-hydroxybenzoate See methylsalicylate.

methyl iodide See iodomethane.

methyl methacrylate (CH2:CCH3-COOCH3) The methyl ester ofmethacrylic acid, used to make acrylicpolymers such as Plexiglas (polymethyl-methacrylate).

methyl orange An acid–base indicatorthat is red in solutions below a pH of 3 andyellow above a pH of 4.4. As the transitionrange is clearly on the acid side, methyl or-ange is suitable for the titration of an acidwith a moderately weak base, such assodium carbonate.

methylphenol (cresol; HOC6H4CH3) Acompound with both methyl and hydroxylgroups substituted onto the benzene ring.There are three isomers, with the methylgroup in the 2–, 3–, and 4– positions, re-spectively. A mixture of the isomers can beobtained from coal tar. It is used as a ger-micide (known as Lysol).

methyl red An acid–base indicator thatis red in solutions below a pH of 4.2 andyellow above a pH of 6.3. It is often usedfor the same types of titration as methyl or-ange but the transition range of methyl redis nearer neutral (pH7) than that of methylorange. The two molecules are structurallysimilar.

methyl salicylate (oil of wintergreen;methyl 2-hydroxybenzoate; C8H8O3) Themethyl ester of SALICYLIC ACID, which oc-curs in certain plants. It is absorbedthrough the skin and used medicinally torelieve rheumatic symptoms. It is also usedin perfumes and as a flavoring agent in var-ious foods.

metric system A system of units basedon the meter and the kilogram and usingmultiples and submultiples of 10. SI units,

c.g.s. units, and m.k.s. units are all scien-tific metric systems of units.

mho See siemens.

micelle An aggregate of molecules in aCOLLOID.

Michaelis constant Symbol: Km For anenzyme-catalyzed reaction obeying MICH-AELIS KINETICS under steady-state condi-tions (when the reaction intermediateshave reached a steady concentration):

[ES] = [E][S]/Kmwhere [ES] is the concentration of en-zyme–substrate complex, [E] is the concen-tration of enzyme, [S] is the concentrationof substrate, and Km is the Michaelis con-stant.

Km gives the concentration of substrateat which half the active sites are filled andalso gives an indication of the strength ofthe enzyme–substrate complex if the rate of product formation is much slower thanthe rate of dissociation of the enzyme–substrate complex into enzyme and sub-strate. If this is the case then a high Km in-dicates weak binding of the complex and alow Km indicates strong binding.

Michaelis kinetics (Michaelis–Mentenkinetics) A simple and useful model ofthe kinetics of enzyme-catalyzed reactions.It assumes the formation of a specific en-zyme–substrate complex. Many enzymesobey Michaelis kinetics and a plot of reac-tion velocity (V) against substrate concen-tration [S] gives a characteristic curveshowing that the rate increases quickly atfirst and then levels off to a maximumvalue. When substrate concentration islow, the rate of reaction is almost propor-tional to substrate concentration. Whensubstrate concentration is high, the rate isat a maximum, Vmax, and independent ofsubstrate concentration. The Michaelisconstant Km is the concentration of sub-strate at half the maximum rate and can bedetermined experimentally by measuringreaction rate at varying substrate concen-trations. Different types of inhibition canalso be distinguished in this way. Allostericenzymes do not obey Michaelis kinetics.

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Michaelis kinetics

micro- Symbol: µ A prefix denoting10–6. For example, 1 micrometer (µm) =10–6 meter (m).

micron Symbol: µ A unit of length equalto 10–6 meter.

microwaves A form of electromagneticradiation, ranging in wavelength fromabout 1 mm (where it merges with in-frared) to about 120 mm (bordering onradio waves). Microwaves are produced byvarious electronic devices; they are oftencarried over short distances in tubes of rec-tangular section called waveguides. Spectrain the microwave region can give informa-tion on the rotational energy levels of cer-tain molecules. See also electromagneticradiation.

migration 1. The movement of anatom, group, or double bond from one po-sition to another in a molecule.2. The movement of ions in an electricfield.

milk sugar See lactose.

Miller, Stanley Lloyd (1930– )American chemist. Stanley Miller is fa-mous for an experiment, the results ofwhich he published in 1953, concerningthe possible origin of life. Miller, who wasthen a graduate student of Harold Urey,mixed water vapour, ammonia, methaneand hydrogen in a flask so as to simulatethe early atmosphere of the Earth and thenput a powerful electric discharge through itto simulate lightning. He discovered thatafter a short time organic molecules of bio-logical interest, including some simpleamino acids, were formed. Since aminoacids are the ‘building blocks’ for proteinsit has been suggested that this experimentmay give a clue as to the origin of life onEarth.

milli- Symbol: m A prefix denoting 10–3.For example, 1 millimeter (mm) = 10–3

meter (m).

millimeter of mercury See mmHg.

mineral acid An inorganic acid, espe-cially an acid used commercially in largequantities. Examples are hydrochloric, ni-tric, and sulfuric acids.

mirror image A shape that is identicalto another except that its structure is re-versed as if viewed in a mirror. If an objectis not symmetrical it cannot be superim-posed on its mirror image. For example,the left hand is the mirror image of theright hand. See chirality.

miscible Denoting combinations of sub-stances that, when mixed, give rise to onlyone phase; i.e. substances that dissolve ineach other.

mixed indicator A mixture of two ormore indicators so as to decrease the pHrange or heighten the color change, etc.

mixture Two or more substances form-ing a system in which there is no chemicalbonding between the two. In homogeneousmixtures (e.g. solutions or mixtures ofgases) the molecules of the substances aremixed, and there is only one phase. In het-erogeneous mixtures (e.g. gunpowder orcertain alloys) different phases can be dis-tinguished. Mixtures differ from chemicalcompounds in that:1. The chemical properties of the compo-

nents of a mixture are the same as thoseof the pure substances.

2. The mixture can be separated by physi-cal means (e.g. distillation or crystalliza-tion) or mechanically.

3. The proportions of the components canvary. Some mixtures (e.g. certain solu-tions) can only vary in proportions be-tween definite limits.

m.k.s. system A system of units basedon the meter, the kilogram, and the second.It formed the basis for SI units.

mmHg (millimeter of mercury) A for-mer unit of pressure defined as the pressurethat will support a column of mercury onemillimeter high under specified conditions.It is equal to 133.322 4 Pa, and is almostidentical to the torr.

micro-

144

moiety A part of a molecule; for exam-ple, the sugar moiety in a nucleoside.

molal concentration See concentra-tion.

molar 1. Denoting a physical quantitydivided by the amount of substance. In al-most all cases the amount of substance willbe in moles. For example, volume (V) di-vided by the number of moles (n) is molarvolume Vm = v/n.2. Denoting a solution that contains onemole of solute per cubic decimeter of sol-vent.

molarity A measure of the concentra-tion of solutions based upon the number ofmolecules or ions present, rather than onthe mass of solute, in any particular vol-ume of solution. The molarity (M) is thenumber of moles of solute in one cubicdecimeter (litre). Thus a 0.5M solution ofhydrochloric acid contains 0.5 × (1 +35.5)g HCl per dm3 of solution.

mole Symbol: mol The SI base unit ofamount of substance, defined as theamount of substance that contains as manyelementary entities as there are atoms in0.012 kilogram of 12C. The elementary en-tities may be atoms, molecules, ions, elec-trons, photons, etc., and they must bespecified. The amount of substance isproportional to the number of entities, the constant of proportionality being theAvogadro number. One mole contains6.022 045 × 1023 entities. One mole of an element with relative atomic mass A has a mass of A grams (this mass was for-merly called one gram-atom of the el-ement).

molecular crystal A crystal in whichmolecules, as opposed to atoms, occupylattice points. Because the forces holdingthe molecules together are weak, molecularcrystals have low melting points. When themolecules are small, the crystal structureapproximates to a close-packed arrange-ment.

molecular formula See formula.

molecularity The total number of react-ing molecules in the individual steps of achemical reaction. Thus, a unimolecularstep has molecularity 1, a bimolecular step2, etc. Molecularity is always an integer,whereas the order of a reaction need notnecessarily be so. The molecularity of a re-action gives no information about themechanism by which it takes place.

molecular orbital See orbital.

molecular sieve A substance throughwhich molecules of a limited range of sizescan pass, enabling volatile mixtures to beseparated. Zeolites and other metal alu-minum silicates can be manufactured withpores of constant dimensions in their mo-lecular structure. When a sample is passedthrough a column packed with granules ofthis material, some of the molecules enterthese pores and become trapped. The re-mainder of the mixture passes through theinterstices in the column. The trapped mol-ecules can be recovered by heating. Mo-lecular-sieve chromatography is widelyused in chemistry and biochemistry labora-tories. A modified form of molecular sieveis used in gel filtration. The sieve is a con-tinuous gel made from a polysaccharide. Inthis case, molecules larger than the largestpore size are totally excluded from the col-umn.

molecular-sieve chromatography Seemolecular sieve.

molecular spectrum The absorption oremission SPECTRUM that is characteristic ofa molecule. Molecular spectra are usuallyband spectra.

molecular weight See relative molecu-lar mass.

molecule A particle formed by the com-bination of atoms in a whole-number ratio.A molecule of an element (combiningatoms are the same, e.g. O2) or of a com-pound (different combining atoms, e.g.HCl) retains the properties of that elementor compound. Thus, any quantity of acompound is a collection of many identical

145

molecule

molecules. Molecular sizes are characteris-tically 10–10 to 10–9 m.

Many molecules of natural products areso large that they are regarded as giantmolecules (macromolecules); they maycontain thousands of atoms and have com-plex structural formulae that require veryadvanced techniques to identify. See alsoformula; relative molecular mass.

mole fraction The number of moles of agiven component in a mixture divided bythe total number of moles present of all thecomponents. The mole fraction of compo-nent A is

nA/(nA + nB + nC + …)where nA is the number of moles of A, etc.

Molisch’s test See alpha-naphthol test.

monobasic acid An acid that has onlyone acidic hydrogen. Ethanoic acid,CH3COOH, is an example of a monobasicacid. See also dibasic acid.

monochlorobenzene See chloroben-zene.

monohydric alcohol see alcohol.

monomer The molecule, group, (orcompound) from which a dimer, trimer, orPOLYMER is formed.

monosaccharide A SUGAR that cannotbe hydrolyzed to simpler carbohydrates ofsmaller carbon content. Glucose and fruc-tose are examples.

monosodium glutamate (MSG) Awhite crystalline solid compound, madefrom soya-bean protein. It is a sodium saltof glutamic acid (see amino acid) used as aflavor enhancer, particularly in Chinesefood. Monosodium glutamate can cause anallergic reaction in certain people.

monoterpene See terpene.

monovalent (univalent) Having a va-lence of one.

mordant An inorganic compound usedto fix dye in cloth. The mordant (e.g. alu-minum hydroxide or chromium salts) isprecipitated in the fibers of the cloth, andthe dye then absorbs in the particles.

morphine An alkaloid present in opium(from the poppy Papaver somniferum). Itis used for the relief of severe pain. Thedrug heroin is a derivative.

mother liquor The solution remainingafter the formation of crystals.

MSG See monosodium glutamate.

mucopolysaccharide See glycosamino-glycan.

mucoprotein See proteoglycan.

multicenter bond A two-electron bondformed by the overlap of orbitals frommore than two atoms (usually 3). Thebridging in diborane is believed to takeplace by overlap of an sp3 hybrid orbitalfrom each boron atom with the 1s orbitalon the hydrogen atom. This multicenterbond is called a two-electron three-centerbond. See also electron-deficient com-pound.

multidentate ligand A LIGAND that pos-sesses at least two sites at which it can co-ordinate.

multiple bond A bond between twoatoms involving more than one pair of elec-trons; i.e. a double bond or a triple bond.This additional bonding arises from over-lap of atomic orbitals that are perpendicu-lar to the internuclear axis and gives rise toan increase in electron density above andbelow this axis. Such bonds are called pibonds. (The bond along the axis is a sigmabond.)

multiple proportions, law of Pro-posed by Dalton in 1804, the principle thatwhen two elements A and B combine toform more than one compound, theweights of B that combine with a fixedweight of A are in small whole-number ra-

mole fraction

146

tios. For example, in dinitrogen oxide,N2O, nitrogen monoxide, NO, and dini-trogen tetroxide, N2O4, the amounts of ni-trogen combined with a fixed weight ofoxygen are in the ratio 4:2:1.

multiple-range indicator See universalindicator.

mustard gas ((CH2ClCH2)2S) A poiso-nous vesicant gas used as a war gas. The

systematic name is 2,2′-dichlorodiethylsulfide.

mutarotation A change in the opticalrotation of a solution with time, caused bythe conversion of one optical isomer intoanother. See optical activity.

myoglobin A globular protein formedof a heme group and a single polypeptidechain. It occurs in muscle tissue, where itacts as an oxygen store.

147

myoglobin

148

NAD (nicotinamide adenine dinucleo-tide) A derivative of nicotinic acid thatacts as a coenzyme in electron-transfer re-actions (e.g. the electron-transport chain).Its role is to carry hydrogen atoms; the re-duced form is written NADH.

NADP Nicotinamide adenine dinu-cleotide phosphate; a coenzyme similar inits action to NAD. The reduced form iswritten NADPH, which acts as an electrondonor in many synthetic reactions.

nano- Symbol: n A prefix denoting 10–9.For example, 1 nanometer (nm) = 10–9

meter (m).

nanotube A tubular structure with a di-ameter of a few nanometers (1nm = 10–9

m). Examples of nanotubes are the carbon

structures known as ‘bucky tubes’, whichhave a structure similar to that of buck-minsterfullerene. Interest has been shownin nanotubes as possible microscopicprobes in experiments, as semiconductormaterials, and as a component of compos-ite materials. Nanotubes can also be pro-duced by joining amino acids to givetubular polypeptide structures. See alsobuckminsterfullerene.

naphtha (solvent naphtha) A mixture ofhydrocarbons obtained from coal and pe-troleum. It has a boiling range of70–160°C and is used as a solvent and as araw material for making various other or-ganic chemicals.

naphthalene (C10H8) A white crys-talline solid with a distinctive smell of

N

+

-

-

--

OPO

O

CH2

O

O CH2

O OP

N

N N

N

NH2

O

O

O

POOH

O

O

OH OH

N

CONH2

NADP

+

OPHO

O

OCH2

N

NN

N

O

O CH2 NO

OHPO

CONH2

OH OH

OH OH

NH2

NAD

mothballs. Naphthalene is found in boththe middle- and heavy-oil fractions ofcrude oil and is obtained by fractional crys-tallization. It is used in the manufacture ofbenzene-1,2-dicarboxylic anhydride (ph-thalic anhydride) and thence in the produc-tion of plastics and dyes.

The structure of naphthalene is ‘ben-zene-like’, having two six-membered ringsfused together. The reactions are charac-teristic of AROMATIC COMPOUNDS.

nascent hydrogen A particularly reac-tive form of hydrogen, which is believed toexist briefly between its generation (e.g. bythe action of dilute acid on magnesium)and its appearance as bubbles of normalhydrogen gas. It is thought that part of thefree energy of the production reaction re-mains with the hydrogen molecules for ashort time. Nascent hydrogen may be usedto produce the hydrides of phosphorus, ar-senic, and antimony, which are not readilyformed from ordinary hydrogen.

Natta, Giulio (1903–79) Italianchemist. The early part of Natta’s careerwas devoted to x-ray crystallography andcatalysts. In 1938 he started research onsynthetic rubbers. In 1953 he developedmethods for using catalysts to producepolymers which had been initiated by KarlZIEGLER to form polypropene. In 1954 hefound that polymers produced in this wayare very specific and regular in their stere-ochemistry. This meant that these poly-mers have desirable properties such as highmelting points and high strength. After1954 he continued to investigate this typeof polymerization. Natta and Zieglershared the 1963 Nobel Prize for chemistryfor their work on polymers.

Natta process A method for the manu-facture of isotactic polypropene usingZiegler catalysts. It is named for the Italianchemist Giulio Natta. See Ziegler process.

natural gas Gas obtained from under-ground deposits and often associated withsources of petroleum. It contains a highproportion of methane (about 85%) and

other volatile hydrocarbons (ethane,propane, and butane).

neighboring-group participation Aneffect in an organic reaction in whichgroups close to the point at which reactionoccurs affect the rate of reaction or stereo-chemistry of the products in some way.

neoprene A type of synthetic RUBBER

made by polymerization of 2-chlorobuta-1,2,-diene (H2C:CHCCl:CH2). It is moreresistant to oil, solvents, and temperaturethan natural rubbers.

neutralization The stoichiometric reac-tion of an acid and a base in volumetricanalysis. The neutralization point or endpoint is detected with indicators.

neutron diffraction A method of struc-ture determination used for solids, liquids,and gases that makes use of the quantummechanical wave nature of neutrons. Ther-mal neutrons with average kinetic energiesof about 0.025eV have a wavelength ofabout 0.1 nanometer, making them suit-able for investigating the structure of mat-ter at the atomic level.

Since a neutron has a nonzero magneticmoment, it interacts both with nuclearmagnetic moments and with the magneticmoments of unpaired electrons. This prop-erty is particularly useful in identifying thepositions of hydrogen atoms in a molecule.These positions are difficult to establishusing x-rays because x-rays interact withelectrons, and hence are scattered weaklyby hydrogen atoms. Protons scatter neu-trons strongly, and the positions of protonscan readily be determined by neutron dif-fraction.

neutron number Symbol: N The num-ber of neutrons in the nucleus of an atom;i.e. the nucleon number (A) minus the pro-ton number (Z).

Newman projection A type of projec-tion FORMULA in which the molecule isviewed along a bond between two of itsatoms. See illustration at conformation.

149

Newman projection

newton Symbol: N The SI unit of force,equal to the force needed to accelerate onekilogram by one meter second–2. 1 N = 1 kgm s–2. It is named for Sir Isaac Newton(1642–1727).

niacin See nicotinic acid.

nicotinamide adenine dinucleotideSee NAD.

nicotinic acid (niacin) One of the water-soluble B-group of vitamins. Its deficiencyin man causes pellagra. Nicotinic acidfunctions as a constituent of two coen-zymes, NAD and NADP, which operate ashydrogen and electron transfer agents andplay a vital role in metabolism. See alsovitamin B complex.

ninhydrin A colorless organic com-pound that gives a blue coloration withamino acids. It is used as a test for aminoacids, in particular to show the positions ofspots of amino acids in paper chromatog-raphy.

nitrate A salt or ester of nitric acid.

nitration A reaction introducing thenitro (–NO2) group into an organic com-pound. Nitration of aromatic compoundsis usually carried out using a mixture ofconcentrated nitric and sulfuric acids, al-though the precise conditions differ fromcompound to compound. The attackingspecies is NO2

+ (the nitryl ion), and the re-action is an example of electrophilic substi-tution.

nitric acid (HNO3) A colorless fumingcorrosive liquid that is a strong acid. Nitricacid can be made in a laboratory by the dis-tillation of a mixture of an alkali metal ni-trate and concentrated sulfuric acid.Commercially it is prepared by the cat-alytic oxidation of ammonia and is sup-plied as concentrated nitric acid, whichcontains 68% of the acid and is often col-ored yellow by dissolved oxides of nitro-gen.

Nitric acid is a strong oxidizing agent.Most metals are converted to their nitrates

with the evolution of oxides of nitrogen(the composition of the mixture of the ox-ides depends on the temperature and on theconcentration of the nitric acid used).Some nonmetals (e.g. sulfur and phospho-rus) react to produce oxyacids. Organicsubstances (e.g. sawdust and ethanol) reactviolently, but the more stable aromaticcompounds, such as benzene and toluene,can be converted to NITRO COMPOUNDS incontrollable reactions.

nitrile (cyanide) A type of organic com-pound containing the –CN group. Nitrilesare colorless liquids with pleasant smells.They can be prepared by refluxing an or-ganic halogen compound with an alcoholicsolution of potassium cyanide:

R+Cl + KCN → RCN + KClAlternatively it is possible to dehydrate anamide using a dehydrating agent such asphosphorus(V) oxide:

RCONH2 – H2O → RCNNitriles can be hydrolyzed to give theamide. Another reaction is hydrogenationto give amines:

RCN + 2H2 → RCH2NH2

nitrile rubber A copolymer of butadi-ene and propenonitrile (acrylonitrile;CH2=CHCN). It is a useful type of rubberbecause of its resistance to oil and solvents.

nitrobenzene (C6H5NO2) A yellow or-ganic oil obtained by refluxing benzenewith a mixture of concentrated nitric andsulfuric acids. The reaction is a typical elec-trophilic substitution on the benzene ringby the nitryl ion (NO2

+).

nitrocellulose See cellulose trinitrate.

nitro compound A type of organiccompound containing the nitro (–NO2)group attached to an aromatic ring. Nitrocompounds can be prepared by nitrationusing a mixture of concentrated nitric andsulfuric acids. They can be reduced to aro-matic amines:

RNO2 + 3H2 → RNH2 + 2H2OThey can also undergo further substitutionon the benzene ring. The nitro group di-rects substituents into the 3 position.

newton

150

nitrogen The first element of group 15(formerly group VA) of the periodic table;a very electronegative element existing inthe uncombined state as gaseous diatomicN2 molecules. The nitrogen atom has theelectronic configuration [He]2s22p3. It istypically nonmetallic and its bonding isprimarily by polarized covalent bonds.With electropositive elements the nitrideion N3– may be formed. It is present inmany organic compounds includingamines, amides, nitriles, and nitro com-pounds.

Nitrogen has two isotopes; 14N, thecommon isotope, and 15N (natural abun-dance 0.366%), which is used as a label inmass spectrometric studies.

Symbol: N; m.p. –209.86°C; b.p.–195.8°C; d. 1.2506 kg m–3 (0°C); p.n. 7;r.a.m. 14.

nitrogenous base A basic compound inwhich a nitrogen atom can accept a proton.The term is used especially for the cyclicring compounds adenine, guanine, cyto-sine, thymine, and uracil, which occur innucleic acids. See also quaternary ammo-nium compound.

nitroglycerine (glyceryl trinitrate) Ahighly explosive substance used in dyna-mite. It is obtained by treating glycerol(1,2,3-trihydroxypropane) with a mixtureof concentrated nitric and sulfuric acids. Itis not a nitro compound, but a nitrate ester

CH2(NO3)CH(NO3)CH2(NO3)

nitro group The group –NO2; the func-tional group of nitro compounds.

nitronium ion See nitryl ion.

nitrophenols (C6H4(OH)NO2) Organiccompounds formed directly or indirectlyby the nitration of phenol. Three isomericforms are possible. The 2 and 4 isomers areproduced by the direct nitration of phenoland can be separated by steam distillation,the 2 isomer being steam volatile. The 3isomer is produced from nitrobenzene byformation of 1,3-dinitrobenzene, conver-sion to 3-nitrophenylamine, and thence bydiazotization to 3-nitrophenol.

nitryl ion (nitronium ion) The ion NO2+,

occurring in NITRATION reactions.

NMR See nuclear magnetic resonance.

nonbenzenoid aromatic See aromaticcompound.

nonessential amino acid See aminoacid.

nonlocalized bond See delocalizedbond.

nonmetal Any of a class of chemical el-ements. Non-metals lie in the top right-hand region of the periodic table. They areelectronegative elements with a tendencyto form covalent compounds or negativeions. They have acidic oxides and hydrox-ides.

nonpolar compound A compoundthat has molecules with no permanent di-pole moment. Examples of nonpolar com-pounds are hydrogen, tetrachloromethane,and carbon dioxide.

nonpolar solvent See solvent.

noradrenaline See norepinephrine.

norepinephrine (noradrenaline) A cat-echolamine, secreted as a hormone by theadrenal medulla, that regulates heart mus-cle, smooth muscle, and glands. It causesnarrowing of arterioles and hence raisesblood pressure. It is also secreted by nerveendings of the sympathetic nervous systemin which it acts as a neurotransmitter. Inthe brain, levels of norepinephrine are re-lated to mental function; lowered levelslead to mental depression.

normality The number of gram equiva-lents per cubic decimeter of a given solu-tion.

normal solution A solution that con-tains one gram equivalent weight per literof solution. Values are designated by thesymbol N, e.g. 0.2N, N/10, etc. Becausethere is not a clear definition of equivalent

151

normal solution

weight suitable for all reactions, a solutionmay have one value of normality for onereaction and another value in a different re-action. Because of this it is now more usualto use the molar solution notation.

NTP See STP.

nuclear magnetic resonance (NMR) Amethod of investigating nuclear spin. In anexternal magnetic field the nucleus canhave certain quantized energy states, corre-sponding to certain orientations of the spinmagnetic moment. Hydrogen nuclei, forinstance, can have two energy states, andtransitions between the two occur by ab-sorption of radiofrequency radiation. Inchemistry, this is the basis of a spectro-scopic technique for investigating thestructure of molecules. In the basic tech-nique, radiofrequency radiation is fed to asample and the magnetic field is changedslowly. Absorption of the radiation is de-tected when the difference between the nu-clear levels corresponds to absorption of aquantum of radiation. This difference de-pends slightly on the electrons around thenucleus – i.e. the position of the atom in themolecule. The difference in frequency ofabsorption caused by the distribution ofelectrons is known as a chemical shift.Thus a different absorption frequency isseen for each type of hydrogen atom. Inethanol, for example, there are three fre-quencies, corresponding to hydrogenatoms on the CH3, the CH2, and the OH.The intensity of absorption also dependson the number of hydrogen atoms (3:2:1).NMR spectroscopy is a powerful methodof finding the structures of organic com-pounds. It is most often used to detect hy-drogen atoms but certain other nuclei canbe investigated (e.g. 13C).

nucleic acids Organic acids whose mol-ecules consist of chains of alternating sugarand phosphate units, with nitrogenousbases attached to the sugar units. Theyoccur in the cells of all organisms. In DNAthe sugar is deoxyribose; in RNA it is ri-bose. See DNA; RNA.

nucleon number (mass number) Sym-bol: A The number of nucleons (protonsplus neutrons) in an atomic nucleus.

nucleophile An electron-rich ion ormolecule that takes part in an organic re-action. The nucleophile can be a negativeion (Br–, CN–) or a molecule with a lonepair of electrons (NH3, H2O). The nucle-ophile attacks positively charged parts ofmolecules, which usually arise from thepresence of an electronegative atom else-where in the molecule. Compare elec-trophile.

nucleophilic addition A class of reac-tion involving the addition of a small mol-ecule to the double bond in an unsaturatedorganic compound. The initial part of thereaction is attack by a nucleophile and theunsaturated bond must contain an elec-tronegative atom, which creates an elec-tron-deficient area in the molecule.Nucleophilic addition is a characteristic re-action of aldehydes and ketones where po-larization of the C=O carbonyl causes apositive charge on the carbon. This is thesite at which the nucleophile attacks. Addi-tion is often followed by the subsequentelimination of a different small molecule,particularly water. See condensation reac-tion.

nucleophilic substitution A reactioninvolving the substitution of an atom orgroup of atoms in an organic compoundwith a nucleophile as the attacking sub-stituent. Since nucleophiles are electron-rich species, nucleophilic substitutionoccurs in compounds in which a stronglyelectronegative atom or group leads to adipolar bond. The electron-deficient centercan then be attacked by the electron-richnucleophile causing the electronegativeatom or group to be displaced. In generalterms:

R–Le + Nu– → R–Nu + Le–

where Nu– represents the incoming nucle-ophile and Le– represents the LEAVING

GROUP.There are two possible mechanisms for

nucleophilic substitution. In the SN1 (sub-stitution, nucleophilic, monomolecular) re-

NTP

152

action the molecule first forms a carbo-nium ion; for example:

RCH2Cl → RCH2+ + Cl–

The nucleophile then attaches itself to thiscarbonium ion:

RCH2+ + OH– → RCH2OH

In the SN2 (substitution, nucleophilic,bimolecular) reaction the nucleophile ap-proaches as the other group leaves, form-ing a transition state in which the carbonhas five attached groups.

The preferred mechanism depends onseveral factors:1. The stability of the intermediate in the

SN1 mechanism.2. Steric factors affecting the formation of

the transition state in the SN2 mecha-nism.

3. The solvent in which the reaction oc-curs: polar solvents will stabilize polar

intermediates and so favor the SN1mechanism.There is a difference in the two mecha-

nisms in that, for an optically active reac-tant, the SN1 mechanism gives a racemicmixture of products, whereas an SN2mechanism gives an optically active prod-uct (see illustration).

See also substitution reaction.

nucleoside A molecule consisting of apurine or pyrimidine base linked to asugar, either ribose or deoxyribose. ADENO-SINE, CYTIDINE, GUANOSINE, THYMIDINE, andURIDINE are common nucleosides.

nucleotide The compound formed bycondensation of a nitrogenous base (apurine, pyrimidine, or pyridine) with asugar (ribose or deoxyribose) and phos-

153

nucleotide

C Cl

X

Y

Z Y Z

C OH

X

Y

Z

X

Y

Z

C

X

+ Cl–+

HO C

OH– OH–

Nucleophilic substitution: the SN1 reaction

C Cl

X

Y

Z Y Z

X

Y

Z

X

HO C

OH–

+ Cl–

... Cl...HO C–

Nucleophilic substitution: the SN2 reaction

phoric acid. The coenzymes NAD andFAD are dinucleotides (consisting of twolinked nucleotides) while the nucleic acidsare polynucleotides (consisting of chains ofmany linked nucleotides).

nucleus The compact positively chargedcenter of an atom made up of one or morenucleons (protons and neutrons) aroundwhich is a cloud of electrons. The densityof nuclei is about 1015 kg m–3. The numberof protons in the nucleus defines the el-ement, being its proton number (or atomicnumber). The nucleon number, or atomicmass number, is the sum of the protons andneutrons. The simplest nucleus is that of ahydrogen atom, 1H, being simply one pro-ton (mass 1.67 × 10–27 kg). The most mas-sive naturally occurring nucleus is 238U of92 protons and 146 protons (mass 4 ×10–25 kg, radius 9.54 × 10–15 m). Only cer-tain combinations of protons and neutronsform stable nuclei. Others undergo sponta-neous decay.

A nucleus is depicted by a symbol indi-cating nucleon number (mass number),proton number (atomic number), and el-ement name. For example, 1

213Na repre-

sents a nucleus of sodium having 11protons and mass 23, hence there are (23 –11) = 12 neutrons.

nuclide A nuclear species with a givennumber of protons and neutrons; for ex-ample, 23Na, 24Na, and 24Mg are all differ-ent nuclides. Thus:

12

13Na has 11 protons and 12 neutrons

12

14Na has 11 protons and 13 neutrons

12

24Mg has 12 protons and 12 neutrons

The term is applied to the nucleus andoften also to the atom.

nylon A type of synthetic polymerlinked by amide groups –NH.CO–. Nylonpolymers can be made by copolymeriza-tion of a molecule containing two aminegroups with one containing two carboxylicacid groups.

nucleus

154

N N(CH2)6

O

HO

(CH2)4C C

OH

O

HN (CH2)6 NH C

O

C

O

(CH2)4 NH (CH2)6

1,6-diaminohexane hexanedioic acid

H

H H

H

Nylon: formation of nylon by a condensation reaction

OAA See oxaloacetic acid.

occlusion 1. The process in which smallamounts of one substance are trapped inthe crystals of another; for example, pock-ets of liquid occluded during crystallizationfrom a solution.2. Absorption of a gas by a solid; for ex-ample, the occlusion of hydrogen by palla-dium.

octadecanoic acid (stearic acid;CH3(CH2)16COOH) A solid carboxylicacid present in fats and oils as the glyc-eride.

octadecenoic acid (oleic acid) A natu-rally occurring unsaturated carboxylic acidpresent (as glycerides) in fats and oils:

CH3(CH2)7CH:CH(CH2)7COOHThe naturally occurring form is cis-9-octadecenoic acid.

octane (C8H18) A liquid alkane ob-tained from the light fraction of crude oil.Octane and its isomers are the principalconstituents of gasoline, which is obtainedas the refined light fraction from crude oil.See also octane rating.

octane number See octane rating.

octane rating (octane number) A ratingfor the performance of gasoline in internal-combustion engines. The octane ratingmeasures the freedom from ‘knocking’ –i.e. preignition of the fuel in the engine.This depends on the relative proportions ofbranched-chain and straight-chain hydro-carbons present. High proportions ofbranched-chain alkanes are better in high-performance engines. In rating fuels, 2,2,4-trimethylpentane (isooctane) is given a

value of 100 and heptane is given a value 0.The performance of a fuel is comparedwith a mixture of these hydrocarbons.

octet A stable shell of eight electrons inan atom. The completion of the octet givesrise to particular stability and this is thebasis of the Lewis octet theory, thus: 1. The rare gases have complete octets and

are chemically inert.2. The bonding in small covalent molecules

is frequently achieved by the centralatom completing its octet by sharingelectrons with surrounding atoms, e.g.CH4, H2O.

3. The ions formed by electropositive andelectronegative elements are generallythose with a complete octet, e.g. Na+,Ca2+, O2–, Cl–.

ohm Symbol: Ω The SI unit of electricalresistance, equal to a resistance that passesa current of one ampere when there is anelectric potential difference of one voltacross it. 1 Ω = 1 V A–1. Formerly, it wasdefined in terms of the resistance of a col-umn of mercury under specified condi-tions. The unit is named for the Germanphysicist Georg Ohm (1787–1854).

oil Any of various viscous liquids. Min-eral oils are mainly composed of mixturesof hydrocarbons (see petroleum). Naturaloils secreted by plants and animals are ei-ther mixtures of esters and terpenes (see es-sential oil) or are GLYCERIDES of fatty acids.

oil of wintergreen See methyl salicy-late.

oil shale A sedimentary rock that in-cludes in its structure 30–60% of organicmatter, mainly in the form of bitumen.

155

O

oleate

156

Heated in the absence of air it produces anoily substance resembling petroleum,which is rich in nitrogen and sulfur com-pounds.

oleate A salt or ester of oleic acid; i.e. anoctadecenoate.

olefin See alkene.

oleic acid See octadecenoic acid.

oligomer A polymer formed from a rel-atively few monomer molecules. See poly-mer; polymerization.

oligopeptide See peptide.

oligosaccharide A carbohydrate formedof a small number of monosaccharide units(up to around 20). See sugar.

one-pot synthesis A synthesis of achemical compound in which the reactantsform the required product in a single reac-tion mixture.

onium ion An ion formed by additionof a proton (H+) to a molecule. The hydro-nium (or hydroxonium) ion (H3O+) andammonium ion (NH4

+) are examples.

Oparin, Alexandr Ivanovich (1894–1980) Russian biochemist. Oparin was oneof the first people to put forward a theoryof the origin of life on Earth. In 1922 hepostulated that life originated in the seas,with there being many suitable organicmolecules being present there, that there isa large supply of external energy and thatlife is characterized by a high degree oforder. He expounded his views in more de-tail in his book The Origin of Life on Earth(1936). His ideas stimulated a great deal offurther work such as the experiment ofStanley MILLER.

optical activity The ability of certaincompounds to rotate the plane of POLAR-IZATION of plane-polarized light when thelight is passed through them. Optical activ-ity can be observed in crystals, gases,liquids, and solutions. The amount of rota-

(S)-lactic acid(R)-lactic acid

COOH

COH

H

CH3

COOH

CH

OH

CH3

Optical activity: enantiomers of lactic acid

C

HCOOH

OH

H

COOHOH

C

C

HOCOOH

H

C

HOCOOH

H

OH

C

HCOOH

H

COOHOH

C

D-form L-form meso-form

Optical activity: tartaric acid

tion depends on the concentration of theactive compound.

Optical activity is caused by the interac-tion of the varying electric field of the lightwith the electrons in the molecule. It occurswhen the molecules are asymmetric – i.e.they have no plane of symmetry. Such mol-ecules have a mirror image that cannot besuperimposed on the original molecule.The two forms of the molecule are opticalisomers or enantiomers. (Stereoisomersthat are not mirror images of each otherare called diastereoisomers.) In organiccompounds this usually means that themolecule contains a carbon atom attachedto four different groups, forming a chiralcenter. One isomer will rotate the polar-ized light in one sense and the other by thesame amount in the opposite sense. Suchisomers are described as dextrorotatory orlevorotatory, according to whether theyrotate the plane to the ‘right’ or ‘left’ re-spectively (rotation to the left is clockwiseto an observer viewing the light coming to-ward the observer). Dextrorotatory com-pounds are given the symbol d or (+) and

levorotatory compounds l or (–). A mix-ture of the two isomers in equal amountsdoes not show optical activity. Such a mix-ture is sometimes called the (±) or dl-form,a racemate, or a racemic mixture

Optical isomers have identical physicalproperties (apart from optical activity) andcannot be separated by fractional crystal-lization or distillation. Their general chem-ical behavior is also the same, althoughthey do differ in reactions involving otheroptical isomers. Many naturally occurringsubstances are optically active (only oneoptical isomer exists naturally) and bio-chemical reactions occur only with the nat-ural isomer. For instance, the natural formof glucose is d-glucose and living organ-isms cannot metabolize the l-form.

The terms ‘dextrorotatory’ and ‘levoro-tatory’ refer to the effect on polarized light.A more common method of distinguishingtwo optical isomers is by their D-form(dextro-form) or L-form (levo-form). Thisconvention refers to the absolute structureof the isomer according to specific rules.Sugars are related to a particular configu-

157

optical activity

The corn rule for absolute configuration of alpha amino acids

R–configuration S–configuration

H

C

R

COOH

H2N

1

C

3 2

1

C

2 3

Corn rule

ration of glyceraldehyde (2,3-dihydroxy-propanal). For alpha amino acids the cornrule is used: the structure of the acidRC(NH2)(COOH) H is drawn with H atthe top; viewed from the top the groupsspell CORN in a clockwise direction for allD-amino acids (i.e. the clockwise order is–COOH,R,NH2). The opposite is true forL-amino acids. Note that this conventionrefers to absolute configuration, not to op-tical activity: D-alanine is dextrorotatorybut D-cystine is levorotatory.

An alternative is the R-S convention forshowing configuration. There is an orderof priority of attached groups based on theproton number of the attached atom:I, Br, Cl, SO3H, OCOCH3, OCH3, OH,NO2, NH2, COOCH3, CONH2, COCH3,CHO, CH2OH, C6H5, C2H5, CH3, H

Hydrogen has the lowest priority (seeCIP system). The chiral carbon is viewedsuch that the group of lowest priority ishidden behind it. If the other three groupsare in descending priority in a clockwise di-rection, the compound is R-. If descendingpriority is anticlockwise it is S-.

The existence of a carbon atom boundto four different groups is not the strictcondition for optical activity. The essentialpoint is that the molecule should be asym-metric. Inorganic octahedral complexes,for example, can show optical isomerism.It is also possible for a molecule to containasymmetric carbon atoms and still have a plane of symmetry. One structure of tartaric acid has two parts of the mol-ecule that are mirror images, thus having aplane of symmetry. This (called the meso-form) is not optically active. See also reso-lution.

optical isomer See optical activity.

optical rotary dispersion (ORD) Thephenomenon in which the amount of rota-tion of plane-polarized light by an opticallyactive substance depends on the wave-length of the light. Plots of rotation againstwavelength can be used to give informa-tion about the molecular structure of opti-cally active compounds.

optical rotation Rotation of the planeof polarization of plane-polarized light byan optically active substance.

orbit The path of an electron as it movesaround the nucleus in an atom.

orbital A region around an atomic nu-cleus in which there is a high probability offinding an electron. The modern picture ofthe atom according to quantum mechanicsdoes not have electrons moving in fixed el-liptical orbits. Instead, there is a finiteprobability that the electron will be foundin any small region at all possible distancesfrom the nucleus. In the hydrogen atom theprobability is low near the nucleus, in-creases to a maximum, and falls off to in-finity. It is useful to think of a region inspace around the nucleus – in the case ofhydrogen the region within a sphere –within which there is a high chance of find-ing the electron. Each of these, called anatomic orbital, corresponds to a subshelland can ‘contain’ a single electron or twoelectrons with opposite spins. Another wayof visualizing an orbital is as a cloud ofelectron charge (the average distributionwith time).

Similarly, in molecules the electronsmove in the combined field of the nucleiand can be assigned to molecular orbitals.In considering bonding between atoms it isuseful to treat molecular orbitals as formedby overlap of atomic orbitals.

It is possible to calculate the shapes andenergies of atomic and molecular orbitalsby quantum theory. The shapes of atomicorbitals depend on the orbital angular mo-mentum (the subshell). For each shell thereis one s orbital, three p orbitals, five d or-bitals, etc. The s orbitals are spherical, thep orbitals each have two lobes; d orbitalshave more complex shapes, typically withfour lobes.

Molecular orbitals are formed by over-lap of atomic orbitals, and again there aredifferent types. If the orbital is completelysymmetrical about an axis between the nu-clei, it is a sigma orbital. This can occur,for instance, by overlap of two s orbitals,as in the hydrogen atom, or two p orbitalswith their lobes along the axis. However,

optical isomer

158

two p orbitals overlapping at right anglesto the axis form a different type of molecu-lar orbital – a pi orbital – with regionsabove and below the axis. Pi orbitals arealso formed by overlap of d orbitals. Eachmolecular orbital can contain a pair ofelectrons, forming a sigma bond or pibond. A double bond, for example thebond in ethene, is a combination of a sigmabond and a pi bond. The triple bond inethyne is one sigma bond and two pibonds.

Hybrid orbitals are atomic orbitalsformed by combinations of s, p, and datomic orbitals, and are useful in describ-ing the bonding in compounds. There arevarious types. In carbon, for instance, theelectron configuration is 1s22s22p2. Car-bon, in its outer (valence) shell, has onefilled s orbital, two filled p orbitals, andone ‘empty’ p orbital. These four orbitalsmay hybridize (sp3 hybridization) to act asfour equal orbitals arranged tetrahedrally,each with one electron. In methane, eachhybrid orbital overlaps with a hydrogen sorbital to form a sigma bond. Alterna-tively, the s and two of the p orbitals mayhybridize (sp2 hybridization) and act asthree orbitals in a plane at 120°. The re-

maining p orbital is at right angles to theplane, and can form pi bonds. Finally, sphybridization may occur, giving two or-bitals in a line. More complex types of hy-bridization, involving d orbitals, explainthe geometries of inorganic complexes.

The combination of two atomic orbitalsin fact produces two molecular orbitals.One – the bonding orbital – has a concen-tration of electron density between the nu-clei, and thus tends to hold the atomstogether. The other – the antibonding or-bital – has slightly higher energy and tendsto repel the atoms. If both atomic orbitalsare filled, the two molecular orbitals arealso filled and cancel each other out – thereis no net bonding effect. If each atomic or-bital has one electron, the pair occupies thelower energy bonding orbital, producing anet attraction.

order The sum of the indices of the con-centration terms in the expression that de-termines the rate of a chemical reaction.For example, in the expression:

rate = k[A]x[B]y

x is called the order with respect to A, y theorder with respect to B, and (x + y) theorder overall. The values of x and y are not

159

order

s orbitaldxy orbital

px orbital py orbital pz orbital

z

y

x

z

y

x

z

y

x

z

y

x

z

y

x

z

y

x

dz2 orbital

Orbital: atomic orbitals

160

z

A

B

C

x x

x x

x x

s orbitals sigma

px orbitals

pz orbitals

sigma

pi

z

Orbital: molecular orbitals

one s

one s

one s

+

+

+

ATOMIC ORBITALS HYBRID ORBITALS SHAPE

one p two sp3 linear

two p three sp2 planar

sp

three p tetrahedralfour sp3

sp3

sp2

Orbital: hybrid orbitals

necessarily equal to the coefficients of Aand B in the molecular equation. Order isan experimentally determined quantity de-rived without reference to any equation ormechanism. Fractional orders do occur.For example, in the reaction:

CH3CHO → CH4 + COthe rate is proportional to [CH3CHO]1.5

i.e. it is of order 1.5.

organic acid An organic compoundthat can release hydrogen ions (H+) to abase, such as a carboxylic acid or a phenol.See acid; carboxylic acid; phenol.

organic base An organic compoundthat can function as a base. Organic basesare typically amines that gain H+ ions. Seeamine.

organic chemistry The chemistry ofcompounds of carbon. Originally the termorganic chemical referred to chemical com-pounds present in living matter, but now itcovers any carbon compound with the ex-ception of certain simple ones, such as thecarbon oxides, carbonates, cyanides, andcyanates. These are generally studied in in-organic chemistry. The vast numbers ofsynthetic and natural organic compoundsexist because of the ability of carbon toform chains of atoms (catenation). Otherelements are involved in organic com-pounds: principally hydrogen and oxygenbut also nitrogen, halogens, sulfur, andphosphorus.

organometallic compound An organiccompound containing a carbon–metalbond. Tetraethyl lead, (C2H5)4Pb, is an ex-ample of an organometallic compound,formerly used as an additive in petrol.

ornithine cycle (urea cycle) The se-quence of enzyme-controlled reactions bywhich urea is formed as a breakdownproduct of amino acids. It occurs in cells ofthe liver. The amino acid ornithine is com-bined with ammonia (from amino acids)and carbon dioxide, forming anotheramino acid, arginine, which is then splitinto urea (which is excreted) and ornithine.

ortho- 1. Designating a benzene com-pound with substituents in the 1,2 posi-tions. The position next to a substituent isthe ortho position on the benzene ring.This was used in the systematic naming ofbenzene derivatives. For example, ortho-dinitrobenzene (or o-dinitrobenzene) is1,2-dinitrobenzene.2. Certain acids, regarded as formed froman anhydride and water, were named orthoacids to distinguish them from the less hy-drated meta acids. For example, H4SiO4(from SiO2 + 2H2O) is orthosilicic acid;H2SiO3 (SiO2 + H2O) is metasilicic acid.3. Designating the form of a diatomic mol-ecule in which both nuclei have the samespin direction; e.g. orthohydrogen, orth-odeuterium.

See also meta-; para-.

osmium(IV) oxide (osmium tetroxide;OsO4) A volatile yellow crystalline solidwith a penetrating odor, used as an oxidiz-ing agent and, in aqueous solution, as acatalyst for organic reactions.

osmium tetroxide See osmium(IV)oxide.

osmosis Systems in which a solvent isseparated from a solution by a SEMIPERME-ABLE MEMBRANE approach equilibrium bysolvent molecules on the solvent side of themembrane migrating through it to the so-lution side; this process is called osmosisand always leads to dilution of the solu-tion. The phenomenon is quantified bymeasurement of the osmotic pressure. Theprocess of osmosis is of fundamental im-portance in transport and control mecha-nisms in biological systems; for example,plant growth and general cell function. Seeosmotic pressure.

osmotic pressure Symbol: π The pres-sure that must be exerted on a solution toprevent the passage of solvent moleculesinto it when the solvent and solution areseparated by a semipermeable membrane.The osmotic pressure is therefore the pres-sure required to maintain equilibrium be-tween the passage of solvent moleculesthrough the membrane in either direction

161

osmotic pressure

and thus prevent the process of osmosisproceeding. The osmotic pressure can bemeasured by placing the solution, con-tained in a small perforated thimble cov-ered by a semipermeable membrane andfitted with a length of glass tubing, in abeaker of the pure solvent. Solvent mol-ecules pass through the membrane, dilut-ing the solution and thereby increasing thevolume on the solution side and forcing thesolution to rise up the glass tubing. Theprocess continues until the pressure ex-erted by the solvent molecules on the mem-brane is balanced by the hydrostaticpressure of the solution in the tubing. Asample of the solution is then removed andits concentration determined. Osmosis is acolligative property; therefore the methodcan be applied to the determination of rel-ative molecular masses, particularly forlarge molecules, such as proteins, but it isrestricted by the difficulty of preparinggood semipermeable membranes.

As the osmotic pressure is a colligativeproperty it is directly proportional to themolar concentration of the solute if thetemperature remains constant; thus π isproportional to the concentration n/V,where n is the number of moles of solute,and V the solvent volume. The osmoticpressure is also proportional to the ab-solute temperature. Combining these twoproportionalities gives πV = nCT, whichhas the same form as the gas equation, PV= nRT, and experimental values of C aresimilar to those for R, the universal gasconstant. This gives considerable supportto the kinetic theory of colligative proper-ties.

Ostwald’s dilution law See dissocia-tion constant.

oxalate A salt or ester of ethanedioicacid (oxalic acid). See ethanedioic acid.

oxalic acid See ethanedioic acid.

oxaloacetic acid (OAA) A water-soluble carboxylic acid, structurally relatedto fumaric acid and maleic acid. Oxalo-acetic acid forms part of the Krebs cycle, it is produced from L-malate in an NAD-

requiring reaction and itself is a step to-wards the formation of citric acid in areaction involving pyruvate ion and coen-zyme A.

oxidant An oxidizing agent. In rocketfuels, the oxidant is the substance that pro-vides the oxygen for combustion (e.g.liquid oxygen or hydrogen peroxide).

oxidative metabolism See aerobic res-piration.

oxidative phosphorylation The pro-duction of ATP from phosphate and ADPas electrons are transferred along the electron-transport chain from NADH orFADH2 to oxygen. Most of the NADH andFADH2 is formed in the mitochondrial ma-trix by the Krebs cycle and fatty acid oxi-dation. Oxidative phosphorylation occursin mitochondria and is the main source ofATP in aerobes. See electron-transportchain.

oxidation An atom, an ion, or a mol-ecule is said to undergo oxidation or to beoxidized when it loses electrons. Theprocess may be effected chemically, i.e. byreaction with an oxidizing agent, or electri-cally, in which case oxidation occurs at theanode. For example,

2Na + Cl2 → 2Na+ + 2Cl–

where chlorine is the oxidizing agent andsodium is oxidized, and

4CN– + 2Cu2+ → C2N2 + 2CuCNwhere Cu2+ is the oxidizing agent and CN–

is oxidized.The oxidation state of an atom is indi-

cated by the number of electrons lost or ef-fectively lost by the neutral atom, i.e. theoxidation number. The oxidation numberof a negative ion is negative. The process ofoxidation is the converse of reduction. Seealso redox.

oxidizing agent See oxidation.

oxime A type of organic compound con-taining the C:NOH group, formed by reac-tion of an ALDEHYDE or KETONE withhydroxylamine (NH2OH). The reaction is

Ostwald’s dilution law

162

a condensation reaction, in which a mol-ecule of water is lost.

oxo process A method of manufactur-ing aldehydes by passing a mixture of car-bon monoxide, hydrogen, and alkanesover a cobalt catalyst at high pressure (100atmospheres and 150°C). The aldehydescan subsequently be reduced to alcohols,making the process a useful source of alco-hols of high molecular weight.

2-oxopropanoic acid See pyruvic acid.

oxyacid An acid in which the replace-able hydrogen atom is part of a hydroxylgroup, including carboxylic acids, phenolsand inorganic acids such as phosphoric(V)acid and sulfuric(VI) acid. See acid.

oxygen A colorless odorless diatomicgas; the first member of group 16 (formerlyVIA) of the periodic table. It has the elec-tronic configuration [He]2s22p4 and itschemistry involves the acquisition of elec-trons to form either the di-negative ion,O2–, or two covalent bonds. In each casethe oxygen atom attains the configurationof the rare gas neon. Oxygen is the mostplentiful element in the Earth’s crust ac-counting for over 40% by weight. It is pre-sent in the atmosphere (20%) and is aconstituent of the majority of minerals androcks (e.g. sandstones, SiO2, carbonates,CaCO3, aluminosilicates) as well as themajor constituent of the sea. Oxygen is anessential element for almost all livingthings. Elemental oxygen has two forms:the diatomic molecule O2 and the less sta-

ble molecule trioxygen (OZONE), O3, whichis formed by passing an electric dischargethrough oxygen gas.

Oxygen occurs in three natural isotopicforms, 16O (99.76%), 17O (0.0374%), 18O(0.2039%); the rarer isotopes are used indetailed studies of the behavior of oxygen-containing groups during reactions (tracerstudies).

Symbol: O; m.p. –218.4°C; b.p.–182.962°C; d. 1.429 kg m–3 (0°C); p.n. 8;r.a.m. 15.9994.

ozone (trioxygen; O3) A poisonous,blue-colored allotrope of oxygen made bypassing oxygen through a silent electricdischarge. Ozone is unstable and decom-poses to oxygen on warming. It is a strongoxidizing agent. It is present in the upperlayers of the atmosphere, where it screensthe Earth from harmful short-wave ultravi-olet radiation. There is concern that theozone layer is possibly being depleted byfluorocarbons and other compounds pro-duced by industry.

ozonide See ozonolysis.

ozonolysis The addition of ozone (O3)to alkenes and the subsequent hydrolysis ofthe ozonide into hydrogen peroxide and amixture of carbonyl compounds. The car-bonyl compounds can be separated andidentified, which in turn, identifies thegroups and locates the position of the dou-ble bond in the original alkene. Ozonolysiswas formerly an important analytical tech-nique.

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164

palmitate A salt or ester of palmiticacid.

palmitic acid See hexadecanoic acid.

pantothenic acid (vitamin B5) One ofthe water-soluble B-group of vitamins.Sources of the vitamin include egg yolk,kidney, liver, and yeast. As a constituent ofcoenzyme A, pantothenic acid is essentialfor several fundamental reactions in me-tabolism. A deficiency results in symptomsaffecting a wide range of tissues; the over-all effects include fatigue, poor motor co-ordination, and muscle cramps.

paper chromatography A form ofCHROMATOGRAPHY widely used for theanalysis of mixtures. Paper chromatogra-phy usually employs a specially producedpaper as the stationary phase. A base line ismarked in pencil near the bottom of thepaper and a small sample of the mixture isspotted onto it using a capillary tube. Thepaper is then placed vertically in a suitable

solvent, which rises up to the base line andbeyond by capillary action. The compo-nents within the sample mixture dissolve inthis mobile phase and are carried up thepaper. However, the paper holds a quan-tity of moisture and some components willhave a greater tendency than others to dis-solve in this moisture rather than in themobile phase. In addition, some compo-nents will preferentially cling to the surfaceof the paper. Therefore, as the solventmoves through the paper, certain compo-nents will be left behind and components inthe mixture will become separated fromeach other.

When the solvent has almost reachedthe top of the paper, the paper is removedand quickly dried. The paper is developedto locate the positions of colorless fractionsby spraying with a suitable chemical, e.g.ninhydrin, or by exposure to ultraviolet ra-diation. The components are identified bycomparing the distance they have traveledup the paper with standard solutions thathave been run simultaneously, or by com-puting an RF VALUE. A simplified version ofpaper chromatography uses a piece of filterpaper.

para- 1. Designating a benzene com-pound with substituents in the 1,4 posi-tions. The position on a benzene ringdirectly opposite a substituent is the paraposition. This was used in the systematicnaming of benzene compounds. For exam-ple, para-dinitrobenzene (or p-dinitroben-zene) is 1,4-dinitrobenzene.2. Designating the form of a diatomic mol-ecule in which both nuclei have oppositespin directions; e.g. parahydrogen, para-deuterium.

See also meta-, ortho-.

P

developingchamber

paper strip

mobile phase

spotted samples

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paraffin 1. See petroleum.2. See alkane.

paraffin oil See petroleum.

paraffin wax A solid mixture of hydro-carbons obtained from petroleum. See alsowax.

paraformaldehyde See methanal.

paraldehyde See ethanal.

partial ionic character The electronsof a covalent bond between atoms orgroups with different electronegativitieswill be polarized towards the more elec-tronegative constituent; the magnitude ofthis effect can be measured by the ioniccharacter of the bond. When the effect issmall the bond is referred to simply as apolar bond and is adequately treated usingDIPOLE MOMENTS; as the effect grows thetheoretical treatment requires other contri-butions to ionic character.

partial pressure In a mixture of gases,the contribution that one componentmakes to the total pressure. It is the pres-sure that the gas would have if it alonewere present in the same volume. See Dal-ton’s law.

particulate matter (PM) An airbornepollutant consisting of small particles ofsilicate, carbon, or large polyaromatic hy-drocarbons (PAHs). These pollutants areoften referred to as particulates. They areclassified according to size; e.g. PM10 isparticulate matter formed of particles lessthan 10 µm diameter.

particulates See particulate matter.

partition coefficient If a solute dis-solves in two nonmiscible liquids, the par-tition coefficient is the equilibrium ratio ofthe concentration in one liquid to the con-centration in the other liquid.

pascal Symbol: Pa The SI unit of pres-sure, equal to a pressure of one newton persquare meter (1 Pa = 1 N m–2). The unit is

named for the French mathematicianBlaise Pascal (1623–62).

Pasteur, Louis (1822–95) French chemistand biologist. Pasteur is famous for his in-vestigations on stereochemistry, fermenta-tion, and vaccines. In his early work hediscovered the phenomenon of optical iso-mers and invented methods for separatingsuch isomers. In 1860 he postulated thatoptical isomers existed because of thearrangements of atoms in the molecules.This idea was a major stimulus to the de-velopment of structural chemistry. Hiswork on fermentation led to the processknown as pasteurization, which uses ele-vated temperatures to kill organisms thatspoil milk or wine. He also established therole of micro-organisms such as yeast infermentation.

Pauli exclusion principle See exclu-sion principle.

Pauling, Linus Carl (1901–94) Ameri-can chemist. Pauling was one of the great-est scientists of the twentieth century. Hisearly work was on determining the struc-ture of complex minerals such as molyb-denite by x-ray diffraction. In 1928–29 thisled to Pauling’s rules governing the struc-ture of complex minerals. Pauling was amajor pioneer in the application of quan-tum mechanics to chemical bonding. In1931 he published a classic paper entitledThe Nature of the Chemical Bond that ex-plained how a chemical bond is formedfrom a pair of electrons. Pauling also intro-duced the concept of hybridization to ex-plain the chemical bonding of the carbonatom. Pauling also considered partiallyionic bonds. Pauling put together his ideasabout chemical bonding in his book TheNature of the Chemical Bond, the first edi-tion of which was published in 1939. In themid-1930s Pauling turned his attention tomolecules of biological interest and wasone of the founders of molecular biology.Together with Robert Corey, he showedthat many proteins have helical shapes. Healso worked on sickle-cell anemia. He co-authored the book Introduction to Quan-tum Mechanics (1935) and wrote the

influential books General Chemistry(1948) and college Chemistry (1950). Paul-ing was awarded the 1954 Nobel Prize forchemistry. In the 1950s he became con-cerned with nuclear weapons, particularlytheir testing in the atmosphere. This led tohis winning the 1962 Nobel Prize forpeace.

PCB See polychlorinated biphenyl.

pectic substances Polysaccharides that,together with hemicelluloses, form the ma-trix of plant cell walls. They serve to ce-ment the cellulose fibers together. Fruitsare a rich source.

They are principally made from thegroup of sugar acids known as uronicacids. Pectic acids, the basis of the otherpectic substances, are soluble unbranchedchains of α-1,4 linked galacturonic acidunits (derived from the sugar galactose).The acid is precipitated as insoluble cal-cium or magnesium pectate in the middlelamella of plant cells. Pectinic acids areslightly modified pectic acids. Under suit-able conditions pectinic acids and pectinsform gels with sugar and acid. Pectins areused commercially as gelling agents, e.g. injams. Insoluble pectic substances aretermed protopectin and this is the most im-portant group in normal cell walls. Pro-topectin is hydrolyzed to soluble pectin bypectinase in ripening fruits, changing thefruit consistency.

pectin See pectic substances.

pentane (C5H12) A straight-chain alkaneobtained by distillation of crude oil.

pentanoic acid (valeric acid; CH3(CH2)3-COOH) A colorless liquid carboxylicacid, used in making perfumes.

pentose A SUGAR that has five carbonatoms in its molecules.

pentose phosphate pathway (hexosemonophosphate shunt) A pathway ofglucose breakdown in which pentoses areproduced, in addition to reducing power(NADPH) for many synthetic reactions. It

is an alternative to glycolysis and is muchmore active in adipose tissue (where largeamounts of NADPH are consumed) thanin skeletal muscle.

pentyl group (amyl group) The groupCH3CH2CH2CH2CH2–

peptide A compound formed by linkageof two or more amino-acid groups. Pep-tides can be formed by reaction of the car-boxylic acid group on one amino acid withthe amino group on another amino acid,with elimination of water. The amino acidsare joined together by a bond of the type–CO–NH–, known as a peptide linkage.Simple peptides consisting of a small num-ber of amino-acid units (less than about10) are known as oligopeptides and aredesignated as dipeptides, tripeptides, etc.,according to the number of amino-acidunits present. Peptides with large numbersof amino-acid units are called POLYPEP-TIDES.

peptide linkage See peptide.

percentage composition A way of ex-pressing the composition of a chemicalcompound in terms of the percentage (bymass) of each of the elements that make itup. It is calculated by dividing the mass ofeach element (taking into account the num-ber of atoms present) by the relative mo-lecular mass of the whole molecule. Forexample, methane (CH4) has a relative mo-lecular mass of 16 and its percentage com-position is 12/16 = 75% carbon and (4 ×1)/16 = 25% hydrogen.

perfect gas (ideal gas) See gas laws.

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R

C

H

O

C

H

N

R

C

H

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pericyclic reaction A type of CON-CERTED REACTION in which the TRANSITION

STATE is cyclic and can be regarded asformed by movement of electrons in a cir-cle, from one bond to an adjacent bond. Itis usual to distinguish three types of peri-cyclic reaction:Cycloadditions. In these reactions a conju-gated diene adds to a double bond to forma ring. This involves the formation of twonew sigma bonds. The DIELS–ALDER REAC-TION is a cycloaddition. The reverse reac-tion, in which a ring containing a doublebond breaks to a diene and a compoundcontaining a double bond, is also a peri-cyclic reaction.Sigmatropic rearrangements. In these reac-tions a sigma bond breaks and another isformed.Electrocyclic reactions. In these reactions aring is formed across the ends of a conju-gated system of double bonds. The reversereaction, in which a ring containing twodouble bonds breaks by movement of elec-trons in a cycle, is also an electrocyclic re-action.Pericyclic reactions are often treated usingFRONTIER ORBITAL theory or the WOOD-WARD–HOFFMANN RULES.

period One of the horizontal rows in theconventional periodic table. Each periodrepresents the elements arising from pro-gressive filling of the outer shell (i.e. the ad-dition of one extra electron for each newelement), the elements being arranged inorder of ascending proton number. In astrict sense hydrogen and helium representone period but convention refers to the el-ements lithium to neon (8 elements) as thefirst short period (n = 2), and the elementssodium to argon (8 elements) as the secondshort period (n = 3). With entry to the n =4 level there is filling of the 4s, then backfilling of the 3d, before the 4p are filled.Thus this set contains a total of 18 elec-trons (potassium to krypton) and is calleda long period. The next set, rubidium toxenon, is similarly a long period.

periodic law The law upon which themodern periodic table is based. Enunciatedin 1869 by the Russian chemist Dmitri

Mendeléev (1834–1907), this law statedthat the properties of the elements are a pe-riodic function of their atomic weights: ifarranged in order of increasing atomicweight then elements having similar prop-erties occur at fixed intervals. Certain ex-ceptions or gaps in the table lead to theview that the nuclear charge is a morecharacteristic function, thus the modernstatement of the periodic law is that thephysical and chemical properties of el-ements are a periodic function of their pro-ton number.

periodic table A table of the elementsarranged in order of increasing protonnumber to show similarities in chemicalbehavior between elements. Horizontalrows of elements are called periods. Acrossa period there is a general trend frommetallic to nonmetallic behavior. Verticalcolumns of related elements are calledgroups. Down a group there is an increasein atomic size and in electropositive (metal-lic) behavior.

Originally the periodic table wasarranged in eight groups with the alkalimetals as group I, the halogens as groupVII, and the rare gases as group 0. Thetransition elements were placed in a blockin the middle of the table. Groups weresplit into two sub-groups. For example,group I contained the main-group el-ements, Li, Na, K, Rb, Cs, in subgroup IAand the subgroup IB elements Cu, Ag, Au.The system was confusing because therewas a difference in usage for subgroupsand a current form of the table has 18groups.

See also group; period.

Perkin, Sir William Henry (1838–1907) British organic chemist. Perkin be-came famous for his discovery of mauve,the first synthetic dye. This discovery orig-inated in 1856 when Perkin attempted tosynthesize quinine. He did not succeed indoing so. However, when he used chromicacid to oxidize toluidine he obtained a darkprecipitate. When he repeated the experi-ment using aniline he again obtained adark precipitate. He found that adding al-cohol to this precipitate produced a bright

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purple solution. He also found that thisbright purple substance was a dye whichdid not fade readily in light. This syntheticdye was called mauve. Perkin patented theprocess for the manufacture of mauve andset up a factory for its production. Thisventure was so successful that he was ableto retire in 1874 and devote his life to sci-entific research. His son William HenryPerkin (1860–1929) also became an emi-nent organic chemist.

peroxide 1. A compound containingthe –O–O– group. Organic peroxides tendto be unstable and form free radicals. Theyare used to initiate free-radical polymeriza-tion reactions.2. An oxide containing the –O–O– ion.

peta- Symbol: P A prefix denoting 1015.

petrochemicals Chemicals that are ob-tained from crude oil or from natural gas.

petrol See petroleum.

petroleum A mixture of hydrocarbonsformed originally from marine animals andplants, found beneath the ground trappedbetween layers of rock. It is obtained bydrilling (also called crude oil). Differentoilfields produce petroleum with differingcompositions. The mixture is separatedinto fractions by fractional distillation in avertical column. The main fractions are:Diesel oil (gas oil) in the range 220–350°C,consisting mainly of C13–C25 hydrocar-bons. It is used as fuel in diesel engines.Kerosene (paraffin) in the range160–250°C, consisting mainly of C11 andC12 hydrocarbons. It is a fuel both for do-mestic heating and jet engines.Gasoline (petrol) in the range 40–180°C,consisting mainly of C5–C10 hydrocarbons.It is used as motor fuel and as a raw ma-terial for making other chemicals.Refinery gas, consisting of C1–C4 gaseoushydrocarbons.

In addition lubricating oils and paraffinwax are obtained from the residue. Theblack material left is bitumen tar.

petroleum ether A flammable mixture

of hydrocarbons, mainly pentane andhexane, used as a solvent. Note that it isnot an ether.

pH The logarithm to base 10 of the rec-iprocal of the hydrogen-ion concentrationof a solution. In pure water at 25°C theconcentration of hydrogen ions is 1.00 ×10–7 mol l–1, thus the pH equals 7 at neu-trality. An increase in acidity increases thevalue of [H+], decreasing the value of thepH below 7. An increase in the concentra-tion of hydroxide ion [OH–] proportion-ately decreases [H+], therefore increasingthe value of the pH above 7 in basic solu-tions. pH values can be obtained approxi-mately by using indicators. More precisemeasurements use electrode systems. Theterm ‘pH’ is short for ‘potential of hydro-gen’.

phase One of the physically separableparts of a chemical system. For example, amixture of ice (solid phase) and water (liquid phase) consists of two phases. Asystem consisting of only one phase is saidto be homogeneous. A system consisting ofmore than one phase is said to be heteroge-neous.

phase diagram A graphical representa-tion of the state in which a substance willoccur at a given pressure and temperature.The lines show the conditions under whichmore than one phase can coexist at equi-librium. For one-component systems (e.g.water) the point at which all three phasescan coexist at equilibrium is called thetriple point and is the point on the graph atwhich the pressure–temperature curves in-tersect.

phenol 1. (carbolic acid; hydroxyben-zene; C6H5OH) A white crystalline solidused to make a variety of other organiccompounds. It is usually made using theCUMENE PROCESS or the RASHIG PROCESS.2. A type of organic compound in which atleast one hydroxyl group is bound directlyto one of the carbon atoms of an aromaticring. Phenols do not show the behaviortypical of alcohols. In particular they are

more acidic because of the electron-with-drawing effect of the aromatic ring. Phenol ionizes in water:

C6H5OH → C6H5O– + H+

The preparation of phenol itself is by fus-ing the sodium salt of the sulfonic acid withsodium hydroxide:

C6H5SO2.ONa + 2NaOH → C6H5ONa+ Na2SO3 + H2O

The phenol is then liberated by sulfuricacid:

2C6H5ONa + H2SO4 → 2C6H5OH +Na2SO4

Reactions of phenol include:1. Replacement of the hydroxyl group with

a chlorine atom using phosphorus(V)chloride:

ROH → RCl2. Reaction with acyl halides to form esters

of carboxylic acids:R1OH + R2COCl → R2COOR1

3. Reaction with haloalkanes under alka-line conditions to give mixed alkyl–arylethers:

R1OH + R2Cl → R1OR2

In addition phenol can undergo furthersubstitution on the benzene ring. The hy-droxyl group directs other substituentsinto the 2- and 4-positions.

phenolphthalein An acid–base indica-tor that is colorless in acid solutions andbecomes red if the pH rises above the tran-sition range of 8–9.6. It is used as the indi-cator in titrations for which the end pointlies clearly on the basic side (pH > 7), e.g.oxalic acid or potassium hydrogentartrateagainst caustic soda.

phenoxy resin A type of thermoplasticresin made by condensation of phenols.

phenylalanine See amino acid.

phenylamine See aniline.

phenylethene (styrene; C6H5CHCH2) Aliquid hydrocarbon used as the startingmaterial for the production of polystyreneand some synthetic rubbers. The manufac-ture of phenylethene is by dehydrogenationof ethyl benzene using various metal oxidecatalysts:

C6H5C2H5 → C6H5CH:CH2 + H2

phenyl group The group C6H5–, de-rived from benzene.

phenylhydrazone See hydrazone.

phenylmethanol (benzyl alcohol; C6H5-CH2OH) An aromatic primary alcoholused as a solvent. Phenylmethanol is syn-thesized by the CANNIZZARO REACTION,which involves the simultaneous oxidationand reduction of benzenecarbaldehyde(benzaldehyde) by refluxing in an aqueoussolution of sodium hydroxide:

2C6H5CHO → C6H5CH2OH +C6H5COOH

Benzoic acid is the other product.Phenylmethanol undergoes the reac-

tions characteristic of alcohols, especiallythose in which the formation of a stablecarbonium ion as an intermediate(C6H5CH2

+) enhances the reaction. Substi-tution onto the benzene ring is also possi-ble; the –CH2OH group directs into the 2-or 4-position by the donation of electronsto the ring.

phenyl methyl ketone (acetophenone;C6H5COCH3) A colorless sweet-smellingorganic liquid, which solidifies below20°C. It is used as a solvent for methyl andethyl cellulose plastics.

3-phenylpropenoic acid (cinnamic acid)A white pleasant-smelling crystalline car-boxylic acid, C6H5CH:CHCOOH. It oc-curs in amber but can be synthesized and isused in perfumes and flavorings.

pheromone A substance that is excretedby an animal and causes a response inother animals of the same species (e.g. sex-ual attraction, development). Comparekairomone.

Phillips process A method for the man-ufacture of high-density polyethene using acatalyst of chromium(III) oxide on a pro-moter of silica and alumina. The reactionconditions are 150°C and 30 atm pressure.See also Ziegler process.

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Phillips process

phosgene See carbonyl chloride.

phosphate A salt or ester of a phos-phorus(V) oxoacid, especially one of phosphoric(V) acid, H3PO4. Polymericphosphates occur containing P–O–Pbridges.

phosphide A compound of phosphoruswith a more electropositive element.

phosphine (phosphorus(III) hydride; PH3)A colorless gas that is slightly soluble inwater. It has a characteristic fishy smell. Itcan be made by reacting water and calciumphosphide or by the action of yellow phos-phorus on a concentrated alkali. Phosphineusually ignites spontaneously in air be-cause of contamination with diphosphine.

phospholipid A lipid with a phosphategroup attached by an ester linkage. Theyare the major class of lipid in all biologicalmembranes and, together with glycolipidsand cholesterol, are the main structuralcomponents. All membrane phosphlipids,except sphingosine, are derived from glyc-erol and are called glycerophospholipids.They consist of a glycerol backbone withtwo fatty acid chains esterified to carbons1 and 2 and a phosphorylated alcoholesterified at carbon 3. The simplest glyc-erophospholipid is diacylglycerol 3-phos-phate or phosphatidate, which has noalcohol part. Only small amounts existnaturally, but it is a key intermediate in thebiosynthesis of other glycerophospholipidswith the phosphate being esterified to oneof several alcohols (serine, ethanolamine,choline, or glycerol) to form the majormembrane glycerophospholipids: phos-phatidylserine, phosphatidylethanolamine,phosphatidylcholine, and phosphatidyl-glycerol. Sphingomylein (like glycolipids)is derived from sphingosine and has aphosphorylcholine esterified to the pri-mary hydroxyl group of sphingosine.

Another important glycerophospho-lipid is phosphatidylinositol, which is phos-phorylated by specific kinases to phos-phatidylinositol 4,5-bisphosphate. This is akey molecule in signal transduction thatmediates the action of several hormones

and other effectors that control importantcellular functions, e.g. glycogenolysis, in-sulin secretion, the aggregation of platelets,and smooth muscle contraction.

phosphonic acid (phosphorous acid; H3PO3) A colorless deliquescent solidthat can be prepared by the action of wateron phosphorus(III) oxide orphosphorus(III) chloride. It is a dibasicacid producing the anions H2PO3

– andHPO3

2– in water. The acid and its salts areslow reducing agents.

phosphonium ion The ion PH4+ de-

rived from phosphine.

phosphoprotein A conjugated proteinformed by the combination of protein withphosphate groups. Casein is an example.

phosphoric(V) acid (orthophosphoricacid; H3PO4) A white solid that can bemade by reacting phosphorus(V) oxidewith water or by heating yellow phospho-rus with nitric acid. The naturally occur-ring phosphates (orthophosphates,M3PO4) are salts of phosphoric(V) acid.

phosphorous acid See phosphonicacid.

phosphorescence 1. The absorption ofenergy by atoms followed by emission ofelectromagnetic radiation. Phosphores-cence is a type of luminescence, and is dis-tinguished from fluorescence by the factthat the emitted radiation continues forsome time after the source of excitation hasbeen removed. In phosphorescence the ex-cited atoms have relatively long lifetimesbefore they make transitions to lower en-ergy states. However, there is no definedtime distinguishing phosphorescence fromfluorescence.2. In general usage the term is applied tothe emission of ‘cold light’ – light producedwithout a high temperature. The namecomes from the fact that white phosphorusglows slightly in the dark as a result of achemical reaction with oxygen. The lightcomes from excited atoms produced di-rectly in the reaction – not from the heat

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170

produced. It is thus an example of chemi-luminescence. There are also a number ofbiochemical examples termed biolumines-cence; for example, phosphorescence issometimes seen in the sea from marine or-ganisms, or on rotting wood from certainfungi (known as ‘fox fire’).

phosphorus A reactive solid nonmetal-lic element; the second element in group 15(formerly VA) of the periodic table. It hasthe electronic configuration [Ne]3s23p3

and is therefore formally similar to nitro-gen. It is however very much more reactivethan nitrogen and is never found in naturein the uncombined state. Phosphorus iswidespread throughout the world; eco-nomic sources are phosphate rock(Ca3(PO4)2) and the apatites, variously oc-curring as both fluoroapatite(3Ca3(PO4)2.CaF2) and as chloroapatite(3Ca3(PO4)2CaCl2). Guano formed fromthe skeletal phosphate of fish in sea-birddroppings is also an important source ofphosphorus. The largest amounts of phos-phorus compounds produced are used asfertilizers, with the detergents industryproducing increasingly large tonnages ofphosphates. Phosphorus is an essentialconstituent of living tissue and bones, andit plays a very important part in metabolicprocesses and muscle action.

Symbol: P; m.p. 44.1°C (white) 410°C(red under pressure); b.p. 280.5°C; r.d.1.82 (white) 2.2 (red) 2.69 (black) (all at20°C); p.n. 15; r.a.m. 30.973762.

phosphorus(III) bromide (phosphorustribromide; PBr3) A colorless liquidmade by reacting phosphorus withbromine. It is readily hydrolyzed by waterto phosphonic acid and hydrogen bromide.Phosphorus(III) bromide is important inorganic chemistry, being used to replace ahydroxyl group with a bromine atom.

phosphorus(V) bromide (phosphoruspentabromide; PBr5) A yellow crystallinesolid that sublimes easily. It can be madeby the reaction of bromine and phospho-rus(III) bromide. Phosphorus(V) bromideis readily hydrolyzed by water to phos-phoric(V) acid and hydrogen bromide. Its

main use is in organic chemistry to replacea hydroxyl group with a bromine atom.

phosphorus(III) chloride (phosphorustrichloride; PCl3) A colorless liquidformed from the reaction of phosphoruswith chlorine. It is rapidly hydrolyzed bywater to phosphonic acid and hydrogenchloride. Phosphorus(III) chloride is usedin organic chemistry to replace a hydroxylgroup with a chlorine atom.

phosphorus(V) chloride (phosphoruspentachloride; PCl5) A white easily sub-limed solid formed by the action of chlo-rine on phosphorus(III) chloride. It ishydrolyzed by water to phosphoric(V) acidand hydrogen chloride. Its main use is as achlorinating agent in organic chemistry toreplace a hydroxyl group with a chlorineatom.

phosphorus(III) chloride oxide (phos-phorus trichloride oxide; phosphorus oxy-chloride, phosphoryl chloride, POCl3) Acolorless liquid that can be obtained by re-acting phosphorus(III) chloride with oxy-gen or by distilling phosphorus(III)chloride with potassium chlorate. The re-actions of phosphorus(III) chloride oxideare similar to those of phosphorus(III)chloride. The chlorine atoms can be re-placed by alkyl groups using Grignardreagents or by alkoxo groups using alco-hols. Water hydrolysis yields phos-phoric(V) acid.

phosphorus(III) hydride See phos-phine.

phosphorus(V) oxide (phosphorus pen-toxide, P2O5) A white powder that is sol-uble in organic solvents. It usually exists asP4O10 molecules. Phosphorus(V) oxide canbe prepared by burning phosphorus in aplentiful supply of oxygen. It readily com-bines with water to form phosphoric(V)acid and is therefore used as a drying agentfor gases. It is a useful dehydrating agentbecause it is able to remove the elements ofwater from compounds containing oxygenand hydrogen

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phosphorus(V) oxide

phosphorus oxychloride See phos-phorus(III) chloride oxide.

phosphorus pentabromide See phos-phorus(V) bromide.

phosphorus pentachloride See phos-phorus(V) chloride.

phosphorus pentoxide See phospho-rus(V) oxide.

phosphorus tribromide See phospho-rus(III) bromide.

phosphorus trichloride See phospho-rus(III) chloride.

phosphorus trichloride oxide Seephosphorus(III) chloride oxide.

phosphorylation The transfer of aphosphoryl group –PO(OH)2 from ATP toa protein by a protein kinase. Many meta-bolic enzymes are regulated by phosphory-lation as are several signal pathwaysinvolved in cell growth. The removal of thephosphoryl group (dephosphorylation) isbrought about by enzymes known as phos-phatases.

phosphoryl chloride See phospho-rus(III) chloride oxide.

photochemical reaction A reactionbrought about by light or ultraviolet radia-tion; examples include the bleaching of col-ored material, the reduction of silverhalides (in photography), and the photo-synthesis of carbohydrates. Chemicalchanges occur only when the reactingatoms or molecules absorb photons of theappropriate energy to produce excitedspecies or when the photons have sufficientenergy to produce free radicals or ions. Theamount of substance that reacts is propor-tional to the quantity of energy absorbed.For example, in the reaction between hy-drogen and chlorine, it is not the concen-trations of hydrogen or chlorine thatdictate the rate of reaction but the intensityof the radiation.

photochemistry The branch of chem-istry dealing with reactions induced bylight or ultraviolet radiation.

photoelectron spectroscopy See photo-ionization.

photoemission The emission of photo-electrons by the photoelectric effect or byphotoionization.

photoionization The ionization ofatoms or molecules by electromagnetic ra-diation. Photons absorbed by an atom mayhave sufficient photon energy to free anelectron from its attraction by the nucleus.The process is

M + hv → M+ + e–

As in the photoelectric effect, the radia-tion must have a certain minimum thresh-old frequency. The energy of thephotoelectrons ejected is given by W = hv –I, where I is the ionization potential of theatom or molecule. Analysis of the energiesof the emitted electrons gives informationon the ionization potentials of the sub-stance – a technique known as photoelec-tron spectroscopy.

photolysis A chemical reaction that isproduced by electromagnetic radiation(light or ultraviolet radiation). Many pho-tolytic reactions involve the formation offree radicals. See also flash photolysis.

photosynthesis The synthesis of or-ganic compounds using light energy ab-sorbed by chlorophyll. With the exceptionof a small group of bacteria, organismsphotosynthesize from inorganic materials.All green plants photosynthesize as well ascertain prokaryotes (some bacteria). Ingreen plants, photosynthesis takes place inchloroplasts, mainly in leaves. Directly orindirectly, photosynthesis is the source ofcarbon and energy for all except chemoau-totrophic organisms. The mechanism iscomplex and involves two sets of stages:light reactions followed by dark reactions.The overall reaction in green plants can besummarized by the equation:

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172

CO2 + 4H2O → [CH2O] + 3H2O + O2

In the light reactions, light energy is ab-sorbed by chlorophyll (and other pig-ments), setting off a chain of chemicalreactions in which water is split andgaseous oxygen evolved. The hydrogenfrom the water is attached to other mol-ecules, and used to reduce carbon dioxideto carbohydrates in the later dark reac-tions. These involve a complex cycle of re-actions (the Calvin cycle) in which sugarphosphates are formed.

See electron-transport chain; photosyn-thetic pigments.

photosynthetic pigments Pigments thatabsorb the light energy required in photo-synthesis. They are located in the chloro-plasts of plants and algae, whereas in mostphotosynthetic bacteria they are located inthylakoid membranes, typically distributedaround the cell periphery. All photosyn-thetic organisms contain chlorophylls andcarotenoids; some also contain phyco-bilins. Chlorophyll a is the primary pig-ment since energy absorbed by this is useddirectly to drive the light reactions of pho-tosynthesis. The chlorophyll a that formsthe reaction center of photosystem II hasan absorption peak at 680 nm and that ofphotosystem I at 700 nm. The other pig-ments (chlorophylls b, c, and d, and thecarotenoids and phycobilins) are accessorypigments that pass the energy they absorbon to chlorophyll a. They broaden thespectrum of light used in photosynthesis.See absorption spectrum.

phthalic acid See benzene-1,2-dicar-boxylic acid.

phthalic anhydride See benzene-1,2-dicarboxylic acid.

phycobilins A group of accessory pho-tosynthetic pigments found in Cyanobacte-ria and red algae. Chemically they arelinear tetrapyrroles in contrast to chloro-phyll, which is a cyclic tetrapyrrole. Theyabsorb light in the middle of the spectrumnot absorbed by chlorophyll, an importantfunction in algae living under water where

blue and red light are absorbed in the sur-face layers. They comprise the blue phyco-cyanins, which absorb extra orange andred light, and the red phycoerythrins,which absorb green light, enabling redalgae to grow at depth in the sea. See alsoabsorption spectrum; photosynthetic pig-ments.

phycocyanin A photosynthetic pig-ment. See phycobilins.

phycoerythrin A photosynthetic pig-ment. See phycobilins.

phylloquinone See vitamin K.

physical change A change to a sub-stance that does not alter its chemical prop-erties. Physical changes (e.g. melting,boiling, and dissolving) are comparativelyeasy to reverse.

physical chemistry The branch ofchemistry concerned with the physicalproperties of compounds and how thesedepend on the chemical bonding. It in-cludes such topics as chemical thermody-namics and electrochemistry.

physisorption See adsorption.

phytohormone See plant hormone.

pi bond See orbital.

pico- Symbol: p A prefix denoting 10–12.For example, 1 picofarad (pF) = 10–12

farad (F).

picrate A salt of picric acid; metal pi-crates are explosive. See also picric acid.

picric acid (2,4,6-trinitrophenol;C6H2(NO3)3OH) A yellow crystallinesolid made by nitrating phenolsulfonicacid. It is used as a dye and as an explosive.With aromatic hydrocarbons picric acidforms characteristic charge-transfer com-plexes (misleadingly called picrates), usedin analysis for identifying the hydrocarbon.

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picric acid

pine-cone oil See turpentine.

pi orbital See orbital.

pipette A device used to transfer aknown volume of solution from one con-tainer to another; in general, several sam-ples of equal volume are transferred forindividual analysis from one stock solu-tion. Pipettes are of two types, bulbpipettes, which transfer a known and fixedvolume, and graduated pipettes, which cantransfer variable volumes. Pipettes were atone time universally mouth-operated butsafety pipettes using a plunger or rubberbulb are now preferred.

pK The logarithm to the base 10 of thereciprocal of an acid’s dissociation con-stant:

log10(1/Ka)

Planck constant Symbol: h A funda-mental constant; the ratio of the energy(W) carried by a photon to its frequency(v). A basic relationship in the quantumtheory of radiation is W = hv. The value ofh is 6.626 196 × 10–34 J s. The Planck con-stant appears in many relationships inwhich some observable measurement isquantized (i.e. can take only specific dis-crete values rather than any of a range ofvalues).

plane polarization A type of POLARIZA-TION of electromagnetic radiation in whichthe vibrations take place entirely in oneplane.

plant hormone (phytohormone) Oneof a group of essential organic substancesproduced in plants. They are effective invery low concentrations and controlgrowth and development by their interac-tions. Examples are auxins, gibberellins,cytokinins, abscisic acid, and ethylene.

plastic A substance that can be shapedby heat and pressure. Most plastics aremade from synthetic POLYMERS, althoughsome are based on natural materials suchas cellulose. In a plastic, the synthetic resinis usually mixed with other substances

such as plasticizers, fillers, stabilizers, andcolorants. See also resin.

plasticizer A substance added to a syn-thetic resin to make it more flexible.

platinum A silvery-white malleable duc-tile transition metal. It occurs naturally inAustralia and Canada, either free or in as-sociation with other platinum metals. It isresistant to oxidation and is not attackedby acids (except aqua regia) or alkalis. Plat-inum is used as a catalyst for ammonia ox-idation (to make nitric acid), hydrocarboncracking, and in catalytic converters. It isalso used in jewelry.

Symbol: Pt; m.p. 1772°C; b.p. 3830 ±100°C; r.d. 21.45 (20°C); p.n. 78; r.a.m.195.08.

platinum black A finely divided blackform of platinum produced, as a coating,by evaporating platinum onto a surface inan inert atmosphere. Platinum-black coat-ings are used as absorbents and as cata-lysts.

Plexiglas (Trademark) A widely-usedacrylic resin, polymethylmethacrylate.

PM See particulate matter.

poison 1. A substance that destroys cat-alyst activity.2. Any substance that endangers biologicalactivity, whether by physical or chemicalmeans.

polar Describing a compound with mol-ecules that have a permanent dipole mo-ment. Hydrogen chloride and water areexamples of polar compounds.

polar bond A covalent bond in whichthe bonding electrons are not sharedequally between the two atoms. A bond be-tween two atoms of different electronega-tivity is said to be polarized in the directionof the more electronegative atom, i.e. theelectrons are drawn preferentially towardsthe atom. This leads to a small separationof charge and the development of a bonddipole moment as in, for example, hydro-

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pollution

gen fluoride, represented as H→F or asHδ+–Fδ– (F is more electronegative).

The charge separation is much smallerthan in ionic compounds; molecules inwhich bonds are strongly polar are said todisplay partial ionic character. The effectof the electronegative element can be trans-mitted beyond adjacent atoms, thus theC–C bonds in, for example, CCl3CH3 andCH3CHO are slightly polar. See also di-pole moment; intermolecular force.

polarimeter (polariscope) An instru-ment for measuring optical activity. Seeoptical activity.

polariscope See polarimeter.

polarizability The ease with which anelectron cloud is deformed (polarized). Inions, an increase in size or negative chargeleads to an increase in polarizability.

polarization 1. The restriction of thevibrations in a transverse wave so that thevibration occurs in a single plane. Electro-magnetic radiation, for instance, is a trans-verse wave motion. It can be thought of asan oscillating electric field and an oscillat-ing magnetic field, both at right angles tothe direction of propagation and at rightangles to each other. Usually, the electricvector is considered since it is the electricfield that interacts with charged particlesof matter and causes the effects. In ‘nor-mal’ unpolarized radiation, the electricfield oscillates in all possible directions per-pendicular to the wave direction. On re-flection or on transmission through certainsubstances (e.g. Polaroid) the field is con-fined to a single plane. The radiation isthen said to be plane-polarized. If the tip ofthe electric vector describes a circular helixas the wave propagates, the light is said tobe circularly polarized.2. See polarizability.

polar molecule A molecule in which theindividual polar bonds are not perfectlysymmetrically arranged and are thereforenot ‘in balance’. Thus the charge separa-tion in the bonds gives rise to an overallcharge separation in the molecule as, for

example, in water. Such molecules possessa DIPOLE MOMENT.

polarography An analytical method inwhich current is measured as a function ofpotential. A special type of cell is used inwhich there is a small easily polarizablecathode (the dropping mercury electrode)and a large non-polarizable anode (refer-ence cell). The analytical reaction takesplace at the cathode and is essentially a re-duction of the cations, which are dis-charged according to the order of theirelectrode potential values. The data is ex-pressed in the form of a polarogram, whichis a plot of current against applied voltage.As the applied potential is increased a pointis reached at which the ion is discharged.There is a step-wise increase in current,which levels off because of polarizationeffects. The potential at half the step height(called the half-wave potential) is used toidentify the ion. Most elements can be de-termined by polarography. The optimumconcentrations are in the range10–2–10–4M; modified techniques allowdeterminations in the parts per millionrange.

polar solvent See solvent.

pollution Any damaging or unpleasantchange in the environment that resultsfrom the physical, chemical, or biologicalside-effects of human industrial or socialactivities. Pollution can affect the atmos-phere, rivers, seas, and the soil.

Air pollution is caused by the domesticand industrial burning of carbonaceousfuels, by industrial processes, and by carexhausts. Among recent problems are in-dustrial emissions of sulfur(IV) oxide caus-ing acid rain, and the release into theatmosphere of chlorofluorocarbons, usedin refrigeration, aerosols, etc., has beenlinked to the depletion of ozone in thestratosphere. Carbon dioxide, produced byburning fuel and by car exhausts, is slowlybuilding up in the atmosphere, whichcould result in an overall increase in thetemperature of the atmosphere (green-house effect). Car exhausts also containcarbon monoxide and lead. The former has

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not yet reached dangerous levels, but vege-tation near main roads contains a high pro-portion of lead and levels are sufficientlyhigh in urban areas to cause concern aboutthe effects on children. Lead-free gasolineis widely available. Photochemical smog,caused by the action of sunlight on hydro-carbons and nitrogen oxides from car ex-hausts, is a problem in several countries.Catalytic converters reduce harmful emis-sions from car exhausts.

Water pollutants include those that arebiodegradable, such as sewage effluent,which cause no permanent harm if ade-quately treated and dispersed, as well asthose that are nonbiodegradable, such ascertain chlorinated hydrocarbon pesticides(e.g. DDT) and heavy metals, such as lead,copper, and zinc in some industrial efflu-ents (causing heavy-metal pollution).When these accumulate in the environmentthey can become very concentrated in foodchains. The pesticides DDT, aldrin, anddieldrin are now banned in many coun-tries. Water supplies can become pollutedby leaching of nitrates from agriculturalland. The discharge of waste heat can causethermal pollution of the environment, butthis is reduced by the use of cooling towers.In the sea, oil spillage from tankers and theinadequate discharge of sewage effluentare the main problems.

Other forms of pollution are noise fromaircraft, traffic, and industry and the dis-posal of radioactive waste.

polyamide A synthetic polymer inwhich the monomers are linked by thegroup –NH–CO–. Nylon is an example ofa polyamide.

polyamine An aliphatic compound whichhas two or more amino and/or iminogroups. Polyamines are often found associ-ated with DNA and RNA in bacteria andviruses. This may stabilize the nucleic acidmolecule in a way analogous to the actionof histones on DNA in eukaryote cells. Ex-amples of polyamines include spermine,spermidine, cadaverine, and putrescine.

polyatomic molecule A molecule thatconsists of several atoms (three or more).

Examples are benzene (C6H6) and methane(CH4).

polybasic acid An acid that has two ormore replaceable hydrogen atoms. For ex-ample, phosphorus(V) acid, H3PO4, is trib-asic.

polycarbonate A thermoplastic poly-mer consisting of polyesters of carbonicacid and dihydroxy compounds. Polycar-bonates are tough and transparent, usedfor making soft-drink bottles and electricalconnectors.

polychlorinated biphenyl (PCB) Atype of compound based on biphenyl(C6H5C6H5), in which some of the hydro-gen atoms have been replaced by chlorineatoms. They are used in certain polymersused for electrical insulators. PCBs arehighly toxic and concern has been causedby the fact that they can accumulate in thefood chain.

polychloroethene (polyvinyl chloride;PVC) A synthetic polymer made fromchloroethene. It is a strong material with awide variety of uses.

polycyclic Describing a compound thathas two or more rings in its molecules.

polyene An alkene with more than twodouble bonds in its molecules.

polyester A synthetic polymer made byreacting alcohols with acids, so that themonomers are linked by the group–O–CO–. Synthetic fibers such as Dacronare polyesters.

polyethene (polyethylene; polythene) Asynthetic polymer made from ethene. It isproduced in two forms – a soft material oflow density and a harder, higher densityform, which is more rigid. It can be madeby the ZIEGLER PROCESS and the PHILLIPS

PROCESS.

polyethylene See polyethene.

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polysaccharide

polyhydric alcohol An alcohol that hasseveral –OH groups in its molecules.

polymer A compound in which thereare very large molecules made up of re-peating molecular units (monomers). Apolymer has a repeated structural unit,known as a mer. Polymers do not usuallyhave a definite relative molecular mass, be-cause there are variations in the lengths ofdifferent chains. They may be natural sub-stances (e.g. polysaccharides and proteins)or synthetic materials (e.g. nylon and poly-ethene). The two major classes of syntheticpolymers are thermosetting (e.g. Bakelite)and thermoplastic (e.g. polyethene). Theformer are infusible, and heat may onlymake them harder, whereas the lattersoften on heating. See also polymerization.

polymerization The process in whichone or more compounds react to form aPOLYMER. Homopolymers are formed bypolymerization of one monomer (e.g. theformation of polyethene). Heteropolymersor copolymers come from two or moremonomers (as for nylon). Heteropolymersmay be of different types depending on thearrangement of units. An alternatingcopolymer of two units A and B has anarrangement:

–A–B–A–B–A–B–A block copolymer has an arrangement inwhich blocks of one monomer alternatewith blocks of the other; for example:

–A–A–A–B–B–B–A–A–A–In a graft copolymer there is a main choiceof one monomer (–A–A–A–A–), with shortside chains of the other monomer attachedat regular intervals (–B–B–).

Stereospecific polymers have the sub-unit repeated along the chain in a regular

way. These are tactic polymers. If one par-ticular group is always on the same side ofthe chain, the polymer is said to be isotac-tic. If the group alternates in position alongthe chain the polymer is syndiotactic. Ifthere is no regular pattern, the polymer isatactic.

Polymerization reactions are also classi-fied according to the type of reaction. Ad-dition polymerization occurs when themonomers undergo addition reactions,with no other substance formed. Conden-sation polymerization involves the elimina-tion of small molecules in the formation ofthe polymer. See also cross linkage.

polymethanal See methanal.

polymethylmethacrylate A transpar-ent ACRYLIC RESIN made by polymerizingmethyl methacrylate, trade name Plexi-glass.

polypeptide A PEPTIDE composed of alarge number of amino-acid units. PRO-TEINS are polypeptides containing a fewhundred amino-acid units.

polypropene (polypropylene) A syn-thetic polymer made from propene. It issimilar in properties to polyethene, butstronger and lighter. The propene is poly-merized by the Ziegler process.

polypropylene See polypropene.

polysaccharide A high-molecular-weightpolymer of a monosaccharide (see sugar).The polysaccharides contain many re-peated units in their molecular structures.They can be broken down to smaller polysaccharides, disaccharides, and mono-

isotactic syndiotactic

H H HH H H

H H HX X X

H H HH H H

HH

HX

XX

XH

Polymerization: examples of stereospecific polymers

saccharides by hydrolysis or by the appro-priate enzyme. Important polysaccharidesare heparin, inulin, starch, glycogen (some-times known as animal starch), and cellu-lose. See also carbohydrates; sugar.

polystyrene A synthetic polymer madefrom styrene (phenylethene). Expandedpolystyrene is a rigid foam used in packingand insulation.

polytetrafluoroethene (PTFE) A syn-thetic polymer made from tetrafluoro-ethene (i.e. CF2:CF2). It is able to withstandhigh temperatures without decomposingand also has a very low coefficient of fric-tion, hence its use in non-stick pans, bear-ings, etc.

polythene See polyethene.

polyunsaturated Describing a com-pound that has a number of C=C bonds inits compounds.

polyurethane A synthetic polymer con-taining the group –NH–CO–O– linking themonomers. Polyurethanes are made bycondensation of isocyanates (–NCO) withalcohols.

polyvinyl acetate (PVA; polyvinylethanoate) A thermoplastic polymermade by the polymerization of vinylethanoate, CH2:CHOOCH3. It is used as acoating for paper and cloth and as anadhesive.

polyvinyl chloride See polychloroethene.

polyvinyl ethanoate See polyvinyl ac-etate.

p orbital See orbital.

porphyrin Any of several cyclic organicstructures that have the important charac-teristic property of forming complexeswith metal ions. Examples of such metallo-porphyrins are the iron porphyrins (e.g.heme in hemoglobin) and the magnesiumporphyrin, chlorophyll, the photosyntheticpigment in plants. In nature, the majority

of metalloporphyrins are conjugated toproteins to form a number of very impor-tant molecules, e.g. hemoglobin, myoglo-bin, and the cytochromes.

potentiometric titration A titration inwhich an electrode is used in the reactionmixture. The end point can be found bymonitoring the electrode potential of thisduring the titration.

precipitate A suspension of small parti-cles of a solid in a liquid formed by a chem-ical reaction.

precursor A substance from which an-other substance is formed in a chemical re-action.

Prelog, Vladimir (1906–98) Yugoslav-born Swiss organic chemist. The early partof Prelog’s career was devoted to the struc-ture of the alkaloids. This resulted in thedetermination of the structure of anti-malarial alkaloids. He also corrected theformulae for strychnine alkaloids found bySir Robert ROBINSON. Prelog subsequentlybecame interested in the relation betweenstereochemistry and chemical reactivity ofring molecules with about 10 members ofthe ring. He showed that specific stereo-chemistry has an important effect on thecourse of chemical reactions. Togetherwith Sir Christopher INGOLD he found asystem for defining chirality of molecules.Prelog shared the 1975 Nobel Prize forchemistry with Sir John CORNFORTH for hiswork on the ‘stereochemistry of organicmolecules and reactions’.

pressure Symbol: p The pressure on asurface due to forces from another surfaceor from a fluid is the force acting at 90° tounit area of the surface:

pressure = force/areaThe unit is the pascal (Pa).

primary alcohol See alcohol.

primary amine See amine.

primary standard A substance that canbe used directly for the preparation of stan-

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dard solutions without reference to someother concentration standard. Primarystandards should be easy to purify, dry, ca-pable of preservation in a pure state, unaf-fected by air or CO2, of a high molecularweight (to reduce the significance of weigh-ing errors), stoichiometric, and readily sol-uble. Any likely impurities should be easilyidentifiable.

producer gas (air gas) A mixture of car-bon monoxide (25–30%), nitrogen (50–55%), and hydrogen (10–15%), preparedby passing air with a little steam through athick layer of white-hot coke in a furnaceor ‘producer’. The air gas is used while stillhot to prevent heat loss and finds uses inindustrial heating, for example the firing ofretorts and in glass furnaces. Comparewater gas.

projection formula See formula.

proline See amino acid.

promoter (activator) A substance thatimproves the efficiency of a catalyst. It doesnot itself catalyze the reaction but assiststhe catalytic activity.

proof A measure of the ethanol contentof intoxicating drinks. In the United States,proof spirit contains 40% ethanol by vol-ume. In the UK, it contains 49.28% ofethanol by weight (57.1% by volume at16°C). The degree of proof gives the num-ber of parts of proof spirit per 100 parts ofthe total. 100° of proof is 50% by volume,80° of proof is 0.8 × 50%, etc.

propanal (propionaldehyde, C2H5CHO)A colorless liquid aldehyde.

propane (C3H8) A gaseous alkane ob-tained either from the gaseous fraction ofcrude oil or by the cracking of heavier frac-tions. The principal use of propane is as afuel for heating and cooking, since it can beliquefied under pressure, stored in cylin-ders, and transported easily. Propane is thethird member of the homologous series ofalkanes.

propanedioic acid (malonic acid;CH2(COOH)2) A white crystalline di-basic carboxylic acid.

propane-1,2,3-triol (glycerol; glycerine;CH2(OH)CH(OH)CH2(OH)) A color-less viscous liquid obtained as a by-productfrom the manufacture of soap by the reac-tion of animal fats with sodium hydroxide.It is used as a solvent and plasticizer. Seealso glyceride.

propanoate A salt or ester of propanoicacid.

propanoic acid (propionic acid,C2H5COOH) A colorless liquid car-boxylic acid.

propanol Either of two alcohols:propan-1-ol (CH3CH2CH2OH) andpropan-2-ol (CH3CH2(OH)CH3). Both arecolorless volatile flammable liquids.

propanone (acetone; CH3COCH3) Acolorless liquid ketone, used as a solventand in the manufacture of methyl 2-methyl-propanoate (from which poly-methylmethacrylate is produced). Pro-panone is manufactured from propene, either by the air-oxidation of propan-2-olor as a by-product from the CUMENE

PROCESS.

propenal (acrolein; CH2:CHCHO) Acolorless liquid unsaturated aldehyde witha pungent odor. It can be polymerized tomake acrylic resins.

propene (propylene; C3H6) A gaseousalkene. Propene is not normally present inthe gaseous crude-oil fraction but can beobtained from heavier fractions by cat-alytic cracking. This is the principal indus-trial source. Propene is the organic startingmaterial for the production of propan-2-ol,required for the manufacture of propanone(acetone), and the starting material for theproduction of polypropene (polypropy-lene).

propenoate A salt or ester of propenoicacid.

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propenoate

propenoic acid (acrylic acid, CH2:CH-COOH) An unsaturated liquid car-boxylic acid with a pungent odor. The acidand its esters are used to make acrylicresins. See acrylic resin.

propenonitrile (acrylonitrile, H2C:-CH(CN)) An organic compound fromwhich acrylic-type polymers are produced.

propenyl group (allyl group) The or-ganic group, CH2=CH–CH2–, derived byremoving one hydrogen atom frompropene.

propionaldehyde See propanal.

propionic acid See propanoic acid.

propylene See propene.

propyl group The group CH3CH2CH2–.

prosthetic group The nonprotein com-ponent of a conjugated protein. Thus theheme group in hemoglobin is an exampleof a prosthetic group, as are the coenzymecomponents of a wide range of enzymes.

protamine One of a group of polypep-tides formed from a few amino acids. Theyare soluble in water, dilute acids, andbases. On heating they do not coagulate.When protamines are hydrolyzed theyyield a large proportion of basic aminoacids, particularly arginine, alanine, andserine. They occur in the sperm of verte-brates, packing the DNA into a condensedform.

protein One of a large number of sub-stances that are important in the structureand function of all living organisms. Pro-teins are polypeptides; i.e. they are madeup of AMINO ACID molecules joined to-gether by peptide links. Their molecularweight may vary from a few thousand toseveral million. About 20 amino acids arepresent in proteins. Simple proteins con-tain only amino acids. In conjugated pro-teins, the amino acids are joined to othergroups. Proteins may consist of one or sev-eral polypeptide chains.

The primary structure of a protein is theparticular sequence of amino acids present.The secondary structure is the way inwhich this chain is arranged; for example,coiled in an ALPHA HELIX or held in betapleated sheets. The secondary structure isheld by hydrogen bonds. The TERTIARY

STRUCTURE of the protein is the way inwhich the polypeptide chain is folded intoa three-dimensional structure. This may beheld by cystine bonds and by attractiveforces between atoms. A protein must havethe correct tertiary structure in order tofunction properly and abnormally foldedproteins can cause disease, e.g. priondiseases, Alzheimer’s. QUATERNARY STRUC-TURE is the interaction between polypep-tide chains (or subunits).

proteoglycan (mucoprotein) A type ofglycoprotein consisting of long branchedheterogeneous chains of glycosaminogly-can molecules linked to a protein core ofamino acids. Unlike more typical glyco-proteins, they have a greater carbohy-drate content, the protein core is rich inserine, and they have a higher molecularweight.

protic acid See acid.

proton An elementary particle with apositive charge (+1.602 192 × 10–19 C) andrest mass 1.672 614 × 10–27 kg. Protonsare nucleons, found in all nuclides.

proton number (atomic number) Symbol:Z The number of protons in the nucleusof an atom. The proton number determinesthe chemical properties of the element be-cause the electron structure, which deter-mines chemical bonding, depends on theelectrostatic attraction to the positivelycharged nucleus.

prussic acid See hydrocyanic acid.

pseudoaromatic See aromatic com-pound.

pseudo-first order Describing a reac-tion that appears to exhibit first-order ki-netics under special conditions, even

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180

though the ‘true’ order is greater than one.For example, in the hydrolysis of an esterin the presence of a large volume of water,the concentration of water remains ap-proximately constant. The rate of reactionis thus found experimentally to be propor-tional to the concentration of the ester only(even though it also depends on theamount of water present). Such a reactionis described as ‘bimolecular of the firstorder’.

pteroylglutamic acid See folic acid.

PTFE See polytetrafluoroethene.

purine A simple nitrogenous organicmolecule with a double ring structure.Members of the purine group includeadenine and guanine, which are con-stituents of the nucleic acids, and certainplant alkaloids, e.g. caffeine and theo-bromine.

putrescine An amine, H2N[CH2]4NH2,produced from ornithine in decaying meator fish.

PVA See polyvinyl acetate.

PVC See polychloroethene.

pyranose A sugar that has a six-mem-bered ring form (five carbon atoms and oneoxygen atom). See also sugar.

pyrazine (1,4-diazine; C4H4N2) A hetero-cyclic aromatic compound with a six-membered ring containing four carbonatoms and two nitrogen atoms.

pyrazole (1,2-diazole; C3H4N2) A hete-rocyclic crystalline aromatic compoundwith a five-membered ring containing threecarbon atoms and two nitrogen atoms.

pyrene (C16H10) A solid aromatic com-pound whose molecules consist of fourbenzene rings joined together. It is carcino-genic.

pyridine (C5H5N) An organic liquid offormula C5H5N. The molecules have ahexagonal planar ring and are isoelectronicwith benzene. Pyridine is an example of anaromatic heterocyclic compound, with theelectrons in the carbon–carbon pi bondsand the lone pair of the nitrogen delocal-ized over the ring of atoms. The compoundis extracted from coal tar and used as a sol-vent and as a raw material for organic syn-thesis.

pyridoxine (vitamin B6) One of thewater-soluble B-group of vitamins. Goodsources include yeast and certain seeds (e.g.wheat and corn), liver, and to a limited ex-tent, milk, eggs, and leafy green vegetables.There is also some bacterial synthesis of thevitamin in the intestine. Pyridoxine givesrise to a coenzyme involved in various as-pects of amino acid metabolism. See alsovitamin B complex.

pyrimidine A simple nitrogenous or-ganic molecule whose ring structure is con-tained in the pyrimidine bases cytosine,thymine, and uracil, which are constituentsof the nucleic acids, and in thiamine (vita-min B1).

pyrolysis The decomposition of chemi-

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pyrolysis

N

N

Pyrimidine

NNN

Pyridine

N

N

N

NH

1

3 9

7

Purine

cal compounds by subjecting them to veryhigh temperature.

pyrometer An instrument used in thechemical industry to measure high temper-ature, e.g. in reactor vessels.

pyrone (C5H4O2) A compound having asix-membered ring of five carbon atomsand one oxygen, with one of the carbonatoms attached to a second oxygen in acarbonyl group. The pyrone ring systemhas two forms depending on the position ofthe carbonyl relative to the oxygen heteroatom. It occurs in many natural products.

pyrrhole ((CH)4NH) A heterocyclic liq-

uid aromatic compound with a five-mem-bered ring containing four carbon atomsand one nitrogen atom. It has importantbiochemical derivatives, including chloro-phyll and heme.

pyruvate An intermediate in severalmetaoblic pathways, including glycolysisand gluconeogenesis, with the formulaCH3COCOO–. Pyruvate is also the precur-sor for the synthesis of the amino acids ala-nine, valine, and leucine.

pyruvic acid A carboxylic acid, CH3-COCOOH. The systematic name is 2-oxopropanoic acid.

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qualitative analysis Analysis carriedout with the purpose of identifying thecomponents of a sample. Classical meth-ods involved simple preliminary tests fol-lowed by a carefully devised scheme ofsystematic tests and procedures. Modernmethods include the use of a range of spec-troscopic techniques. Compare quantita-tive analysis.

quanta See quantum.

quantitative analysis Analysis carriedout with the purpose of determining theconcentration of one or more componentsof a sample. Classical wet methods includevolumetric and gravimetric analysis. Awide range of instrumental techniques arealso used, including polarography and var-ious types of chromatography and spec-troscopy. Compare qualitative analysis.

quantized Describing a physical quan-tity that can take only certain discrete val-ues, and not a continuous range of values.Thus, in an atom or molecule the electronsaround the nucleus can have certain ener-gies, E1, E2, etc., and cannot have interme-diate values. Similarly, in atoms andmolecules, the electrons have quantizedvalues of spin angular momentum and or-bital angular momentum.

quantum (plural quanta) A definiteamount of energy released or absorbed in aprocess. Energy often behaves as if it were‘quantized’ in this way. The quantum ofelectromagnetic radiation is the photon.

quantum mechanics See quantumtheory.

quantum number An integer or half in-teger that specifies the value of a quantizedphysical quantity (energy, angular momen-tum, etc.). See atom.

quantum states States of an atom, elec-tron, particle, etc., specified by a unique setof quantum numbers. For example, the hy-drogen atom in its ground state has an elec-tron in the K shell specified by the fourquantum numbers: n = 1, l = 0, m = 0, ms =½. In the helium atom there are two elec-trons:

n = 1, l = 0, m = 0, ms = ½n = 1, l = 0, m = 0, ms = –½

quantum theory A mathematicaltheory originally introduced by the Ger-man physicist Max Planck (1848–1947) toexplain the radiation emitted from hotbodies. Quantum theory is based on theidea that energy (or certain other physicalquantities) can be changed only in certaindiscrete amounts for a given system. Otherearly applications were the explanations ofthe photoelectric effect and the Bohrtheory of the atom.

Quantum mechanics is a system of me-chanics that developed from quantumtheory and is used to explain the behaviorof atoms, molecules, etc. In one form it isbased on de Broglie’s idea that particles canhave wavelike properties – this branch ofquantum mechanics is called wave me-chanics. See orbital.

quantum yield The number of reactiveevents per absorbed photon in a photo-chemical reaction.

quaternary ammonium compound Acompound formed from an amine by addi-

Q

quaternary structure

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tion of a proton to produce a positive ion.Quaternary compounds are salts, the sim-plest example being ammonium com-pounds formed from ammonia and anacid, for example:

NH3 + HCl → NH4+Cl–

Other amines can also add protons to giveanalogous compounds. For instance,methylamine (CH3NH2) forms the com-pound

[CH3NH3]+X–

where X– is an acid radical.The formation of quarternary com-

pounds occurs because the lone pair on thenitrogen atom can form a coordinate bondwith a proton. This can also occur withheterogeneous nitrogen compounds, suchas adenine, cytosine, thymine, and gua-nine. Such compounds are known as ni-trogenous bases. See also amine salt.

quaternary structure See protein.

quinhydrone See quinone.

quinine A poisonous ALKALOID found inthe bark of the cinchona tree of SouthAmerica. It was used in treating malaria.

quinol See benzene-1,4-diol.

quinoline (C9H7N) A colorless two-ringheterocyclic compound with an unpleasantodor, which acts as a base and forms saltswith acids. First made from the alkaloidquinine, it is found in bone oil and coal tar and used for making drugs and dye-stuffs.

quinone (cyclohexadiene-1,4-dione; ben-zoquinone; C6H4O2) A yellow crystallineorganic compound with a pungent odor.Its molecules contain a nonaromatic six-carbon ring and it behaves as an unsatu-rated diketone with conjugated doublebonds. It is used in making dyestuffs. Thesystematic name is cyclohexadiene-1,4-dione. A platinum electrode in an equi-molar solution of quinone and hydro-quinone (benzene-1,4-diol; C6H4(OH)2) isused as a standard electrode in electro-chemistry. The reaction is:

C6H4(OH)2 ˆ C6H4O2 + 2H+ + 2e

This type of electrode is called a quinhy-drone electrode.

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racemate See optical activity.

racemic mixture See optical activity.

racemization The conversion of an op-tical isomer into an equal mixture of enan-tiomers, which is not optically active. Seeoptical activity.

rad A unit of absorbed dose of ionizingradiation, defined as being equivalent to anabsorption of 10–2 joule of energy in onekilogram of material.

radian Symbol: rad The SI unit of planeangle; 2π radian is one complete revolution(360°).

radiation In general the emission of en-ergy from a source, either as waves (light,sound, etc.) or as moving particles (betarays or alpha rays).

radical 1. See free radical.2. A group of atoms in a molecule. See alsofunctional group.

radioactive Describing an element ornuclide that exhibits natural radioactivity.

radioactive dating (radiometric dating)A technique for dating archaeological spec-imens, rocks, etc., by measuring the extentto which some radionuclide has decayed togive a product.

radioactive decay See radioactivity.

radioactivity The disintegration of cer-tain unstable nuclides with emission of ra-diation. The emission of alpha particles,beta particles, and gamma rays are the

three most important forms of radiationthat occur. The process by which one nu-clide changes to another is radioactivedecay.

radiocarbon dating See carbon dating.

radiochemistry The chemistry of radio-active isotopes of elements. Radiochem-istry involves such topics as thepreparation of radioactive compounds, theseparation of isotopes by chemical reac-tions, the use of radioactive LABELS in stud-ies of mechanisms, and experiments on thechemical reactions and compounds oftransuranic elements.

radiogenic Caused by radioactivedecay.

radioisotope A radioactive isotope ofan element. Tritium, for instance, is a ra-dioisotope of hydrogen. Radioisotopes areextensively used in research as souces of ra-diation and as tracers in studies of chemi-cal reactions. Thus, if an atom in acompound is replaced by a radioactive nu-clide of the element (a label) it is possible tofollow the course of the chemical reaction.Radioisotopes are also used in medicine fordiagnosis and treatment.

radiolysis A chemical reaction pro-duced by high-energy radiation (x-rays,gamma rays, or particles).

radiometric dating See radioactive dat-ing.

radio waves A form of electromagneticradiation with wavelengths greater than a

R

raffinate

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few millimeters. See also electromagneticradiation.

raffinate The liquid remaining after thesolvent extraction of a dissolved substance.See solvent extraction.

raffinose A SUGAR occurring in sugarbeet. It is a trisaccharide consisting of fruc-tose, galactose, and glucose units.

r.a.m. See relative atomic mass.

Raman effect A change in the fre-quency of electromagnetic radiation, suchas light, which occurs when a photon of ra-diation undergoes an inelastic collisionwith a molecule. This type of scattering iscalled Raman scattering, in contrast tonormal (Rayleigh) scattering. The intensityof Raman scattering is much smaller(about 1/1000) than the intensity ofRayleigh scattering. The Raman effect wasfirst observed by the Indian physicists SirChandrasekhara Venkata Raman (1888–1970) and his colleague Sir Karia-manikkam Srinivasa Krishnan in 1928,having been predicted theoretically on thebasis of quantum mechanics by HendrikAnton Kramers and Werner Heisenberg in1925. The Raman effect has been used ex-tensively in Raman spectroscopy for thedetermination of molecular structure, par-ticularly since then advent of the laser. Insome cases it is possible to draw conclu-sions about molecular structure by observ-ing whether certain lines are present orabsent.

Raney nickel A catalytic form of nickelproduced by treating a nickel–aluminumalloy with caustic soda. The aluminum dis-solves (as aluminate) and Raney nickel isleft as a spongy mass, which is pyrophoricwhen dry. It is used especially for catalyz-ing hydrogenation reactions. It was discov-ered by the US chemist M. Raney in 1927.

Raoult’s law A relationship betweenthe pressure exerted by the vapor of a solu-tion and the presence of a solute. It statesthat the partial vapor pressure of a solventabove a solution (p) is proportional to the

mole fraction of the solvent in the solution(X) and that the proportionality constant isthe vapor pressure of pure solvent, (p0), atthe given temperature: i.e. p = p0X. Solu-tions that obey Raoult’s law are said to beideal. There are some binary solutions forwhich Raoult’s law holds over all values ofX for either component. Such solutions aresaid to be perfect and this behavior occurswhen the intermolecular attraction be-tween molecules within one component isalmost identical to the attraction of mol-ecules of one component for molecules ofthe other (e.g. chlorobenzene and bro-mobenzene). Because of solvation forcesthis behavior is rare and in general Raoult’slaw holds only for dilute solutions.

For solutions that are ideal but not per-fect the solute behavior is similar in thatthe partial pressure of the solute, ps, is pro-portional to the mole fraction of the solute,Xs, but in this case the proportionality con-stant, p′, is not the vapor pressure of thepure solute but must be determined exper-imentally for each system. This soluteequivalent of Raoult’s law has the form ps = p′Xs, and is called HENRY’S LAW. Be-cause of intermolecular attractions p′ isusually less than p0. It is named for theFrench chemist François-Marie Raoult(1830–1901), who formulated it in 1882.

Rashig process A method for the man-ufacture of chlorobenzene, and thence phenol, from benzene. Benzene vapor, hy-drogen chloride, and air are passed over acopper(II) chloride catalyst (230°C):

2C6H6 + 2HCl + O2 → 2C6H5Cl +2H2O

The chlorobenzene is converted to phenolby reaction with water over a silicon cata-lyst (425°C):

C6H5Cl + H2O → HCl + C6H5OHIt is named for the German chemist FritzRashig (1863–1928).

rate constant (velocity constant; specificreaction rate) Symbol: k The constant ofproportionality in the rate expression for achemical reaction. For example, in a reac-tion A + B → C, the rate may be propor-tional to the concentration of A multipliedby that of B; i.e.

rate = k[A][B]where k is the rate constant for this partic-ular reaction. The constant is independentof the concentrations of the reactants butdepends on temperature; consequently thetemperature at which k is recorded must bestated. The units of k vary depending onthe number of terms in the rate expression,but are easily determined rememberingthat rate has the units s–1.

rate-determining step (limiting step)The slowest step in a multistep reaction.Many chemical reactions are made up of anumber of steps in which the one with thelowest rate is the one that determines therate of the overall process.

The overall rate of a reaction cannot ex-ceed the rate of the slowest step. For exam-ple, the first step in the reaction betweenacidified potassium iodide solution and hy-drogen peroxide is the rate-determiningstep:

H2O2 + I– → H2O + OI– (slow)H+ + OI– → HOI (fast)

HOI + H+ + I– → I2 + H2O (fast)

rate of reaction A measure of theamount of reactant consumed in a chemi-cal reaction in unit time. It is thus a meas-ure of the number of effective collisionsbetween reactant molecules. The rate atwhich a reaction proceeds can be measuredby the rate the reactants disappear or bythe rate at which the products are formed.The principal factors affecting the rate ofreaction are temperature, pressure, concen-tration of reactants, light, and the action ofa catalyst. The units usually used to meas-ure the rate of a reaction are mol dm–3 s–1.See also mass action, law of.

rationalized units A system of units inwhich the equations have a logical form re-lated to the shape of the system. SI unitsform a rationalized system of units. In itformulae concerned with circular symme-try contain a factor of 2π; those concernedwith radial symmetry contain a factor of4π.

raw material A substance from whichother substances are made. In the chemical

industry it may be simple (such as nitrogenfrom air used to make ammonia) or com-plex (such as coal and petroleum, used tomake a wide range of products).

rayon An artificial fiber formed fromwood pulp (cellulose). There are two types.Viscose rayon is made by dissolving the cel-lulose in carbon disulfide and sodium hy-droxide. The solution is forced through afine nozzle into an acid bath, which regen-erates the fibers. Acetate rayon is made bydissolving cellulose acetate in an organicsolvent, and forcing the solution through anozzle. The solvent is evaporated, and thecellulose acetate thus obtained as fibers.

reactant A compound taking part in aCHEMICAL REACTION.

reaction See chemical reaction.

reactive dye A dye that sticks to thefibers of a fabric by forming covalentchemical bonds with the substance of thefabric. The dyes used to color the cellulosefibers in rayon are examples of reactivedyes.

reagent A compound that reacts withanother (the substrate). The term is oftenalso used for common laboratory chemi-cals – sodium hydroxide, hydrochloricacid, etc. – used for experiment and analy-sis.

rearrangement A reaction in which thegroups of a compound rearrange them-selves to form a different compound. Anexample is the BECKMANN REARRANGE-MENT.

reciprocal proportions, law of Seeequivalent proportions, law of.

recrystallization The repeated crystal-lization of a compound to ensure purity ofthe sample.

rectified spirit A constant-boiling mix-ture of ethanol and water that containsabout 6% water; no more water can be re-

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rectified spirit

moved by further distillation. It is used asan industrial solvent.

redox Relating to the process of oxida-tion and reduction, which are intimatelyconnected in that during oxidation bychemical agents the oxidizing agent itselfbecomes reduced, and vice versa. Thus anoxidation process is always accompaniedby a reduction process. In electrochemicalprocesses this is equally true, oxidationtaking place at the anode and reduction atthe cathode. These systems are often calledredox systems, particularly when the inter-est centers on both compounds.

Oxidizing and reducing power is in-dicated quantitatively by the redox poten-tial or standard electrode potential, EŠ.Redox potentials are normally expressedas reduction potentials. They are obtainedby electrochemical measurements and thevalues are referred to the H+/H2 couple forwhich EŠ is set equal to zero. Thus in-creasingly negative potentials indicate in-creasing ease of oxidation or difficulty ofreduction. Thus in a redox reaction the halfreaction with the most positive value of EŠ

is the reduction half and the half reactionwith the least value of EŠ (or most highlynegative) becomes the oxidation half.

reducing agent See reduction.

reduction The gain of electrons byatoms, molecules, ions, etc. It often in-volves the loss of oxygen from a com-pound, or addition of hydrogen. Reductioncan be effected chemically, i.e. by the use ofreducing agents (electron donors), or elec-trically, in which case the reductionprocess occurs at the cathode. See alsoredox.

refining The process of removing impu-rities from a substance or of extracting asubstance from a mixture. The refining ofPETROLEUM is a particularly important in-dustrial process.

refluxing The process of boiling a liquidin a vessel connected to a condenser, sothat the condensed liquid runs back intothe vessel. By using a reflux condenser, the

liquid can be maintained at its boilingpoint for long periods of time, withoutloss. The technique is a standard method ofcarrying out reactions in organic chem-istry.

reforming The cyclization of straight-chain hydrocarbons from crude oil byheating under pressure with a catalyst, usu-ally platinum on alumina. For example, themanufacture of methylbenzene from hep-tane:

C7H16 → C6H11CH3This first step is the production of methylcyclohexane, which then loses six hydro-gen atoms to give methylbenzene:

C6H11CH3 → C6H5CH3 + 3H2See also steam reforming.

Regnault’s method A method used forthe determination of the density of gases. Abulb of known volume is evacuated andweighed then the gas is admitted at aknown pressure (from a vacuum line) andthe bulb weighed again. The temperature isalso noted and the data corrected to STP.The method is readily applicable to the de-termination of approximate relative mo-lecular masses of gaseous samples. It isnamed for the French chemist Henri VictorRegnault (1810–78).

relative atomic mass (r.a.m.) Symbol: ArThe ratio of the average mass per atom ofthe naturally occurring element to 1/12 ofthe mass of an atom of nuclide 12C. It wasformerly called atomic weight

relative density Symbol: d The ratio ofthe density of a given substance to the den-sity of some reference substance. The rela-tive densities of liquids are usuallymeasured with reference to the density ofwater at 4°C. Relative densities are alsospecified for gases; usually with respect toair at STP. The temperature of the sub-stance is stated or is understood to be20°C. Relative density was formerly calledspecific gravity.

relative molecular mass Symbol: Mr The ratio of the average mass per moleculeof the naturally occurring form of an el-

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ement or compound to 1/12 of the mass ofan atom of nuclide 12C. This was formerlycalled molecular weight. It does not have tobe used only for compounds that have dis-crete molecules; for ionic compounds (e.g.NaCl) and giant-molecular structures (e.g.BN) the formula unit is used.

relaxation The process by which an ex-cited species loses energy and falls to alower energy level (such as the groundstate).

rem (radiation equivalent man) A unitfor measuring the effects of radiation doseon the human body. One rem is equivalentto an average adult male absorbing one radof radiation. The biological effects dependon the type of radiation as well as the en-ergy deposited per kilogram.

resin A yellowish insoluble organic com-pound exuded by trees as a viscous liquidthat gradually hardens on exposure to airto form a brittle amorphous solid. Syn-thetic resins are artificial polymers used inmaking adhesives, insulators, and paints.See also rosin.

resolution The separation of a racemateinto the two optical isomers. This cannotbe done by normal methods, such as crys-tallization or distillation, because the iso-mers have identical physical properties.The main methods are:1. Mechanical separation. Certain opti-cally active compounds form crystals withdistinct left- and right-handed shapes. Thecrystals can be sorted by hand.2. Chemical separation. The mixture is re-acted with an optical isomer. The productsare then not optical isomers of each other,and can be separated by physical means.For instance, a mixture of D- and L-formsof an acid, acting with a pure L-base, pro-duces two salts that can be separated byfractional crystallization and then recon-verted into the acids.3. Biochemical separation. Certain organiccompounds can be separated by using bac-teria that feed on one form only, leavingthe other.

resonance (mesomerism) The behaviorof many compounds cannot be adequatelyexplained by a single structure using simplesingle and double bonds. The bonding elec-trons of the compound have a different dis-tribution in the molecules. The actualbonding in the molecule can be regarded asa hybrid of two or more conventionalforms of the molecule, called resonanceforms or canonical forms. The result is aresonance hybrid. For example, the car-bonyl group in a ketone has negativecharge on the oxygen atom. It can be de-scribed as a resonance hybrid, somewherebetween =C=O, in which a pair of electronsis shared between the C and the O, and=C+–O–, in which the electrons are local-ized on the O atom. Note that the twocanonical forms do not contribute equallyin the hybrid. The bonding of benzene canbe represented by a resonance hybrid oftwo Kekulé structures and, to a lesser ex-tent, three Dewar structures. It is conven-tional to represent a resonance hybrid bytwo or more conventional structures joinedby double-headed arrows, as in:

R2C=O ↔ R2C+O–

The double-headed arrow should not beconfused with the equilibrium symbol(ˆ). The two forms are not in equilibriumbut represent different classical structuresthat contribute to the actual structure.

resonance ionization spectroscopy(RIS) A type of spectroscopy that detectsspecific types of atoms using lasers. Thelaser ionizes the atoms of interest. The fre-quency of the laser is chosen so that onlythe atoms of interest in a sample are ex-cited by the laser. This method is very se-lective because ionization only occurs forthose atoms those whose energy levels fit inwith the frequency of the laser light. Thisselectivity has led to many practical usesfor this technique.

resorcinol See benzene-1,3-diol.

respiration The oxidation of organicmolecules to provide energy in plants andanimals. In animals, food molecules arerespired, but autotrophic plants respiremolecules that they have themselves syn-

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respiration

thesized by photosynthesis. The energyfrom respiration is used to attach a high-energy phosphate group to ADP to formthe short-term energy carrier ATP, whichcan then be used to power energy-requiringprocesses within the cell. The actual chem-ical reactions of respiration are known asinternal (cell or tissue) respiration and theynormally require oxygen from the environ-ment (aerobic respiration). Some organ-isms are able to respire, at least for a shortperiod, without the use of oxygen (an-aerobic respiration), although this processproduces far less energy than aerobic respi-ration, e.g. 38 molecules of ATP are gener-ated for each molecule of glucose oxidizedin aerobic respiration, compared with only2 in anerobic respiration. Respiration usu-ally involves an exchange of gases with theenvironment; this is known as external res-piration. In small animals and all plants ex-change by diffusion is adequate, but largeranimals generally have special respiratoryorgans with large moist and ventilated sur-faces (e.g. lungs, gills) and there is often acirculatory system to transport gases inter-nally to and from the respiratory organs.See also electron-transport chain; glycoly-sis; Krebs cycle.

respiratory chain The electron-trans-port chain in aerobic respiration.

respiratory pigments Colored com-pounds that can combine reversibly withoxygen. HEMOGLOBIN is the blood pigmentin all vertebrates and a wide range of in-vertebrates. Other blood pigments, such ashaemoerythrin (containing iron) and he-mocyanin (containing copper), are foundin lower animals, and in many cases aredissolved in the plasma rather than presentin cells. Their affinity for oxygen is compa-rable with hemoglobin, though oxygen ca-pacity is generally lower.

retinal (retinene) An aldehyde derivativeof retinol (vitamin A). Retinal is a con-stituent of the light-sensitive conjugatedprotein, rhodopsin, which occurs in therod cells of the retina. See rhodopsin.

retinene See retinal.

retinol See vitamin A.

retort A piece of laboratory apparatusconsisting of a glass bulb with a long nar-row neck. In industrial chemistry, variousmetallic vessels in which distillations or re-actions take place are called retorts.

retrosynthetic analysis A techniquefor planning the synthesis of an organicmolecule. The structure of the molecule isconsidered and it is divided into imaginaryparts, which could combine by known re-actions. These disconnections of the mol-ecule suggest charged fragments, known assynthons, which give a guide to possiblereagents for the synthesis.

reversible change In thermodynamics,a change in the pressure, volume, or otherproperties of a system, in which the systemremains at equilibrium throughout thechange. Such processes could be reversed;i.e. returned to the original starting posi-tion through the same series of stages. Theyare never realized in practice. An isother-mal reversible compression of a gas, for ex-ample, would have to be carried outinfinitely slowly and involve no friction,etc. Ideal energy transfer would have totake place between the gas and the sur-roundings to maintain a constant tempera-ture.

In practice, all real processes are irre-versible changes in which there is not anequilibrium throughout the change. In anirreversible change, the system can still bereturned to its original state, but notthrough the same series of stages. For aclosed system, there is always an entropyincrease involved in an irreversible change.

reversible reaction A chemical reactionthat can proceed in either direction. An ex-ample is the reaction of an acid with an al-cohol to form an ester and water:

R1COOH + R2OH ˆ R1COOR2 +H2O

In general, there will be an equilibriummixture of reactants and products.

respiratory chain

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rotary dryers

RF value The distance traveled by agiven component divided by the distancetravelled by the solvent front in chro-matography.

rheology The study of the ways inwhich matter can flow. This topic is of par-ticular interest in the study of polymers.

riboflavin (vitamin B2) One of thewater-soluble B-group of vitamins. It isfound in cereal grains, peas, beans, liver,kidney, and milk. Riboflavin is a con-stituent of several enzyme systems (flavo-proteins), acting as a coenzyme forhydrogen transfer in the reactions cat-alyzed by these enzymes. Two forms ofphosphorylated riboflavin are known toexist in various enzyme systems: FMN(flavin mononucleotide) and FAD (flavinadenine dinucleotide). See also vitamin Bcomplex.

ribonucleic acid See RNA.

ribose (C5H10O5) A monosaccharideSUGAR. It rarely occurs naturally in the freestate but ribose is an important componentof RNA. The derivative compound, de-oxyribose, is a component of DNA.

ring A closed loop of atoms in a mol-ecule, as in benzene or cyclohexane. Afused ring is one joined to another ring insuch a way that they share two atoms.Naphthalene is an example of a fused-ringcompound.

ring closure A reaction in which onepart of an open chain in a molecule reactswith another part, so that a ring of atomsis formed. For example, an amino group onone end of a long molecule can react witha carboxyl group on the other end to forma cyclic amide containing the –NH.CO–group (see lactam). Another example ofring closure is the conversion of thestraight-chain form of a SUGAR into thecyclic form. There are many examples ofring-closure reactions in organic chemistry.

RNA (ribonucleic acid) A nucleic acidfound mainly in the cytoplasm and in-

volved in protein synthesis. It is a singlepolynucleotide chain similar in composi-tion to a single strand of DNA except thatthe sugar ribose replaces deoxyribose andthe pyrimidine base uracil replacesthymine. RNA is synthesized on DNA inthe nucleus and exists in three forms. Incertain viruses, RNA is the genetic ma-terial.

Robinson, Sir Robert (1886–1975)British organic chemist. Sir Robert Robin-son made many important contributions toexperimental and theoretical organicchemistry. His experimental work con-cerned plant products, particularly the al-kaloids. Robinson won the 1947 NobelPrize for chemistry for this work. His theo-retic work concerned the role of electronsin organic reactions. He originated the rep-resentation of electronic transfer by curlyarrows. In particular, he applied theseideas for benzene and its chemical reac-tions using concepts such as electrophilicreagents and nucleophilic reagents.

roentgen Symbol: R A unit of radiation,used for x-rays and gamma rays, defined interms of the ionizing effect on air. Oneroentgen induces 2.58 × 10–4 coulomb ofcharge in one kilogram of dry air. The unitis named for the German physicist W. K.Roentgen (1845–1923).

rosin A brittle yellow or brown resinthat remains after the distillation of tur-pentine. It is used as a flux in soldering andin making paints and varnishes. Powderedrosin gives a ‘grip’ to violin bows and box-ers’ shoes. See resin.

rotamer See conformation.

rotary dryers Devices commonly usedin the chemical industry for the drying,mixing, and sintering of solids. They con-sist essentially of a rotating inclined cylin-der, which is longer in length than indiameter. Gases flow through the cylinderin either a countercurrent or cocurrent di-rection to regulate the flow of solids, whichare fed into the end of the cylinder. Rotary

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dryers can be applied to both batch andcontinuous processes.

R-S convention See optical activity.

rubber A natural or synthetic polymeric

elastic material. Natural rubber is a poly-mer of methylbuta-1,3-diene (isoprene).Various synthetic rubbers are made bypolymerization; for example CHLOROPRENE

rubber (from 2-chlorobuta-1,3-diene) andsilicone rubbers. See also vulcanization.

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Sabatier, Paul (1854–1941) French or-ganic chemist. Starting in 1897, Sabatierperformed experiments in which he studiedthe hydrogenation of ethene when it ispassed over nickel. When nickel is finely di-vided it catalyses this reaction. It alsocatalyses other hydrogenation reactionssuch as the conversion of benzene into cy-clohexane. Sabatier explained his results interms of chemisorption of unstable com-pounds onto the surface of a catalyst andproduced evidence for this idea. He dis-cussed his findings in the book Catalysis inOrganic Chemistry (1912). Sabatier sharedthe 1912 Nobel Prize with François GRIG-NARD for his work on catalysis.

Sabatier–Senderens process A methodof hydrogenating unsaturated vegetableoils to make margarine, using hydrogenand a nickel catalyst. It is named for theFrench chemists Paul Sabatier and Jean-Baptiste Senderens (1856–1937). See alsohardening.

saccharide See sugar.

saccharin (C7H5NO3S) A white crys-talline organic compound used as an artifi-cial sweetener; it is about 550 times assweet as sugar (sucrose). It is nearly insolu-ble in water and so generally used in theform of its sodium salt. Possible links withcancer in animals has restricted its use insome countries.

saccharose See sucrose.

Sachse reaction A process for the man-ufacture of ethyne (acetylene) from naturalgas (methane). Part of the methane isburned in two stages to raise the furnace

temperature to about 1500°C. Under theseconditions, the rest of the methane is con-verted to ethyne and hydrogen:

2CH4 → C2H2 + 3H2The process is important because it pro-vides a source of ethyne from readily avail-able natural gas, thus avoiding theexpensive carbide process.

salicylate (hydroxybenzoate) A salt orester of salicylic acid.

salicylic acid (2-hydroxybenzoic acid;C6H4(OH)COOH) A crystalline aro-matic carboxylic acid. It is used in medi-cines, as an antiseptic, and in themanufacture of azo dyes. Its ethanoyl(acetyl) ester is aspirin. See also aspirin;methyl salicylate.

Sandmeyer reaction A method of pro-ducing chloro- and bromo-substituted de-rivatives of aromatic compounds by usingthe DIAZONIUM SALT with a copper halide.The reaction starts with an amine, which isdiazotized at low temperature by using hy-drochloric acid and sodium nitrite (to pro-duce nitrous acid, HNO2). For example:

C6H5NH2 + NaNO2 + HCl →C6H5N2

+ + Cl– + OH– + Na+ + H2OThe copper halide (e.g. CuCl) acts as a cat-alyst to give the substituted benzene deriv-ative:

C6H5N2+ + Cl– → C6H5Cl + N2

This reaction was discovered by the Ger-man chemist Traugott Sandmeyer (1854–1922) in 1884. A variation of the reaction,in which the catalyst is freshly precipitatedcopper powder, was reported in 1890 bythe German chemist Ludwig Gatterman(1860–1920). This is known as the Gatter-

S

man reaction (or Gatterman–Sandmeyerreaction).

sandwich compound A type of com-plex formed between transition-metal ionsand aromatic compounds, in which themetal ion is ‘sandwiched’ between therings. The bonding is between the d or-bitals of the metal and the pi electrons onthe ring. Four-, five-, six-, seven-, andeight-membered rings are known to com-plex with a number of elements includingV, Cr, Mn, Co, Ni, and Fe. Ferrocene(Fe(C5H5)2) is the best-known example.Compounds of this type are also known asmetallocenes.

Sanger, Frederick (1918– ) Britishbiochemist. Sanger is one of the very fewpeople to win two Nobel Prizes. He wasawarded the 1958 Nobel Prize for chem-istry for determining the sequence ofamino acids in the insulin of cows. Thiswork started in 1943. By 1945 he had dis-covered a compound now known asSanger’s reagent which attaches itself tothe amino acids, and breaks up the proteinchain. After the discovery of the structureof DNA in 1953 Sanger and his colleaguesstarted to investigate the sequence of thenucleotides in DNA. Sanger and his col-leagues developed techniques for splittingup the DNA. In 1977 they announced thecomplete sequence of more than 5000 nu-cleotides of the DNA of a bacterial virus.This resulted in Sanger sharing the 1980Nobel Prize for chemistry with Paul Bergand Walter Gilbert.

saponification The process of hy-drolyzing an ester with a hydroxide. Thecarboxylic acid forms a salt. For instance,with sodium hydroxide:

R1COOR2 + NaOH → NaOOCR1 +R2OH

The saponification of fats, which are estersof propane-1,2,3-triol (glycerol) with long-chain carboxylic acids, forms soaps.

saturated compound An organic com-pound that does not contain any double ortriple bonds in its structure. A saturatedcompound will undergo substitution reac-

tions but not addition reactions since eachatom in the structure will already have itsmaximum possible number of singlebonds. Compare unsaturated compound.

saturated solution A solution that con-tains the maximum equilibrium amount ofsolute at a given temperature. A solution issaturated if it is in equilibrium with itssolute. If a saturated solution of a solid iscooled slowly, the solid may stay tem-porarily in solution; i.e. the solution maycontain more than the equilibrium amountof solute. Such solutions are said to besupersaturated.

saturated vapor A vapor that is in equi-librium with the solid or liquid. A satu-rated vapor is at the maximum pressure(the saturated vapor pressure) at a giventemperature. If the temperature of a satu-rated vapor is lowered, the vapor con-denses. Under certain circumstances, thesubstance may stay temporarily in thevapor phase; i.e. the vapor contains morethan the equilibrium concentration of thesubstance. The vapor is then said to besupersaturated.

scavenger A compound or chemicalspecies that removes a trace componentfrom a system or that removes a reactiveintermediate from a reaction.

Schiff’s base A type of compoundformed by reacting an aldehyde or ketone(e.g. RCOR) with an aryl amine (e.g.ArNH2). The product, an N-arylimide,which is usually crystalline, has the for-mula R2C:NAr. Schiff’s bases were for-merly used to identify aldehydes andketones (by forming the crystalline baseand measuring its melting point). They arenamed after the German organic chemistHugo Schiff (1834–1915).

Schiff’s reagent An aqueous solution ofmagenta dye decolorized by reduction withsulfur(IV) oxide (sulfur dioxide; SO2). It isa test for aldehydes and ketones. Aliphaticaldehydes restore the color quickly;aliphatic ketones and aromatic aldehydesslowly; aromatic ketones give no reaction.

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195

sequence rule

Schultze’s solution A solution of zincchloride, potassium iodide, and iodineused mainly for testing for cellulose andhemicellulose. Both materials stain a bluecolor with the reagent, that of hemicellu-lose being weaker.

scleroprotein One of a group of pro-teins obtained from the exoskeletal struc-tures of animals. They are insoluble inwater, salt solutions, dilute acids, and al-kalis. This group exhibits a wide range ofboth physical and chemical properties.Typical examples of scleroproteins are ker-atin (hair), elastin (elastic tissue), and col-lagen (connective tissue).

SCP See single-cell protein.

scrubber A part of an industrial chemi-cal plant that removes impurities from agas by passing it through a liquid.

secondary alcohol See alcohol.

secondary amine See amine.

secondary structure See protein.

second-order reaction A reaction inwhich the rate of reaction is proportionalto the product of the concentrations of twoof the reactants or to the square of the con-centration of one of the reactants:

rate = k[A][B]or

rate =k[A]2

For example, the hydrolysis by dilutealkali of an ester is a second-order reaction:

rate = k[ester][alkali]The rate constant for a second-order re-

action has the units mol–1 dm3 s–1. Unlike afirst-order reaction, the time for a definitefraction of the reactants to be consumed isdependent on the original concentrations.

sedimentation The settling of a suspen-sion, either under gravity or in a centrifuge.The speed of sedimentation can be used toestimate the average size of the particles.This technique is used with an ULTRA-CENTRIFUGE to find the relative molecularmasses of macromolecules.

seed A small crystal added to a gas orliquid to assist solidification or precipita-tion from a solution. The seed, usually acrystal of the substance to be formed,enables particles to pack into predeter-mined positions so that a larger crystal canform.

self-assembly See supramolecular chem-istry.

Seliwanoff’s test A test for ketose SUG-ARS in solution. The reagent used consistsof benzene-1,3-diol (resorcinol) dissolvedin hydrochloric acid. A few drops areadded to the solution and a red precipitateindicates a ketonic sugar. The test is namedfor the Russian chemist F. F. Seliwanoff.

semicarbazide See semicarbazone.

semicarbazone A type of organic com-pound containing the C:N.NH.CO.NH2grouping, formed by reaction of an ALDE-HYDE or KETONE with semicarbazide(H2N.NH.CO.NH2). The compounds arecrystalline solids with sharp meltingpoints, which were formerly used to char-acterize the original aldehyde or ketone.

semipermeable membrane A mem-brane that, when separating a solutionfrom a pure solvent, permits the solventmolecules to pass through but does notallow the transfer of solute molecules. Syn-thetic semipermeable membranes are gen-erally supported on a porous material,such as unglazed porcelain or fine wirescreens, and are commonly formed of cel-lulose or related materials. They are used inosmotic studies, gas separations, and med-ical applications.

Equilibrium is reached at a semiperme-able membrane if the chemical potentialson both sides become identical; migrationof solvent molecules towards the solutionis an attempt by the system to reach equi-librium. The pressure required to halt thismigration is the OSMOTIC PRESSURE.

semipolar bond A coordinate bond.

sequence rule See CIP system.

serine See amino acid.

serotonin (hydroxytryptamine) A sub-stance that serves as a neurohormone thatacts on muscles and nerves, and a neuro-transmitter found in both the central andperipheral nervous systems. It controls di-lation and constriction of blood vessels andaffects peristalsis and gastrointestinal tractmotility. Within the brain it plays a role inmood behavior. Many hallucinogenic com-pounds (e.g. LSD) antagonize the effects ofserotonin in the brain.

sesquiterpene See terpene.

shell A group of electrons that share thesame principal quantum number. Earlywork on x-ray emission studies used theterms K, L, M, and these are still some-times used for the first three shells: n = 1,K-shell; n = 2, L-shell; n = 3, M-shell.

shikimic acid pathway The mainmetabolic pathway in the biosynthesis ofthe aromatic amino acids tyrosine, pheny-lalanine, and tryptophan. The first step iserythrose-4-phosphate, which comes fromthe pentose phosphate pathway, condens-ing with phosphenolpyruvate, from glycol-ysis. This product cyclizes to shikimate,which is then converted by various path-ways into the amino acids.

short period See period.

SI See SI units.

side chain In an organic compound, analiphatic group or radical attached to alonger straight chain of atoms in an acycliccompound or to one of the atoms in thering of a cyclic compound. See also chain.

side reaction A chemical reaction thattakes place to a limited extent at the sametime as a main reaction. Thus the mainproduct of a reaction may contain smallamounts of other compounds.

siemens (mho) Symbol: S The SI unit ofelectrical conductance, equal to a conduc-tance of one ohm–1. The unit is named for

the German physicist Ernst Werner vonSiemens (1816–92).

sievert Symbol: Sv The SI unit of doseequivalent. It is the dose equivalent whenthe absorbed dose produced by ionizing ra-diation multiplied by certain dimensionlessfactors is 1 joule per kilogram (1 J kg–1).The dimensionless factors are used to mod-ify the absorbed dose to take account of thefact that different types of radiation causedifferent biological effects. The unit isnamed for the Swedish physicist Rolf Siev-ert (1896–1966).

sigma bond See orbital.

sigmatropic rearrangement See peri-cyclic reaction.

silica gel A gel made by coagulatingsodium silicate sol. The gel is dried by heat-ing and used as a catalyst support and as adrying agent. The silica gel used in desicca-tors and in packaging to remove moistureis often colored with a cobalt salt to indi-cate whether it is still active (blue = dry;pink = moist).

silicones Polymeric synthetic siliconcompounds containing chains of alternat-ing silicon and oxygen atoms, with organicgroups bound to the silicon atoms. Sili-cones are used as lubricants and water re-pellants and in waxes and varnishes.Silicone rubbers are superior to naturalrubbers in their resistance to both high andlow temperatures and chemicals.

silver-mirror test A test for the alde-hyde group. A few drops of the sample arewarmed with TOLLEN’S REAGENT. An alde-hyde reduces Ag+ to silver metal, causing abrilliant silver mirror to coat the insidewall of the tube.

single bond A covalent bond betweentwo elements that involves one pair of elec-trons only. It is represented by a single line,for example H–Br, and is usually a sigmabond, although it can be a pi bond. Com-pare multiple bond. See also orbital.

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single-cell protein (SCP) Protein pro-duced from microorganisms, such as bac-teria, yeasts, mycelial fungi, and unicellularalgae, used as food for man and other ani-mals.

singlet state See carbene.

SI units (Système International d’Unités)The internationally adopted system ofunits used for scientific purposes. It hasseven base units and two dimensionlessunits, formerly called supplementary units.Derived units are formed by multiplicationand/or division of base units. Standard pre-fixes are used for multiples and submulti-ples of SI units. The SI system is a coherentrationalized system of units.

SN1 reaction See nucleophilic substitu-tion.

SN2 reaction See nucleophilic substitu-tion.

SNG Substitute (or synthetic) naturalgas. A mixture of hydrocarbons manufac-tured from coal or the naphtha fraction ofpetroleum for use as a fuel.

soap A substance consisting of sodiumor potassium compounds of fatty acidsused to improve the cleansing properties ofwater. Soap is a SURFACTANT and was theearliest known DETERGENT. The basicmethod of making soap involved treatinganimal fat (mainly beef tallow), which is atriglyceride of octadecanoic acid (stearicacid), with caustic soda (sodium hydrox-ide; NaOH) to produce sodium octade-canoate (sodium stearate). More refinedsoaps are made from vegetable oils, such aspalm oil, which contains hexadecanoicacid (palmitic acid). Liquid soaps (softsoap) are made using potassium hydroxiderather than sodium hydroxide. Othermetal salts of long-chain carboxylic acidsare also known as ‘soaps’.

soft soap See soap.

sol A COLLOID consisting of solid parti-cles distributed in a liquid medium. A wide

variety of sols are known; the colors oftendepend markedly on the particle size. Theterm aerosol is used for solid or liquidphases dispersed in a gaseous medium.

solid The state of matter in which theparticles occupy fixed positions, giving thesubstance a definite shape. The particlesare held in these positions by bonds. Threekinds of attraction fix the positions of theparticles: ionic, covalent, and intermolecu-lar. Since these bonds act over short dis-tances the particles in solids are packedclosely together. The strengths of thesethree types of bonds are different and so,therefore, are the mechanical properties ofdifferent solids.

solid solution A solid composed of twoor more substances mixed together at themolecular level. Atoms, ions, or moleculesof one component in the crystal are at lat-tice positions normally occupied by theother component.

solubility The amount of one substancethat could dissolve in another to form asaturated solution under specified condi-tions of temperature and pressure. Solubil-ities are stated as moles of solute per 100grams of solvent or as mass of solute perunit volume of solvent.

solubility product Symbol: Ks If anionic solid is in contact with its saturatedsolution, there is a dynamic equilibriumbetween solid and solution:

AB(s) ˆ A+(aq) + B–(aq)The equilibrium constant for this is givenby

[A+][B–]/[AB]The concentration of undissolved solid[AB] is also constant, so

Ks = [A+][B–]Ks is the solubility product of the salt (at agiven temperature). For a salt A2B3, for in-stance:

Ks = [A+]2[B–]3, etc.Solubility products are meaningful only forsparingly soluble salts. If the product ofions exceeds the solubility product, precip-itation occurs.

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solubility product

198

BASE AND DIMENSIONLESS SI UNITS

Physical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr

*supplementary units

DERIVED SI UNITS WITH SPECIAL NAMES

Physical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Ωelectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert Sv

DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS

Submultiple Prefix Symbol Multiple Prefix Symbol10–1 deci- d 101 deca- da10–2 centi- c 102 hecto- h10–3 milli- m 103 kilo- k10–6 micro- µ 106 mega- M10–9 nano- n 109 giga- G10–12 pico- p 1012 tera- T10–15 femto- f 1015 peta- P10–18 atto- a 1018 exa- E10–21 zepto- z 1021 zetta- Z10–24 yocto- y 1024 yotta- Y

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specific reaction rate

solute A material that is dissolved in asolvent to form a solution.

solution A liquid system of two or morespecies that are intimately dispersed withineach other at a molecular level. The systemis therefore totally homogeneous. Themajor component is called the solvent(generally liquid in the pure state) and theminor component is called the solute (gas,liquid, or solid).

The process occurs because of a directintermolecular interaction of the solventwith the ions or molecules of the solute.This interaction is called SOLVATION. Partof the energy of this interaction appears asa change in temperature on dissolution. Seealso heat of solution; solid solution; solu-bility.

solvation The attraction of a solutespecies (e.g. an ion) for molecules of sol-vent. In water, for example, a positive ionwill be surrounded by water molecules,which tend to associate around the ion be-cause of attraction between the positivecharge of the ion, and the negative part ofthe polar water molecule. The energy ofthis solvation (hydration in the case ofwater) is the ‘force’ needed to overcome theattraction between positive and negativeions when an ionic solid dissolves. The at-traction of the dissolved ion for solventmolecules may extend for several layers.

solvent A liquid capable of dissolvingother materials (solids, liquids, or gases) toform a solution. The solvent is generallythe major component of the solution. Sol-vents can be divided into classes, the mostimportant being:Polar. A solvent in which the moleculespossess a moderate to high dipole momentand in which polar and ionic compoundsare easily soluble. Polar solvents are usu-ally poor solvents for nonpolar com-pounds. For example, water is a goodsolvent for many ionic species, such assodium chloride or potassium nitrate, andpolar molecules, such as the sugars, butdoes not dissolve paraffin wax.Nonpolar. A solvent in which the mol-ecules do not possess a permanent dipole

moment and consequently will solvatenonpolar species in preference to polarspecies. For example, benzene and tetra-chloromethane are good solvents for io-dine and paraffin wax, but do not dissolvesodium chloride.Amphiprotic. A solvent that undergoesself-ionization and can act both as a protondonator and as an acceptor. Water is agood example and ionizes according to:

2H2O = H3O+ + OH–

Aprotic. A solvent that can neither acceptnor yield protons. An aprotic solvent istherefore the opposite to an amphiproticsolvent.

solvent extraction (liquid–liquid extrac-tion) A method of removing a substancefrom solution by shaking it with and dis-solving it in another (better) solvent that isnot miscible with the original solvent.

solvent naphtha See naphtha.

solvolysis A reaction between a com-pound and the solvent in which it is dis-solved. See also hydrolysis.

s orbital See orbital.

sorbitol (HOCH2(CHOH)4CH2OH) Ahexahydric alcohol that occurs in rose hipsand rowan berries. It can be synthesized bythe reduction of glucose. Sorbitol is used tomake vitamin C (ascorbic acid) and surfac-tants. It is also used in medicines and as asweetener (particularly in foods for diabet-ics). It is an isomer of mannitol.

sorption Absorption of gases by solids.

specific Denoting a physical quantityper unit mass. For example, volume (V) perunit mass (m) is called specific volume:

V = VmIn certain physical quantities the term

does not have this meaning: for example,specific gravity is more properly called rel-ative density.

specific gravity See relative density.

specific reaction rate See rate constant.

specific rotatory power Symbol: αm The rotation of plane-polarized light in de-grees produced by a 10 cm length of solu-tion containing 1 g of a given substance permilliliter of stated solvent. The specific ro-tatory power is a measure of the optical ac-tivity of substances in solution. It ismeasured at 20°C using the D-line ofsodium.

spectra See spectrum.

spectral line A particular wavelength oflight emitted or absorbed by an atom, ion,or molecule. See line spectrum.

spectral series A group of related linesin the absorption or emission spectrum ofa substance. The lines in a spectral seriesoccur when the transitions all occur be-tween one particular energy level and a setof different levels.

spectrograph An instrument for pro-ducing a photographic record of a spec-trum.

spectrographic analysis A method ofanalysis in which the sample is excited elec-trically (by an arc or spark) and emits radi-ation characteristic of its componentatoms. This radiation is passed through aslit, dispersed by a prism or a grating, andrecorded as a spectrum, either photograph-ically or photoelectrically. The photo-graphic method was widely used forqualitative and semiquantitative work butphotoelectric detection also allows widequantitative application.

spectrometer 1. An instrument for ex-amining the different wavelengths presentin electromagnetic radiation. Typically,spectrometers have a source of radiation,which is collimated by a system of lensesand/or slits. The radiation is dispersed by aprism or grating, and recorded photo-graphically or by a photocell. There aremany types for producing and investigat-ing spectra over the whole range of theelectromagnetic spectrum. Often spec-trometers are called spectroscopes.

2. Any of various other instruments for an-alyzing the energies, masses, etc., of parti-cles. See mass spectrometer.

spectrophotometer A form of spec-trometer able to measure the intensity ofradiation at different wavelengths in aspectrum, usually in the visible, infrared,or ultraviolet regions.

spectroscope An instrument for exam-ining the different wavelengths present inelectromagnetic radiation. See also spec-trometer.

spectroscopy 1. The production andanalysis of spectra. There are many spec-troscopic techniques designed for investi-gating the electromagnetic radiationemitted or absorbed by substances. Spec-troscopy, in various forms, is used foranalysis of mixtures, for identifying anddetermining the structures of chemicalcompounds, and for investigating energylevels in atoms, ions, and molecules. In thevisible and longer wavelength ultraviolet,transitions correspond to electronic energylevels in atoms and molecules. The shorterwavelength ultraviolet corresponds totransitions in ions. In the x-ray region,transitions in the inner shells of atoms orions are involved. The infrared region cor-responds to vibrational changes in mol-ecules, with rotational changes at longerwavelengths.2. Any of various techniques for analysingthe energy spectra of beams of particles orfor determining mass spectra.

spectrum (plural spectra) 1. A range ofelectromagnetic radiation emitted or ab-sorbed by a substance under particular cir-cumstances. In an emission spectrum, lightor other radiation emitted by the body isanalyzed to determine the particular wave-lengths produced. The emission of radia-tion may be induced by a variety ofmethods; for example, by high tempera-ture, bombardment by electrons, absorp-tion of higher-frequency radiation, etc. Inan absorption spectrum a continuous flowof radiation is passed through the sample.The radiation is then analyzed to deter-

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201

states of matter

mine which wavelengths are absorbed. Seealso band spectrum; continuous spectrum;line spectrum.2. In general, any distribution of a prop-erty. For instance, a beam of particles mayhave a spectrum of energies. A beam ofions may have a mass spectrum (the distri-bution of masses of ions). See mass spec-trometer.

spin A property of certain elementaryparticles whereby the particle acts as if itwere spinning on an axis; i.e. it has an in-trinsic angular momentum. Such particlesalso have a magnetic moment. In a mag-netic field the spins line up at an angle tothe field direction and precess around thisdirection. Certain definite orientations tothe field direction occur such that msh/2π isthe component of angular momentumalong this direction. ms is the spin quantumnumber, and for an electron it has values+1/2 and –1/2. h is the Planck constant.

spontaneous combustion The self-ignition of a substance that has a low igni-tion temperature. It occurs when slow oxi-dation of the substance (such as a heap ofdamp straw or oily rags) builds up suffi-cient heat for ignition to take place.

stabilization energy The difference inenergy between the delocalized structureand the conventional structure for a com-pound. For example, the stabilization en-ergy of benzene is 150 kJ per mole, whichrepresents the difference in energy betweena Kekulé structure and the delocalizedstructure: the delocalized form being oflower energy is therefore more stable. Thestabilization energy can be determined bycomparing the experimental value for theheat of hydrogenation of benzene with thatcalculated for Kekulé benzene from bond-energy data.

stabilizer A substance added to preventchemical change (i.e. a negative catalyst).

staggered conformation See confor-mation.

standard solution A solution that con-

tains a known weight of the reagent in adefinite volume of solution. A standardflask or volumetric flask is used for thispurpose. The solutions may be prepared bydirect weighing for primary standards. Ifthe reagent is not available in a pure formor is deliquescent the solution must bestandardized by titration against anotherknown standard solution. See primarystandard.

standard state The standard conditionsused as a reference system in thermody-namics: pressure is 101 325 Pa; tempera-ture is 25°C (298.15 K); concentration is 1mol. The substance under investigationmust also be pure and in its usual state,given the above conditions.

standard temperature An internation-ally agreed value for which many measure-ments are quoted. It is the meltingtemperature of water, 0°C (273.15 K). Seealso STP.

starch A polysaccharide that occurs ex-clusively in plants. Starches are extractedcommercially from maize, wheat, barley,rice, potatoes, and sorghum. The starchesare storage reservoirs for plants; they canbe broken down by enzymes to simple sug-ars and then metabolized to supply energyneeds. Starch is a dietary component of an-imals.

Starch is not a single molecule but amixture of two glucose polymers: amylose(water-soluble, blue color with iodine) andamylopectin (not water-soluble, violetcolor with iodine). The composition isamylose 10–20%, amylopectin 80–90%.

states of matter The three physical con-ditions in which substances occur: solid,liquid, and gas. The addition or removal ofenergy (usually in the form of heat) enablesone state to be converted into another.

The major distinctions between thestates of matter depend on the kinetic ener-gies of their particles and the distances be-tween them. In solids, the particles havelow kinetic energy and are closely packed;in gases they have high kinetic energy andare very loosely packed; kinetic energy and

separation of particles in liquids are inter-mediate.

Solids have fixed shapes and volumes,i.e. they do not flow, like liquids and gases,and they are difficult to compress. In solidsthe atoms or molecules occupy fixed posi-tions in space. In most cases there is a reg-ular pattern of atoms – the solid iscrystalline.

Liquids have fixed volumes (i.e. lowcompressibility) but flow to take up theshape of the container. The atoms or mol-ecules move about at random, but they arequite close to one another and the motionis hindered.

Gases have no fixed shape or volume.They expand spontaneously to fill the con-tainer and are easily compressed. The mol-ecules have almost free random motion.

A plasma is sometimes considered to bea fourth state of matter.

stationary phase See chromatography.

Staudinger, Hermann (1881–1965)German organic chemist. Staudinger was apioneer of polymer chemistry. It wasStaudinger who introduced the word‘macromolecule’ into chemistry in 1922.He proposed that polymers are very longmolecules held together by ordinary chem-ical bonds. This correct view was not uni-versally accepted when it was first putforward but within a decade or sodecisive evidence emerged that confirmedStudinger’s view. In a book entitled Macro-molecular Chemistry and Biology (1947)he anticipated some of the key features ofmolecular biology. Staudinger won the1953 Nobel Prize for chemistry for his fun-damental contributions to polymer chem-istry.

steam distillation A method of isolat-ing or purifying substances by exploitingDalton’s law of partial pressures to lowerthe boiling point of the mixture. When twoimmiscible liquids are distilled, the boilingpoint will be lower than that of the morevolatile component and consequently willbe below 100°C if one component is water.The method is particularly useful for re-covering materials from tarry mixtures.

steam reforming The conversion of amethane–steam mixture at 900°C with anickel catalyst into a mixture of carbonmonoxide and hydrogen. The mixture ofgases (synthesis gas) provides a startingmaterial in a number of processes, e.g. themanufacture of methanol.

stearate A salt or ester of stearic acid; anoctadecanoate.

stearic acid See octadecanoic acid.

step An elementary stage in a chemicalreaction, in which energy may be trans-ferred from one molecule to another,bonds may be broken or formed, or elec-trons may be transferred.

steradian Symbol: sr The SI unit of solidangle. The surface of a sphere, for example,subtends a solid angle of 4π at its center.The solid angle of a cone is the area inter-cepted by the cone on the surface of asphere of unit radius.

stereochemistry The branch of chem-istry concerned with the shapes of mol-ecules and the way these affect theirchemical properties.

stereoisomerism See isomerism.

stereospecific Describing a chemical re-action that results in a particular arrange-ment of atoms in space.

stereospecific polymer See polymer-ization.

steric effect An effect in which theshape of a molecule influences its reac-tions. A particular example occurs in mol-ecules containing large groups, whichhinder the approach of a reactant (sterichindrance).

steric hindrance See steric effect.

steroid Any member of a group of com-pounds having a complex basic ring struc-ture. Examples are corticosteroidhormones (produced by the adrenal gland),

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sex hormones (progesterone, androgens,and estrogens), bile acids, and sterols (suchas cholesterol). See also anabolic steroid;sterol.

sterol A steroid with long aliphatic sidechains (8–10 carbons) and at least one hy-droxyl group. They are lipid-soluble andoften occur in membranes (e.g. cholesteroland ergosterol).

still An apparatus for distillation.

stoichiometric coefficient See chemi-cal equation.

stoichiometry The proportions inwhich elements form compounds. A stoi-chiometric compound is one in which theatoms have combined in small whole num-bers.

STP (NTP) Standard temperature andpressure. Conditions used internationallywhen measuring quantities that vary withboth pressure and temperature (such as thedensity of a gas). The values are 101 325Pa (approximately 100 kPa) and 0°C(273.15 K).

straight chain See chain.

Strecker synthesis A method of synthe-sizing amino acids. Hydrogen cyanideforms an addition product (cyanohydrin)with aldehydes:

RCHO + HCN → RCH(CN)OHWith ammonia further substitution occurs:

RCH(CN)OH + NH3 →RCH(CN)(NH2) + H2O

Acid hydrolysis of the cyanide group thenproduces the amino acid

RCH(CN)(NH2) →RCH(COOH)(NH2)

The method is a general synthesis for α-amino acids (with the –NH2 and –COOHgroups on the same carbon atom).

strong acid An ACID that is almost com-pletely dissociated into its component ionsin solution.

strong base A base that is completely oralmost completely dissociated into its com-ponent ions in solution. See acid.

structural formula See formula.

structural isomerism See isomerism.

styrene See phenylethene.

sublimate A solid formed by sublima-tion.

sublimation The conversion of a solidinto a vapor without the solid first melting.For instance (at standard pressure) iodine,solid carbon dioxide, and ammonium chlo-ride sublime. At certain conditions of ex-ternal pressure and temperature anequilibrium can be established between thesolid phase and vapor phase.

subshell A subdivision of an electronshell. It is a division of the orbitals thatmake up a shell into sets of orbitals, whichare degenerate (i.e. have the same energy)in the free atom. For example, in the sec-ond shell (the L shell) there are the 2s and2p subshells.

substituent An atom or group substi-tuted for another in a compound. Often theterm is used for groups that have replacedhydrogen in organic compounds. For ex-ample, in chlorobenzene (C6H5Cl) chlorinecan be regarded as a substituent.

substitution reaction A reaction inwhich an atom or group of atoms in an or-ganic molecule is replaced by another atomor group. The substitution of a hydrogenatom in an alkane by a chlorine atom is anexample. Substitution reactions fall intothree major classes depending upon the na-ture of the attacking substituent.Nucleophilic substitution: the attackingsubstituent is a nucleophile (i.e. a moleculeor ion that can donate electrons). Such re-actions are very common with alcoholsand halogen compounds, in which the elec-tron-deficient carbon atom attracts the nu-cleophile and the leaving group readilyexists alone. Examples are the hydrolysis

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substitution reaction

of a haloalkane and the chlorination of analcohol:

C2H5Cl + OH– → C2H5OH + Cl–

C2H5OH + HCl → C2H5Cl + H2OElectrophilic substitution: the attackingsubstituent is an electrophile (i.e. a mol-ecule or ion that accepts electrons). Suchreactions are common in aromatic com-pounds, in which the electron-rich ring at-tracts the electrophile. The nitration ofbenzene in which the electrophile is NO2

+

is an example:C6H6 + NO2

+ → C6H5NO2 + H+

Free-radical substitution: a free radical isthe attacking substituent. Such reactionscan be used with compounds that are inertto either nucleophiles or electrophiles, forinstance the halogenation of an alkane:

CH4 + Cl2 → CH3Cl + HClThe term ‘substitution’ is very general

and several reactions that can be consid-ered as substitutions are more normallygiven special names (e.g. esterification,hydrolysis, and nitration). See also elec-trophilic substitution; nucleophilic substi-tution.

substrate A material that is acted on bya catalyst.

succinic acid See butanedioic acid.

sucrose (cane sugar; saccharose;C12H22O11) A SUGAR that occurs in manyplants. It is extracted commercially fromsugar cane and sugar beet. Sucrose is a di-saccharide formed from a glucose unit anda fructose unit. It is hydrolyzed to a mix-ture of fructose and glucose by the enzymeinvertase. Since this mixture has a differentoptical rotation (levorotatory) than theoriginal sucrose, the mixture is called in-vert sugar.

sugar (saccharide) One of a class ofsweet-tasting simple carbohydrates. Sugarshave molecules consisting of a chain of car-bon atoms with –OH groups attached, andeither an aldehyde or ketone group. Theycan exist in a chain form or in a ringformed by reaction of the ketone or alde-hyde group with an OH group to form acyclic hemiacetal (see acetal). Monosac-charides are simple sugars that cannot behydrolyzed to sugars with fewer carbonatoms. Two or more monosaccharide unitscan be linked in disaccharides, trisaccha-rides, etc., by a GLYCOSIDIC LINK.

Monosaccharides are also classified ac-cording to the number of carbon atoms: apentose has five carbon atoms and a hex-

substrate

204

CH2OH

OOO

OH

HOCH2

OH

HOOO

OOO

H

OHHO

H

H

H

H

H

HOCH2OH

CH2OH CH2OHOOO OOO

HO

HO

HO

HO

H

H

H

H

OHOH

H

H

H

H

HOH

H

OH

α–D–glucose β–D–glucoseGlucose — a monosaccharide sugar

Sucrose — a disaccharide sugar

205

sulfonic acid

ose six. Monosaccharides with aldehydegroups are aldoses; those with ketonegroups are ketoses. Thus, an aldohexose isa hexose with an aldehyde group; a ke-topentose is a pentose with a ketone group,etc.

The ring forms of monosaccharides arederived by reaction of the aldehyde or ke-tone group with one of the carbons at theother end of the chain. It is possible to havea six-membered (pyranose) ring or a five-membered (furanose) ring. See alsoanomer; carbohydrate; polysaccharide. Seeillustration overleaf.

sugar acid An acid formed from amonosaccharide by oxidation. Oxidationof the aldehyde group (CHO) of the aldosemonosaccharides to a carboxyl group(COOH) gives an aldonic acid; oxidation

of the primary alcohol group (CH2OH) toCOOH yields uronic acid; oxidation ofboth the primary alcohol and carboxylgroups gives an aldaric acid. The uronicacids are biologically important, beingcomponents of many polysaccharides, forexample glucuronic acid (from glucose) is amajor component of gums and cell walls,while galacturonic acid (from galactose)makes up pectin. Ascorbic acid or vitaminC is an important sugar acid found univer-sally in plants, particularly in citrus fruits.

sugar alcohol (alditol) An alcohol de-rived from a monosaccharide by reductionof its carbonyl group (CO) so that eachcarbon atom of the sugar has an alcoholgroup (OH). For example, glucose yieldssorbitol, common in fruits, and mannoseyields mannitol.

sulfa drug See sulfonamide.

sulfate A salt or ester of sulfuric(VI) acid.

sulfide See thioether.

sulfite A salt or ester of sulfurous acid.

sulfonamide A type of organic com-pound with the general formulaR.SO2.NH2. Sulfonamides, which areamides of sulfonic acids, are active againstbacteria, and some are used in pharmaceu-ticals (‘sulfa drugs’).

sulfonate A salt or ester of a SULFONIC

ACID.

sulfonation A reaction introducing the–SO2OH (sulfonic acid) group into an or-ganic compound. Sulfonation of aromaticcompounds is usually accomplished by re-fluxing with concentrated sulfuric acid forseveral hours. The attacking species is SO3(sulfur trioxide; sulfur(VI) oxide) and thereaction is an example of electrophilic sub-stitution.

sulfonic acid A type of organic com-pound containing the –SO2.OH group.The simplest example is benzenesulfonicacid (C6H5SO2OH). Sulfonic acids are

HOCH2

OH

OH

OH

glucose – a pyranose ring

fructose – in a furanose ring form

ribose – a pyranose ring

OOO

CH2OH

OOO

OH

OH

OHHO

HOCH2

OH

HOOH

CH2OHOOO

Sugars

strong acids and ionize in water to form thesulfonate ion (–SO2O–). Electrophilic sub-stitution can introduce other groups ontothe benzene ring; the –SO2.OH group di-rects substituents into the 3-position.

sulfonium compound An organiccompound of general formula R3SX,where R is an organic radical and X is anelectronegative radical or element; it con-tains the ion R3S+. An example is diethyl-methylsulfonium chloride, (C2H5)2-CH3S+Cl–, made by reacting diethyl sulfidewith chloromethane.

sulfoxide An organic compound of gen-eral formula R1SOR2, where R1 and R2 are

organic radicals. An example is dimethylsulfoxide, (CH3)2SO, commonly used as asolvent.

sulfur A low-melting nonmetallic solid,yellow colored in its common forms; thesecond member of group 16 (formerlyVIA) of the periodic table. It has the elec-tronic configuration [Ne]3s23p4.

Sulfur occurs in the elemental form inSicily and some southern states of the USA,and in large quantities in combined formssuch as sulfide ores (FeS2) and sulfate rocks(CaSO4). It forms about 0.5% of theEarth’s crust.

sulfonium compound

206

H

H

HO

H

H

OH

O

OH

OH

O1C

2C

3C

4C

5C

6CH2OH

HO

H

H

O

H

OH

OH

2C

3C

4C

5C

6CH2OH

1CH OH

H

Glucose (an aldohexose)

straight-chain form

Fructose (a ketohexose)

6CH2OH

5C

OHH

3C

4C

HO

H

H

2C

H

OH

OH

C1

HO

α-glucose

ring forms

β-glucose

6CH2OH

5C

OHH

3C

4C

HO

H

H

2C

H

OH

O

straight-chain form ring form

H

4C

5C

HO

OH

HOCH2

3C

OH

H

OH

C2

CH2OHO6 1

Sugar: straight-chain and ring forms

Sulfur forms a wide range of organicsulfur compounds, most of which have thetypically revolting smell of H2S.

Symbol: S; m.p. 112.8°C; b.p. 444.6°C;r.d. 2.07; p.n. 16; r.a.m. 32.066.

sulfur dichloride dioxide (sulfuryl chlo-ride; SO2Cl2) A colorless fuming liquidformed by the reaction of chlorine with sul-fur(IV) oxide in sunlight. It is used as achlorinating agent.

sulfur dichloride oxide (thionyl chlo-ride; SOCl2) A colorless fuming liquidformed by passing sulfur(IV) oxide overphosphorus(V) chloride and distilling themixture obtained. Sulfur dichloride oxideis used in organic chemistry to introducechlorine atoms (for example, into ethanolto form monochloroethane), the organicproduct being easily isolated as the otherproducts are gases.

sulfur dioxide See sulfur(IV) oxide.

sulfuretted hydrogen See hydrogensulfide.

sulfuric(IV) acid See sulfurous acid.

sulfuric(VI) acid (oil of vitriol;H2SO4) A colorless oily liquid manufac-tured by the contact process. The concen-trated acid is diluted by adding it slowly towater, with careful stirring. Concentratedsulfuric acid acts as an oxidizing agent, giv-ing sulfur(IV) oxide as the main product,and also as a dehydrating agent. The di-luted acid acts as a strong dibasic acid, neu-tralizing bases and reacting with activemetals and carbonates to form sulfates.

Sulfuric acid is used in the laboratory todry gases (except ammonia), to prepare ni-tric acid and ethene, and to absorb alkenes.In industry, it is used to manufacture fertil-izers (e.g. ammonium sulfate), rayon, anddetergents, to clean metals, and in vehiclebatteries. See contact process.

sulfur monochloride See disulfur di-chloride.

sulfurous acid (sulfuric(IV) acid;H2SO3) A weak acid found only in solu-tion, made by passing sulfur(IV) oxide intowater. The solution is unstable and smellsof sulfur(IV) oxide. It is a reducing agent,converting iron(III) ions to iron(II) ions,chlorine to chloride ions, and orangedichromate(VI) ions to green chrom-ium(III) ions.

sulfur(IV) oxide (sulfur dioxide; SO2) Acolorless choking gas prepared by burningsulfur or heating metal sulfides in air, or bytreating a sulfite with an acid. It is a pow-erful reducing agent, used as a bleach. Itdissolves in water to form sulfurous acid(sulfuric(IV) acid) and combines with oxy-gen, in the presence of a catalyst, to formsulfur(VI) oxide. This latter reaction is im-portant in the manufacture of sulfuric(VI)acid.

sulfur(VI) oxide (sulfur trioxide; SO3) Afuming volatile white solid prepared bypassing sulfur(IV) oxide and oxygen overhot vanadium(V) oxide (acting as a cata-lyst) and cooling the product in ice. Sul-fur(VI) oxide reacts vigorously with waterto form sulfuric acid. See also contactprocess.

sulfur trioxide See sulfur(VI) oxide.

sulfuryl chloride See sulfur dichloridedioxide.

superacid An acid with a high proton-donating ability. They are substances suchas HF–SbF3 and HSO3–SbF5. Sometimesknown as magic acids, they are able to pro-duce carbenium ions from some saturatedhydrocarbons.

superheating The raising of a liquid’stemperature above its boiling temperature,by increasing the pressure.

supernatant Denoting a clear liquidthat lies above a sediment or a precipitate.

supersaturated solution See saturatedsolution.

207

supersaturated solution

supersaturated vapor

208

supersaturated vapor See saturatedvapor.

supplementary units See dimensionlessunits.

supramolecular chemistry A branchof chemistry concerned with the synthesisand study of large structures consisting ofmolecules assembled together in a definitepattern. In a supramolecule the molecularunits are joined by intermolecular bonds –i.e. by hydrogen bonds or by ionic attrac-tions. A particular interest in supramolecu-lar research is the idea of self-assembly –i.e. that the molecules form well-definedstructures spontaneously as a result of theirgeometry and chemical properties. In thisway, supramolecular chemistry is ‘chem-istry beyond molecules’.

There are various different examples ofsupramolecular structures. For instance,single layers of carboxylic acid moleculesheld by hydrogen bonds may form two-di-mensional crystalline structures with novelelectrical properties. Large organic poly-meric structures may be formed in which anumber of individual chains radiate from acentral point or region. These polymers arecalled dendritic polymers or dendrimersand they have a number of possible appli-cations. Another type of supramolecule is ahelicate, which has a double helix made oftwo chains of bipyridyl units held by cop-per ions along the axis. The structure isanalogous to the double helix of DNA. Seealso host–guest chemistry.

supramolecule See supramolecular chem-istry.

surfactant A substance that lowers sur-face tension and has properties of wetting,foaming, detergency, dispersion, and emul-sification. SOAPS and other DETERGENTS

have surfactant properties.

suspension A system in which smallparticles of a solid or liquid are dispersed ina liquid or gas.

synclinal conformation See conforma-tion.

syndiotactic polymer See polymeriza-tion.

Synge, Richard Laurence Millington(1914–94) British biochemist. Synge is bestknown for his work with Archer MARTIN

on paper chromatography which led tothem sharing the 1952 Nobel Prize forchemistry. Synge used this technique to de-termine the exact structure of a simple anti-biotic peptide gramicidin-S.

synperiplanar conformation See con-formation.

synthesis The preparation of chemicalcompounds from simpler compounds.

synthesis gas A mixture of carbonmonoxide and hydrogen produced bysteam reforming of natural gas.

CH4 + H2O → CO + 3H2Synthesis gas is a useful starting ma-

terial for the manufacture of a number oforganic compounds.

synthon See retrosynthetic analysis.

209

tactic polymer See polymerization.

tannic acid See tannin.

tannin Any of several yellow organiccompounds found in vegetable sourcessuch as bark of trees, oak galls, and tea.They are used in tanning animal skins tomake leather and as mordants in dyeing.Tannic acid (a type of tannin) is a whitesolid heterocyclic organic acid extractedfrom oak galls and used for making dyesand inks.

tar (bitumen) A dark oily viscous liquidobtained by the destructive distillation ofcoal or the fractionation of petroleum.Tars are mixtures of mainly high-molecu-lar weight hydrocarbons and phenols.

tartaric acid A crystalline hydroxy car-boxylic acid with the formula:

HOOC(CHOH)2COOHIts systematic name is 2,3-dihydroxy-

butanedioic acid. It is used as an additive infoodstuffs. See also optical activity.

tartrate A salt or ester of tartaric acid.

tautomerism Isomerism in which eachisomer can convert into the other, so thatthe two isomers are in equilibrium. Theisomers are called tautomers. Tautomerismoften results from the migration of a hy-drogen atom. See keto–enol tautomerism.

TCA cycle (tricarboxylic acid cycle) SeeKrebs cycle.

TCCD See dioxin.

Teflon (Trademark) The synthetic poly-mer polytetrafluoroethane.

temperature scale A practical scale formeasuring temperature. A temperaturescale is determined by fixed temperatures(fixed points), which are reproducible sys-tems assigned an agreed temperature. Onthe Celsius scale the two fixed points arethe temperature of pure melting ice (the icetemperature) and the temperature of pureboiling water (the steam temperature). Thedifference between the fixed points is thefundamental interval of the scale, which issubdivided into temperature units. The In-ternational Temperature Scale has 11 fixedpoints that cover the range 13.81 kelvin to1337.58 kelvin.

tera- Symbol: T A prefix denoting 1012.For example, 1 terawatt (TW) = 1012 watts(W).

terephthalic acid See benzene-1,4-dicarboxylic acid.

ternary compound A chemical com-pound formed from three elements; e.g.Na2SO4 or LiAlH4.

terpene Any of a class of natural unsat-urated hydrocarbons with formulae(C5H8)n, found in plants. Terpenes consistof isoprene units,

CH2=C(CH3)CH=CH2.Monoterpenes have two units (C10H16),diterpenes four units (C20H32), triterpenessix units (C30H48), etc. Sesquiterpenes havethree isoprene units (C15H24).

tertiary alcohol See alcohol.

T

tertiary amine See amine.

tertiary structure See protein.

Terylene (Trademark) A polymer madeby condensing benzene-1,4-dicarboxylicacid (terephthalic acid) and ethane-1,2-diol(ethylene glycol), used for making fibersfor textiles.

tesla Symbol: T The SI unit of magneticflux density, equal to a flux density of oneweber of magnetic flux per square meter. 1T = 1 Wb m–2. The unit is named for theCroatian–US electrical engineer NikolaTesla (1870–1943).

tetrachloroethene (ethylene tetrachlo-ride; tetrachloroethylene; CCl2:CCl2) Acolorless poisonous liquid organic com-pound (a haloalkene) used as a solvent indry cleaning and as a de-greasing agent.

tetrachloroethylene See tetrachloro-ethene.

tetrachloromethane (carbon tetrachlo-ride; CCl4) A colorless nonflammableliquid made by the chlorination ofmethane. Its main use is as a solvent al-though it is being replaced by other com-pounds for safety reasons.

tetraethyl lead See lead tetraethyl.

tetrafluoroethene (CF2:CF2) A gaseousorganic compound (a fluorocarbon and ahaloalkene) used to make the plastic poly-tetrafluoroethene (PTFE). See polytetraflu-oroethene.

tetrahydrofuran (THF; C4H8O) A col-orless liquid widely used as a solvent andfor making polymers.

tetrapyrrole A structure of four pyrrolemolecules linked together, found in heme,chlorophyll, and other compounds. Usu-ally a metal ion is coordinated to the fournitrogen atoms on the pyrrole rings. Seeporphyrin.

theobromine An alkaloid found in thecacao bean. Its action is similar to caffeine.The systematic name is 3,7-dimethylxan-thine.

theophylline An alkaloid similar in ac-tion to caffeine. Its systematic name is 1,3-dimethylxanthine.

thermal dissociation The decomposi-tion of a chemical compound into compo-nent atoms or molecules by the action ofheat. Often it is temporary and reversible.

thermochemistry The branch of chem-istry concerned with heats of reaction, sol-vation, etc.

thermodynamics The study of heat andother forms of energy and the various re-lated changes in physical quantities such astemperature, pressure, density, etc.

The first law of thermodynamics statesthat the total energy in a closed system isconserved (constant). In all processes en-ergy is simply converted from one form toanother, or transferred from one system toanother.

A mathematical statement of the firstlaw is:

δQ = δU + δWHere, δQ is the heat transferred to the

system, δU the change in internal energy(resulting in a rise or fall of temperature),and δW is the external work done by thesystem.

The second law of thermodynamics canbe stated in a number of ways, all of whichare equivalent. One is that heat cannot passfrom a cooler to a hotter body withoutsome other process occurring. Another isthe statement that heat cannot be totallyconverted into mechanical work, i.e. a heatengine cannot be 100% efficient.

The third law of thermodynamics statesthat the entropy of a substance tends tozero as its thermodynamic temperature ap-proaches zero.

Often a zeroth law of thermodynamicsis given: that if two bodies are each in ther-mal equilibrium with a third body, thenthey are in thermal equilibrium with eachother. This is considered to be more funda-

tertiary amine

210

211

thiol

mental than the other laws because they as-sume it.

thermodynamic temperature Symbol:T A temperature measured in kelvins.See also absolute temperature.

thermoplastic polymer See polymer.

thermosetting polymer See polymer.

THF See tetrahydrofuran.

thiamine (vitamin B1) One of the water-soluble B-group of vitamins. Good sourcesof thiamine are unrefined cereal grains,liver, heart, and kidney. Thiamine defi-ciency predominantly affects the peripheralnervous system, the gastrointestinal tract,and the cardiovascular system. Thiaminehas been shown to be of value in the treat-ment of beriberi. Thiamine, in the form ofthiamine diphosphate, is the coenzyme forthe decarboxylation of acids such as pyru-vic acid. See also vitamin B complex.

thiazine (C4H4NS) Any of a group ofheterocyclic organic compounds that havea six-membered ring containing four car-bon atoms, one nitrogen atom, and onesulfur atom.

thiazole (C3H3NS) A colorless volatileliquid, a beterocyclic compound with afive-membered ring containing three car-bon atoms, one nitrogen atom, and onesulfur atom. It resembles PYRIDINE in its re-actions and is used in making dyes.

Thiele, Friedrich Karl Johannes(1865–1918) French organic chemist.Thiele is best known for his concept of par-tial valence which he put forward in 1899.In order to explain the lack of reactivity ofthe benzene molecule Thiele postulatedthat when single and double bonds alter-nate then the overall bonding is less con-ducive to reactivity than purely doublebonds. The concept of partial valence wasnot properly understood until the chemicalbonding and valence of molecules such asbenzene were explained in terms of quan-

tum mechanics. Thiele also worked exten-sively on organic compounds of nitrogen.

thin-layer chromatography A type ofCHROMATOGRAPHY widely used for theanalysis of mixtures. Thin-layer chro-matography employs a solid stationaryphase, such as alumina or silica gel, spreadevenly as a thin layer on a glass plate. Abase line is carefully scratched near the bot-tom of the plate, and a small sample of themixture is spotted onto the base line. Theplate is then stood upright in solvent,which rises up to the base line and beyondby capillary action. The components of thespot of the sample will dissolve in the sol-vent and tend to be carried up the plate.However, some of the components willcling more readily to the solid phase thanothers and will not move up the plate sorapidly. In this way, different fractions ofthe mixture eventually become separated.When the solvent has almost reached thetop, the plate is removed and quickly dried.The plate is developed to locate the posi-tions of colorless fractions by sprayingwith a suitable chemical or by exposure toultraviolet radiation. The components areidentified by comparing the distance theyhave moved up the plate with standard so-lutions that have been run simultaneously,or by computing an RF VALUE.

thio alcohol See thiol.

thiocarbamide (thiourea; NH2CSNH2) A colorless crystalline organic compound(the sulfur analog of urea). It is convertedto the inorganic compound ammoniumthiocyanate on heating. It is used as a sen-sitizer in photography and in medicine.

thioether (sulfide) A compound of thetype RSR′. They are the sulfur analogs ofethers and are generally more reactive thanthe corresponding oxygen compound.With halogen compounds they form sulfo-nium compounds:

CH3SCH3 + CH3Cl → (CH3)3 S+Cl–

They can also be oxidized to sulfoxides:(CH3)2S + O → (CH3)2S=O

thiol (thio alcohol; mercaptan) A com-

pound of the type RSH, similar to an alco-hol with the oxygen atom replaced by sul-fur. Typically they have a strongunpleasant odor. They are more reactivethan the corresponding alcohols and canform salts of the type RS–M+.

thionyl chloride See sulfur dichlorideoxide.

thiophene (C4H4S) A colorless liquidthat smells like benzene, a heterocycliccompound with a five-membered ring con-taining four carbon atoms and one sulfuratom. It occurs as an impurity in commer-cial benzene and is used as a solvent and inorganic syntheses.

thiourea ((NH2)2CS) See thiocarbamide.

threonine See amino acid.

thymidine The NUCLEOSIDE formedwhen thymine is linked to D-ribose by a β-glycosidic bond.

thymine A nitrogenous base found inDNA. It has a PYRIMIDINE ring structure.

tincture A solution in which alcohol(ethanol) is the solvent.

Tiselius, Arne Wilhelm Kaurin(1902–71) Swedish chemist. Tiselius is bestknown for developing electrophoresis as atechnique for studying proteins. He wasable to separate the proteins in horse serumusing this technique and to confirm thatthere are four types of proteins: albuminsand alpha, beta and gamma globulins.Tiselius also used other techniques such aschromatography and partition and gel fil-tration to separate proteins. Tiselius wonthe 1948 Nobel Prize for chemistry for hiswork using electrophoresis and other tech-niques in the analysis of proteins.

titrant See titration.

titration A procedure in volumetricanalysis in which a solution of known con-centration (called the titrant) is added to asolution of unknown concentration from aburette until the equivalence point or endpoint of the titration is reached. See volu-metric analysis.

TNT See trinitrotoluene.

tocopherol See vitamin E.

Todd, Alexander Robertus, Lord(1907–97) British organic chemist. In theearly part of his career, Todd was largelyconcerned with the vitamins B1, B12 and E. In the late 1940s and 1950s he synthe-sized the purine and pyrimidine bases thatoccur in nucleic acids such as DNA and

thionyl chloride

212

NH

NHOCH2 O

H3C

O

O

OH

Thymidine

NH

NH

O

O

H3C

Thymine

RNA and established their structures. Healso synthesized a number of other im-portant compounds such as adenosinetriphosphate (ATP) and adenosine diphos-phate (ADP), which are of crucial impor-tance in energy transfer in biologicalsystems. Todd won the 1957 Nobel Prizefor chemistry for his work on biologicallyimportant molecules. He published an au-tobiography entitled A Time to Rememberin 1983.

Tollen’s reagent A solution of the com-plex ion Ag(NH3)2

+ produced by precipita-tion of silver oxide from silver nitrate witha few drops of sodium hydroxide solution,and subsequent dissolution of the silveroxide in aqueous ammonia. Tollen’sreagent is used in the ‘silver-mirror test’ foraldehydes, where the Ag+ ion is reduced tosilver metal. It is also a test for alkynes witha triple bond in the 1-position. A yellowprecipitate of silver carbide is formed inthis case.

RCCH + Ag+ → RCC–Ag+ + H+

It is named for Bernhard Tollens (1841–1918). See also silver-mirror test.

toluene See methylbenzene.

toluidine (methylaniline; aminotoluene;CH3C6H4NH2) An aromatic amine usedin making dyestuffs and drugs. There arethree isomers; the 1,2- (ortho-amino-toluene) and 1,3- (meta-) forms are liquids,the 1,4- (para-) isomer is a solid.

tonne Symbol: t A unit of mass equal to103 kilograms (i.e. one megagram).

torr A unit of pressure equal to a pres-sure of 101 325/760 pascals (133.322 Pa).One torr is equal to one mmHg. The unit isnamed for the Italian physicist EvangelistaTorricelli (1609–47).

torsion angle See conformation.

tracer An isotope of an element used toinvestigate chemical reactions or physicalprocesses (e.g. diffusion). See isotope.

trans- Designating an isomer withgroups that are on opposite sides of a bondor structure. See isomerism.

trans fat See hardening.

transient species A short-lived interme-diate in a chemical reaction.

transition state (activated complex) Sym-bol: ‡ A short-lived high-energy mol-ecule, radical, or ion formed during areaction between molecules possessing thenecessary activation energy. The transitionstate decomposes at a definite rate to yieldeither the reactants again or the final prod-ucts. The transition state can be consideredto be at the top of the energy profile.

For the reaction,X + YZ = X…Y…Z‡ → XY+ Z

the sequence of events is as follows. X ap-proaches YZ and when it is close enoughthe electrons are rearranged producing aweakening of the bond between Y and Z.A partial bond is now formed between Xand Y producing the transition state. De-pending on the experimental conditions,the transition state either breaks down toform the products or reverts back to the re-actants. See also activation energy.

transition temperature A temperatureat which some definite physical change oc-curs in a substance. Examples of such tran-sitions are change of state, change ofcrystal structure, and change of magneticbehavior.

triacylglycerol See triglyceride.

tricarboxylic acid cycle (TCA cycle)See Krebs cycle.

triaminotriazine See melamine.

triatomic molecule A molecule consist-ing of three atoms, such as O3 or H2O.

triazine (C3H3N3) A heterocyclic or-ganic compound with a six-membered ringcontaining three carbon atoms and threenitrogen atoms. There are three isomers,

213

triazine

used as dyestuffs and herbicides. See alsomelamine.

triazole (C2H3N3) A heterocyclic or-ganic compound with a five-memberedring containing two carbon atoms andthree nitrogen atoms. There are two iso-mers.

tribasic acid An acid with three replace-able hydrogen atoms (such as phos-phoric(V) acid, H3PO4). See acid.

tribromomethane (bromoform; CHBr3)A colorless liquid compound. See halo-form.

trichloroacetic acid See chloroethanoicacid.

trichloroethanal (chloral; CCl3CHO) Acolorless liquid aldehyde made by chlori-nating ethanal. It was used to make the in-secticide DDT. It can be hydrolyzed to give2,2,2-trichloroethanediol (chloral hydrate,CCl3CH(OH)2). Most compounds withtwo –OH groups on the same carbon atomare unstable. However, in this case the ef-fect of the three chlorine atoms stabilizesthe compound. It is used as a sedative.

trichloroethanoic acid See chloro-ethanoic acid.

trichloromethane (chloroform; CHCl3)A colorless volatile liquid formerly used asan anesthetic. Now its main use is as a sol-vent and raw material for making otherchlorinated compounds.Trichloromethane is made by reactingethanal, ethanol, or propanone with chlo-rinated lime. See also haloform.

triglyceride A GLYCERIDE in which estersare formed with all three –OH groups ofglycerol.

trihydric alcohol See triol.

triiodomethane (iodoform; CHI3) Ayellow crystalline compound made bywarming ethanal with an alkaline solutionof an iodide:

CH3CHO + 3I– + 4OH– → CHI3 +HCOO– + 3H2O

The reaction also occurs with all ketones ofgeneral formula CH3COR (R is an alkylgroup) and with secondary alcoholsCH3CH(OH)R. Iodoform is used as a testfor such reactions (the iodoform reaction).See also haloform.

trimer A molecule (or compound)formed by addition of three identical mol-ecules. See ethanal; methanal.

trimethylaluminum (aluminum tri-methyl; (CH3)3Al) A colorless liquid pro-duced by the reduction of dimethylaluminum chloride using sodium. It ignitesspontaneously on contact with air and re-acts violently with water, acids, halogens,alcohols, and amines. Aluminum alkyls areused in the ZIEGLER PROCESS for the manu-facture of high-density polyethene.

trimolecular Describing a reaction orstep that involves three molecules interact-ing simultaneously with the formation of a product. For example, the final step inreaction between hydrogen peroxide andacidified potassium iodide is trimolecu-lar:

HOI + H+ + I– → I2 + H2OIt is uncommon for reactions to take

place involving trimolecular steps.

trinitroglycerine See nitroglycerine.

trinitrophenol See picric acid.

trinitrotoluene (TNT; CH3C6H2(NO2)3)A yellow crystalline solid. It is a highly un-stable substance, used as an explosive. Thecompound is made by nitrating methylben-zene and the nitro groups are in the 2, 4,and 6 positions.

triol (trihydric alcohol) An alcohol thathas three hydroxyl groups (–OH) per mol-ecule of compound.

triose A SUGAR that contains three car-bon atoms.

triazole

214

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tyrosine

trioxygen See ozone.

tripeptide See peptide.

triple bond A covalent bond formed be-tween two atoms in which three pairs ofelectrons contribute to the bond. One pairforms a sigma bond (equivalent to a singlebond) and two pairs give rise to two pibonds. It is conventionally represented asthree lines, thus H–C≡C–H. The bond oc-curs in ALKYNES. See multiple bond.

triple point The only point at which thegas, solid, and liquid phases of a substancecan coexist in equilibrium. The triple pointof water (273.16 K at 101 325 Pa) is usedto define the kelvin.

triplet state See carbene.

triterpene See terpene.

tritiated compound A compound inwhich one or more 1H atoms have been re-placed by tritium (3H) atoms.

tritium Symbol: T, 3H A radioactive iso-tope of hydrogen of mass number 3. Thenucleus contains 1 proton and 2 neutrons.

Tritium decays with emission of low-energy beta radiation to give 3He. The half-life is 12.3 years. It is useful as a tracer instudies of chemical reactions. Compoundsin which 3H atoms replace the usual 1Hatoms are said to be tritiated. A positive tri-tium ion, T+, is a triton.

triton See tritium.

tropylium ion The positive ion C7H7+,

having a symmetrical seven-membered ringof carbon atoms. The tropylium ion ringshows nonbenzenoid aromatic properties.See aromatic compound.

tryptophan See amino acid.

turpentine (pine-cone oil) A yellow vis-cous RESIN obtained from coniferous trees.It can be distilled to produce turpentine oil(also known simply as turpentine), used inmedicine and as a solvent in paints, pol-ishes, and varnishes.

twist-boat conformation See cyclo-hexane.

tyrosine See amino acid.

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ubiquinone An electron-transportingcoenzyme that is a component in the electron-transport chain. It was formerlycalled coenzyme Q.

ultracentrifuge A high-speed centrifugeused for separating out very small parti-cles. The sedimentation rate depends onthe particle size, and the ultracentrifugecan be used to measure the mass of col-loidal particles and large molecules (e.g.proteins).

ultraviolet (UV) A form of electromag-netic radiation, shorter in wavelength thanvisible light. Ultraviolet wavelengths rangebetween about 1 nm and 400 nm. Ordi-nary glass is not transparent to thesewaves; quartz is a much more effective ma-terial for making lenses and prisms for usewith ultraviolet. Like light, ultraviolet radi-ation is produced by electronic transitionsbetween the outer energy levels of atoms.However, having a higher frequency, ultra-violet photons carry more energy thanthose of light and can induce photolysis ofcompounds and photoionization. See alsoelectromagnetic radiation.

unimolecular Describing a reaction (orstep) in which only one molecule is in-volved. For example, radioactive decay is aunimolecular reaction:

Ra → Rn + αOnly one atom is involved in each disinte-gration.

In a unimolecular chemical reaction,the molecule acquires the necessary energyto become activated and then decomposes.The majority of reactions involve only uni-or bimolecular steps. The following reac-tions are all unimolecular:

N2O4 → 2NO2PCl5 → PCl3 + Cl2

CH3CH2Cl → C2H4 + HCl

unit A reference value of a quantity usedto express other values of the same quan-tity. See also SI units.

unit cell The smallest group of atoms,ions, or molecules that, when repeated atregular intervals in three dimensions, willproduce the lattice of a crystal system.There are seven basic types of unit cells,which result in seven CRYSTAL SYSTEMS.

unit processes (chemical conver-sions) The recognized steps used inchemical processes, e.g. alkylation, distilla-tion, hydrogenation, pyrolysis, nitration,etc. Industrial processing and the econom-ics, design, and use of the equipment arebased on these unit processes rather thanconsideration of each reaction separately.

univalent See monovalent.

universal gas constant See gas con-stant.

universal indicator (multiple-range indi-cator) A mixture of indicator dyestuffsthat shows a gradual change in color overa wide pH range. A typical formulationcontains methyl orange, methyl red, bro-mothymol blue, and phenolphthalein andchanges through a red, orange, yellow,green, blue, and violet sequence betweenpH 3 and pH 10. Several commercialpreparations are available as both solu-tions and test papers.

unsaturated compound An organic

U

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UV

compound that contains at least one dou-ble or triple bond between two of its car-bon atoms. The ALKENES and ALKYNES areexamples of unsaturated compounds. Un-saturated compounds typically undergoaddition reactions to form single bonds.Compare saturated compound.

unsaturated solution See saturated so-lution.

unsaturated vapor See saturatedvapor.

UPVC Unplasticized PVC; a hard-wearing form of PVC used in buildingwork (e.g. for window frames).

uracil A nitrogenous base that is foundin RNA, replacing the thymine of DNA. Ithas a pyrimidine ring structure.

urea (carbamide; CO(NH2)2) A whitecrystalline compound made from ammoniaand carbon dioxide. It is used in the manu-facture of urea–formaldehyde (methanal)resins. Urea is the end product of metabo-lism in many animals and is present inurine.

urea cycle See ornithine cycle.

urea–formaldehyde resin A syntheticPOLYMER made by copolymerizing ureawith formaldehyde (methanal, HCHO).

urethane (ethyl carbamate; CO(NH2)-OC2H5) A poisonous flammable organiccompound, used in medicine, as a solvent,and as an intermediate in the manufactureof POLYURETHANE resins.

uric acid A nitrogen compound pro-duced from purines. In certain animals(uricotetic animals), it is the main excre-tory product resulting from breakdown ofamino acids. In humans, uric acid crystalsin the joints are the cause of gout.

uridine The nucleoside formed whenuracil is linked to D-ribose by a β-glycosidicbond.

uronic acid See sugar acid.

UV See ultraviolet.

HN

NH

O

O

Uracil

HN

NH

HN

NH

O

O

O

Uric acid

HN

N

O

O

O

OH OH

HOCH2

Uridine

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vacuum distillation The distillation ofliquids under a reduced pressure, so thatthe boiling point is lowered. Vacuum dis-tillation is a common laboratory techniquefor purifying or separating compoundsthat would decompose at their ‘normal’boiling point.

vacuum flask See Dewar flask.

valence (valency) The combining powerof an element or radical, equal to the num-ber of hydrogen atoms that will combinewith or displace one atom of the element.For simple covalent molecules the valenceis obtained directly, for example C in CH4is tetravalent; N in NH3 is trivalent. Forions the valence is regarded as equivalentto the magnitude of the charge; for exam-ple Ca2+ is divalent, CO3

2– is a divalentradical. The rare gases are zero-valent be-cause they do not form compounds undernormal conditions. As the valence formany elements is constant, the valence ofsome elements can be deduced without ref-erence to compounds formed with hydro-gen. Thus, as the valence of chlorine in HClis 1, the valence of aluminum in AlCl3 is 3;as oxygen is divalent (H2O) silicon in SiO2is tetravalent. The product of the valenceand the number of atoms of each elementin a neutral compound must be equal. Forexample, in Al2O3 for the two aluminumatoms (valence 3) the product is 6 and forthe three oxygen atoms (valence 2) theproduct is also 6.

The valence of an element is generallyequal to either the number of valence elec-trons or eight minus the number of valenceelectrons. Transition metal ions displayvariable valence.

valence electron An outer electron inan atom that can participate in formingchemical bonds.

valency See valence.

valeric acid See pentanoic acid.

valine See amino acid.

van der Waals equation An equationof state for real gases. For n moles of gasthe equation is

(p + n2a/V2)(V – nb) = nRTwhere p is the pressure, V the volume, andT the thermodynamic temperature. a and bare constants for a given substance and R isthe gas constant. The equation gives a bet-ter description of the behavior of real gasesthan the perfect gas equation (pV = nRT).

The equation contains two corrections:b is a correction for the nonnegligible sizeof the molecules; a/V2 corrects for the factthat there are attractive forces between themolecules, thus slightly reducing the pres-sure from that of an ideal gas. It is namedfor the Dutch physicist Johannes Diderikvan der Waals (1837–1923). See also gaslaws.

van der Waals force An intermolecularforce of attraction, considerably weakerthan chemical bonds and arising fromweak electrostatic interactions betweenmolecules (the energies are often less than1 J mol–1).

The van der Waals interaction containscontributions from three effects; perma-nent dipole–dipole interactions found forany polar molecule; dipole–induced dipoleinteractions, where one dipole causes aslight charge separation in bonds that have

V

a high polarizability; and dispersion forces,which result from temporary polarity aris-ing from an asymmetrical distribution ofelectrons around the nucleus. Even atomsof the rare gases exhibit dispersion forces.

Van’t Hoff, Jacobus Henricus (1852–1911) Dutch theoretical chemist. Van’tHoff made important contributions tostereochemistry, thermodynamics, the ki-netics of chemical reactions and the theoryof chemical solutions. In 1874 Van’t Hoffinitiated the subject of stereochemistrywhen he postulated that the four chemicalbonds which a carbon atom can form aredirected toward the corners of a regulartetrahedron. This enabled the phenomenonof optical activity to be understood interms of the structures of optical isomers.Van’t Hoff introduced this idea indepen-dently of Joseph le Bel. Many of the contri-butions of Van’t Hoff to thermodynamics,kinetics and solutions were expounded inhis book Studies of Chemical Dynamics(1884). This included the application ofthermodynamics to chemical equilibrium.He was awarded the first Nobel Prize forchemistry in 1901.

van’t Hoff factor Symbol: i The ratioof the number of particles present in a so-lution to the number of undissociated mol-ecules added. It is used in studies ofcolligative properties, which depend on thenumber of entities present. For example, ifn moles of a compound are dissolved anddissociation into ions occurs, then thenumber of particles present will be in. Os-motic pressure (π), for instance, will begiven by the equation

πV = inRTIt is named for the Dutch theoreticalchemist Jacobus Henricus van’t Hoff.

van’t Hoff isochore The equation:d(logeK)/dT = ∆H/RT2

showing how the equilibrium constant, K,of a reaction varies with thermodynamictemperature, T. ∆H is the enthalpy of reac-tion and R is the gas constant.

vapor A gas formed by the VAPORIZA-TION of a solid or liquid. Some particles

near the surface of a liquid acquire suffi-cient energy in collisions with other parti-cles to escape from the liquid and enter thevapor; some particles in the vapor lose en-ergy in collisions and re-enter the liquid. Ata given temperature an equilibrium is es-tablished, which determines the vaporpressure of the liquid at that temperature.

vapor density The ratio of the mass of acertain volume of a vapor to the mass of anequal volume of hydrogen (measured at thesame temperature and pressure). Determi-nation of vapor densities is one method offinding the relative molecular mass of acompound (equal to twice the vapor den-sity). VICTOR MEYER’S METHOD, DUMAS’METHOD, or HOFMANN’S METHOD can beused.

vaporization The process by which aliquid or solid is converted into a gas orvapor by heat. Unlike boiling, which oc-curs at a fixed temperature, vaporizationcan occur at any temperature. Its rate in-creases as the temperature rises.

vapor pressure The pressure exerted bya vapor. The saturated vapor pressure isthe pressure of a vapor in equilibrium withits liquid or solid. It depends on the natureof the liquid or solid and the temperature.

vat dyes A class of insoluble dyes ap-plied by first reducing them to derivativesthat are soluble in dilute alkali. In this con-dition they have a great attraction for cer-tain fibers, such as cotton. The solution isapplied to the material and the insolubledye is regenerated in the fibers by atmos-pheric oxidation. Indigo and indanthreneare examples of vat dyes.

velocity constant See rate constant.

vesicant A substance that causes blister-ing of the skin. Mustard gas is an example.

vicinal positions Positions in a mol-ecule at adjacent atoms. For example, in1,2-dichloroethane the chlorine atoms arein vicinal positions, and this compoundcan thus be named vic-dichloroethane.

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vicinal positions

Viktor Meyer’s method

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Viktor Meyer’s method A method fordetermining VAPOR DENSITIES in which agiven weight of sample is vaporized and thevolume of air displaced by it is measured.In practice, a bulb in a heating bath is con-nected via a fairly long tube to a water-bath gas-collection arrangement. Thesystem is brought to equilibrium and thesample is then added (without opening theapparatus to the atmosphere). As the airdisplaced by gas is collected over water acorrection for the vapor pressure of wateris necessary and the method may fail if thevapor is soluble in water. It is named forthe German chemist Viktor Meyer (1848–97).

violaxanthin A xanthophyll pigmentfound in the brown algae. See photosyn-thetic pigments.

vinegar A dilute solution (about 4% byvolume) of ethanoic acid (acetic acid),often with added coloring and flavoringsuch as caramel. Natural vinegar is pro-duced by the bacterial fermentation ofcider or wine; it can also be made synthet-ically. See ethanoic acid.

vinylation A catalytic reaction in whicha compound adds across the triple bond ofethyne (acetylene) to form an ethenyl(vinyl) compound. For example, an alcoholcan add as follows:

ROH + HC≡CH → RHC=CH(OH)

vinyl benzene See phenylethene.

vinyl chloride See chloroethene.

vinyl group The group CH2:CH–.

viscose rayon See rayon.

viscosity Symbol: η The resistance toflow of a fluid.

visible radiation See light.

visual purple See rhodopsin.

vitalism See Wöhler’s synthesis.

vitamin One of a number of organiccompounds that are essential in smallquantities for metabolism. The vitaminshave no energy value; most of them seem toact as catalysts for essential chemicalchanges in the body, each one influencing anumber of vital processes. Vitamins A, D,E, and K are the fat-soluble vitamins, oc-curring mainly in animal fats and oils.Vitamins B and C are the water-solublevitamins. If a diet lacks vitamins, this re-sults in the breakdown of normal bodilyactivities and produces disease symptoms.Such deficiency diseases can usually beremedied by including the necessary vita-mins in the diet. Plants can synthesize vita-mins from simple substances, but animalsgenerally require them in their diet, thoughthere are exceptions to this. These includevitamins synthesized by bacteria in the gut,and some that can be manufactured by theanimal itself. A precursor of vitamin D2(ergosterol), for example, can be convertedin the skin by ultraviolet radiation.

vitamin A (vitamin A1; retinol) A fat-soluble vitamin (a derivative of the yellowpigment, carotene) occurring in milk, but-ter, cheese, liver, and cod-liver oil. It canalso be formed in the body by oxidation ofcarotene, which is present in fresh greenvegetables and carrots. Deficiency in vita-min A can result in a reduced resistance todisease and in night blindness.

vitamin B complex A group of ten ormore water-soluble vitamins, which tendto occur together. They can be obtainedfrom whole grains of cereals and frommeat and liver. Since the B vitamins arepresent in most unprocessed food, defi-ciency diseases only occur in populationsliving on restricted diets. Many of the Bvitamins act as coenzymes involved in thenormal oxidation of carbohydrates duringrespiration.

The vitamins of the B complex includethiamine (vitamin B1), riboflavin (vitaminB2), nicotinic acid (niacin), pantothenicacid (vitamin B5), pyridoxine (vitamin B6),cyanocobalamin (vitamin B12), biotin,lipoic acid, and folic acid.

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vulcanization

vitamin C (ascorbic acid) A water-solu-ble vitamin, which is widely required inmetabolism. The major sources of vitaminC are fresh fruit and vegetables and severedeficiency results in scurvy.

vitamin D A fat-soluble vitamin foundin fish-liver oil, butter, milk, cheese, eggyolk, and liver. Its principal action is to in-crease the absorption of calcium and phos-phorus from the intestine. The vitamin alsohas a direct effect on the calcificationprocess in bone. Deficiency results in inad-equate deposition of calcium in the bones,causing rickets in young children andosteomalacia in adults.

The term vitamin D refers, in fact, to agroup of compounds, all sterols, of verysimilar properties. The most important arevitamin D2 (calciferol) and vitamin D3(cholecalciferol). Precursors of these areconverted to the vitamins in the body bythe action of ultraviolet radiation.

vitamin E (tocopherol) A fat-solublevitamin found in wheat germ, dairy prod-ucts, and in meat. Severe deficiency in in-fants may lead to high rates of red-bloodcell destruction and hence to anemia.However, there are very few deficiencyeffects apparent in adults.

vitamin K (phylloquinone; menaquin-one) A fat-soluble vitamin that is re-quired to catalyze the synthesis ofprothrombin, a blood-clotting factor, inthe liver. Intestinal microorganisms are ca-pable of synthesizing considerableamounts of vitamin K in the intestine andthis, together with dietary supply, insuresthat deficiency is unlikely to occur in anybut the newborn. A newborn child may bedeficient because the intestine is sterile atbirth and the level supplied by the motherduring gestation is limited. Thus during thefirst few days of life blood-clotting defi-

ciency may be observed, but this is readilyrectified by a small injection of the vitamin.

VLDL (very low-density lipoprotein) Seelipoprotein.

volatile Easily converted into a vapor.

volt Symbol: V The SI unit of electricalpotential, potential difference, and e.m.f.,defined as the potential difference betweentwo points in a circuit between which aconstant current of one ampere flows whenthe power dissipated is one watt. 1 V = 1 JC–1. The unit is named for the Italian physi-cist Alessandro Volta (1745–1827).

volumetric analysis One of the classi-cal wet methods of quantitative analysis. Itinvolves measuring the volume of a solu-tion of accurately known concentrationthat is required to react with a solution ofthe substance being determined. The solu-tion of known concentration (the standardsolution) is added in small portions from aburette. The process is called a titrationand the equivalence point is called the endpoint. End points are observed with the aidof indicators or by instrumental methods,such as conduction or light absorption.Volumetric analysis can also be applied togases. The gas is typically held over mer-cury in a graduated tube, and volumechanges are measured on reaction or afterabsorption of components of a mixture.

vulcanite (ebonite) A hard black insula-tor made by vulcanizing rubber with alarge amount of sulfur.

vulcanization A process of improvingthe quality of rubber (hardness and resis-tance to temperature changes) by heating itwith sulfur (about 150°C). Acceleratorsare used to speed up the reaction. Certainsulfur compounds can also be used for vul-canization.

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Wacker process An industrial processfor making ethanal (and other carbonylcompounds). To produce ethanal, etheneand air are bubbled through an acid solu-tion of palladium(II) chloride and cop-per(II) chloride (20–60°C and moderatepressure):

C2H4 + Pd2+ + O2 → CH3CHO +Pd + 2H+

The reaction involves an intermediatecomplex between palladium(II) ions andethane in which the palladium bonds to thepi electrons. The purpose of the copper(II)chloride is to oxidize the palladium back toPd2+ ions:

Pd + 2Cu2+ → Pd2+ + 2Cu+

The copper(I) ions spontaneously oxidizeto copper(II) ions in air. The process pro-vides a cheap source of ethanal (and, byoxidation, ethanoic acid) from the readilyavailable ethene. It is named for Alexandervon Wacker (1846–1922).

Walden inversion A reaction in whichan optically active compound reacts to givean optically active product in which theconfiguration has been inverted. This hap-pens in the SN2 mechanism. See nucle-ophilic substitution. It is named for theGerman chemist Paul Walden (1863–1957), who discovered it in 1896.

Walker, John E. (1941– ) British bio-chemist who worked on the enzyme mech-anism underlying the synthesis of ATP. Hewas awarded the 1997 Nobel Prize forchemistry jointly with P. D. Boyer. Theprize was shared with J. C. Skou.

water (H2O) A colorless liquid thatfreezes at 0°C and, at atmospheric pres-sure, boils at 100°C. In the gaseous state

water consists of single H2O molecules.Due to the presence of two lone pairs theatoms do not lie in a straight line, the anglebetween the central oxygen atom and thetwo hydrogen atoms being 105°; the dis-tance between each hydrogen atom and theoxygen atom is 0.099 nm. When ice forms,hydrogen bonds some 0.177 nm long de-velop between the hydrogen atom and oxy-gen atoms in adjacent molecules, giving iceits tetrahedral crystalline structure with adensity of 916.8 kg m–3 at STP. Differentice structures develop under higher pres-sures. When ice melts to form liquid water,the tetrahedral structure breaks down, butsome hydrogen bonds continue to exist;liquid water consists of groups of associ-ated water molecules, (H2O)n, mixed withsome monomers and some dimers. Thismixture of molecular species has a higherdensity than the open-structured crystals.The maximum density of water, 999.97 kgm–3, occurs at 3.98°C. This accounts forthe ability of ice to float on water and forthe fact that water pipes burst as ice ex-pands on freezing.

Although water is predominantly a co-valent compound, a very small amount ofionic dissociation occurs (H2O ˆ H+ +OH–). In every liter of water at STP there isapproximately 10–7 mole of each ionicspecies. It is for this reason that, on the pHscale, a neutral solution has a value of 7.

As a polar liquid, water is the mostpowerful solvent known. This is partly aresult of its high dielectric constant andpartly its ability to hydrate ions. This latterproperty also accounts for the incorpora-tion of water molecules into some ioniccrystals as water of crystallization.

Water is decomposed by reactive metals(e.g. sodium) when cold and by less active

W

metals (e.g. iron) when steam is passedover the hot metal. It is also decomposedby electrolysis.

water gas A mixture of carbon monox-ide and hydrogen produced when steam ispassed over red-hot coke or made to com-bine with hydrocarbons, e.g.

C(s) + H2O(g) → CO(g) + H2(g)The amount of hydrogen can be increasedby combining it with the water gas shift re-action:

CO + H2O ˆ CO2 + H2Water gas was once an important source ofhydrogen for the production of ammonia.Most hydrogen is now obtained frommethane by steam REFORMING.

CH4(g) + H2O(g) → CO(g) + 3H2(g)The production of water gas usingmethane is an important step in the prepa-ration of hydrogen for ammonia synthesis.Compare producer gas.

Watson, James Dewey (1928– )American molecular biologist. James Wat-son is famous for his work in 1953 withFrancis CRICK on the structure of DNA.Watson gave a controversial account ofthis work in his book The Double Helix(1968). He also wrote several other autobi-ographical and popular books as well asco-authoring textbooks on molecular biol-ogy. He shared the 1962 Nobel Prize formedicine with Crick and Maurice WILKINS

for their discovery of the structure of DNA.

watt Symbol: W The SI unit of power,defined as a power of one joule per second.1 W = 1 J s–1. The unit is named for theBritish inventor James Watt (1739–1819).

wave function Symbol: Ψ A functionthat describes the quantum state of a sys-tem in WAVE MECHANICS. The physical sig-nificance of the wave function for a particleis that the square of its absolute value at apoint is proportional to the probability offinding the particle in a small element ofvolume, dxdydz, at that point. See alsoorbital.

wavelength Symbol: λ The distance be-tween the ends of one complete cycle of a

wave. Wavelength is related to the speed(c) and frequency (v) thus:

c = vλ

wave mechanics A formulation ofquantum mechanics put forward by theGerman physicist Erwin Schrödinger(1887-1961) in 1926, following the sug-gestion of the French physicist Louis debroglie (1892-1987) that particles such aselectrons might also have wavelike proper-ties. The basic equation of wave mechanicsis the Schrödinger equation, which is awave equation describing the system. Solu-tions of the equation give rise to WAVE

FUNCTIONS. See also quantum theory;orbital.

wave number Symbol: σ The reciprocalof the wavelength of a wave. It is the num-ber of wave cycles in unit distance, and isoften used in spectroscopy. The unit is themeter–1 (m–1). The circular wave number(symbol: k) is given by:

k = 2πσ

wax One of a group of water-insolublesubstances with a very high molecularweight; they are esters of long-chain alco-hols with fatty acids. Waxes form protec-tive coverings to leaves, stems, fruits, seeds,animal fur, and the cuticles of insects, serv-ing principally as waterproofing. For ex-ample, waxy deposits on some plantorgans add to the efficiency of the cuticle inreducing transpiration, as well as cuttingdown airflow over the surface and forminga highly reflective surface, thus reducingenergy available for evaporation. Theymay also occur in plant cell walls, e.g. leafmesophyll. They are used in varnishes, pol-ishes, and candles.

weak acid An ACID that is not fully dis-sociated in solution.

weak base A base that is not fully disso-ciated in solution. See acid.

weber Symbol: Wb The SI unit of mag-netic flux, equal to the magnetic flux that,linking a circuit of one turn, produces ane.m.f. of one volt when reduced to zero at

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weber

white spirit

224

uniform rate in one second. 1 Wb = 1 V s.The unit is named for the German physicistWilhelm Weber (1804–91).

white spirit A liquid hydrocarbon re-sembling kerosene obtained from petro-leum, used as a solvent and in themanufacture of paints and varnishes.

Wieland, Heinrich Otto (1877–1957)German organic chemist. Much ofWieland’s early work was devoted to theorganic compounds of nitrogen. He startedto study the bile acids in 1912. He foundthat three of these acids: cholic acid, de-oxycholic acid, and lithocholic acid, are allsteroids and are all related to cholesterol.This led him to propose a structure of cho-lesterol. He won the 1927 Nobel Prize forchemistry for his work on steroids. How-ever, subsequent work by Wieland and hiscolleagues in 1932 showed that the struc-ture he proposed was incorrect, leading tothe now generally accepted modified struc-ture being proposed by Wieland et al.

Wilkins, Maurice Hugh Freder-ick (1916– ) New Zealand-bornBritish molecular biologist. MauriceWilkins was one of the key figures in thedetermination of the structure of DNA. Hewas originally a physicist but turned to bio-physics after the end of World War II. Hebegan to study DNA by x-ray diffraction.Some of the x-ray diffraction pictures pro-duced by his colleague Rosalind FRANKLIN

provided essential clues to Francis CRICK

and James WATSON in their search for thestructure of DNA. Wilkins shared the 1962Nobel Prize for Medicine with Crick andWatson. Wilkins also determined the struc-ture of ribonucleic acid (RNA) using x-raydiffraction. In 2003 Wilkins published hisautobiography.

Wilkinson, Sir Geoffrey (1921–96)British chemist. Wilkinson did a lot of no-table work on ‘sandwich compounds’, i.e.molecules such as ferrocene in which aniron atom is sandwiched between two car-bon rings which each have five sides.Wilkinson studied many organo-metalliccompounds including RhCl(P(C6H5)3).

This compound is known as Wilkinson’scatalyst. It is used as a catalyst in the hy-drogenation of alkenes. Wilkinson sharedthe 1973 Nobel Prize for chemistry withErnst Otto Fischer for their work on sand-wich compounds.

Williamson’s synthesis 1. A methodof preparing simple ethers by dehydrationof alcohols with concentrated sulfuric acid.The reaction is carried out at 140°C underreflux with an excess of the alcohol:

2ROH → ROR + H2OThe concentrated sulfuric acid both cat-alyzes the reaction and displaces the equi-librium to the right. Also the ether may bedistilled off during the reaction (in whichcase it is called Wilkinson’s continuousprocess). The product, ether, is termed‘simple’, because the R groups are identi-cal. There are two possible mechanisms forthe process, depending on the nature of thealcohol. In the case of primary alcohols,there is a hydrogensulfate formed. For ex-ample, with ethanol:

C2H5OH + H2SO4 ˆ C2H5O.SO2.OH+ H2O

With another alcohol molecule, an oxo-nium ion is formed:

C2H5OH + C2H5O.SO2.OH →(C2H5)2OH+ + HSO4

–

The oxonium ion loses a proton to give theether:

(C2H5)2OH+ + HSO4– → C2H5O +

H2SO4In the case of tertiary alcohols the first

step is production of a carbocation. For ex-ample, with isobutanol (2-methylpronan-2-ol; (CH3)3COH)):

(CH3)3COH + H+ → H2O + (CH3)3C+

In such cases the ion is stabilized by thealkyl groups. The ion is attached by thelone pair on the oxygen of another alcoholmolecule to form an oxonium ion:

(CH3)3C+ + (CH3)3COH →((CH3)3C)2OH+

As in the above mechanism, this loses aproton to give the ether:

((CH3)3C)2OH+ → (CH3)3C.O.C(CH3)3+ H+

2. A method for the preparation of mixedethers by nucleophilic substitution. Ahaloalkane is refluxed with an alcoholic

solution of sodium alkoxide (from sodiumdissolved in alcohol):

R1Cl + R2O–Na+ → R1OR2 + NaClThe product ether is termed ‘mixed’ if thealkyl groups R1 and R2 are different. Thissynthesis can be used to produce both sim-ple and mixed ethers.

Both reactions are named for the Britishchemist Alexander Williamson (1824–1904).

will-o’-the-wisp See ignis fatuus.

Willstätter, Richard (1872–1942) Ger-man organic chemist. Willstätter’s earlywork was concerned with the structure ofalkaloids such as atropine and cocaine. In1905 he started the work on plant pig-ments such as chlorophyll for which he isbest known. Using the technique of chro-matography he was able to establish thatchlorophyll is not a single compound. Hewas able to work out the chemical formu-lae of these compounds. He won the 1915Nobel Prize for chemistry for his work onplant pigments.

Windaus, Adolf Otto Reinhold (1876–1959) German chemist. He was awardedthe Nobel Prize for chemistry in 1928 forhis research into the constitution of thesterols and their connection with the vita-mins.

Wöhler, Friedrich (1800–82) Germanchemist. Wöhler is most famous for his dis-covery in 1828 that urea can be made byheating ammonium thiocyanate. The sig-nificance of this discovery was that it wasthe first time an organic substance hadbeen synthesized in the laboratory. Previ-ously there was a widespread view, calledvitalism, that organic substances could besynthesized only by living organisms. To-gether with Justus von LIEBIG, he was re-sponsible for a number of importantdiscoveries in organic chemistry such asisomerism. Wöhler also made significantdiscoveries in inorganic chemistry. This in-cluded the isolation of aluminium (1827)and beryllium (1828).

Wöhler’s synthesis A synthesis of ureaby evaporating the inorganic compoundammonium cyanate (NH4

+NCO–), per-formed in 1828 by Friedrich WÖHLER. Atthe time it was believed that there was adistinction between organic and inorganiccompounds in that organic compoundscould be made only be living organisms (anidea known as vitalism). Urea is present inthe urine of mammals and was regarded asdefinitely ‘organic’ in this sense. Ammo-nium cyanate is a definite inorganic com-pound. The discovery is sometimes said tomark the death of vitalism, although, infact, it was many years before the idea wasfinally abandoned.

wood alcohol See methanol.

Woodward, Robert Burns (1917–79)American organic chemist. Robert Wood-ward and his colleagues synthesized manycomplicated molecules. In 1944 togetherwith William von Eggers Doering, Wood-ward succeeded in synthesizing quinine. Inthe late 1940s he determined the structuresof penicilin and strychnine. Among themolecules he synthesized were cholesteroland cortisone, lysergic acid (1954), chloro-phyll (1960), and vitamin B12 (1971). Hiswork on vitamin B12 led Woodward andRoald HOFFMANN to put forward theWoodward–Hoffmann rules governing theconservation of orbital symmetry in themid 1960s. Woodward and Hoffmanngave an account of this work in the bookConservation of Orbital Symmetry (1970).Woodward won the 1965 Nobel Prize forchemistry for his work in synthesizingcomplex organic molecules. He wouldhave shared another Nobel Prize withHoffmann if he had not died in 1979.

Woodward–Hoffmann rules A set ofrules used in analyzing the progress of PER-ICYCLIC REACTIONS. They are based on afundamental analysis of the symmetry ofthe p orbitals in the reactants and how itcorrelates with the symmetry of the prod-ucts. It is an alternative to the related FRON-TIER ORBITAL theory. This approach wasput forward in the 1960s by Woodward

225

Woodward–Hoffmann rules

and the Polish–American theoreticalchemist Roald HOFFMANN.

Wurtz–Fittig reaction See Wurtz reac-tion.

Wurtz reaction A reaction for prepar-ing alkanes by refluxing a haloalkane (RX)with sodium metal in dry ether:

2RX + 2Na → RR + 2NaXThe reaction involves the coupling of twoalkyl radicals. The Fittig reaction is a simi-lar process for preparing alkyl-benzene hy-drocarbons by using a mixture of halogen

compounds. For example, to obtainmethylbenzene:

C6H5Cl + CH3Cl + 2Na → C6H5CH3 +2NaCl

In this mixed reaction phenylbenzene(C6H5C6H5) and ethane (CH3CH3) arealso produced by side reactions.

The Wurtz reaction is named for theFrench chemist Charles Adolphe Wurtz(1817–84), who developed the method in1855. The Fittig reaction is named forRudolph Fittig (1835–1910), a Germanchemist who worked with Wurtz. It isoften called the Wurtz–Fittig reaction.

Wurtz–Fittig reaction

226

227

xanthate A salt or ester of XANTHIC ACID

containing the ion –SCS(OR) or the group–SCS(OR) (where R is an organic group).Cellulose xanthate is used to make RAYON.

xanthene (CH2(C6H4)2O) A yellowcrystalline organic compound, used inmaking dyestuffs and fungicides.

xanthic acid (HSCS(OR)) Any of sev-eral unstable organic acids (where R is anorganic group). The esters and salts of xan-thic acid have various industrial applica-tions. See xanthate.

xanthine (2,6-dioxypurine; C5H4N2O2) A poisonous colorless crystalline organiccompound that occurs in blood, coffeebeans, potatoes, and urine. It is used as achemical intermediate.

xanthone (dibenzo-4-pyrone; CO-(C6H4)2O) A colorless crystalline organiccompound found as a pigment in gentiansand other flowers. It is used as an insecti-cide and in making dyestuffs.

xanthophyll One of a class of yellow toorange pigments derived from carotene,the commonest being lutein. See carot-enoids; photosynthetic pigments.

xanthoproteic test A standard test forproteins. Concentrated nitric acid is addedto the test solution. A yellow precipitateproduced either immediately or on gentleheating indicates a positive result.

x-radiation An energetic form of elec-tromagnetic radiation. The wavelengthrange is 10–11 m to 10–8 m. X-rays are nor-

mally produced by absorbing high-energyelectrons in matter. The radiation can passthrough matter to some extent (hence itsuse in medicine and industry for investigat-ing internal structures). It can be detectedwith photographic emulsions and deviceslike the Geiger-Müller tube.

X-ray photons result from electronictransitions between the inner energy levelsof atoms. When high-energy electrons areabsorbed by matter, an x-ray line spectrumresults. The structure depends on the sub-stance and is thus used in x-ray spec-troscopy. The line spectrum is alwaysformed in conjunction with a continuousbackground spectrum. The minimum (cut-off) wavelength λ0 corresponds to the max-imum x-ray energy, Wmax. This equals themaximum energy of electrons in the beamproducing the x-rays. Wavelengths in thecontinuous spectrum above λ0 are causedby more gradual energy loss by the elec-trons, in the process called bremsstrahlung(braking radiation).

x-ray crystallography The study of theinternal structure of crystals using the tech-nique of x-ray diffraction.

x-ray diffraction A technique used todetermine crystal structure by directing x-rays at the crystals and examining thediffraction patterns produced. At certainangles of incidence a series of spots are pro-duced on a photographic plate; these spotsare caused by interaction between the x-rays and the planes of the atoms, ions, ormolecules in the crystal lattice.

x-rays See x-radiation.

xylene See dimethylbenzene.

X

228

ylid (ylide) A type of ZWITTERION inwhich the two charges are on adjacentatoms.

ylide See ylid.

yocto- Symbol: y A prefix denoting

10–24. For example, 1 yoctometer (ym) =

10–24 meter (m).

yotta- Symbol: Y A prefix denoting

1024. For example, 1 yottameter (Ym) =

1024 meter (m).

Y

229

Zeisel reaction The reaction of an etherwith excess concentrated hydroiodic acid.On refluxing, a mixture of iodoalkanes isformed:

ROR′ + 2HI → H2O + RI + R′IAnalysis to identify the iodoalkanes

gives information about the compositionof the original ether. It was developed by S.Zeisel in 1886.

zeolite A member of a group of hydratedaluminosilicate minerals, which occur innature and are also manufactured for theirion-exchange and selective-absorptionproperties. They are used for water soften-ing and for sugar refining. The zeoliteshave an open crystal structure and can beused as molecular sieves.

See also ion exchange; molecular sieve.

zepto- Symbol: z A prefix denoting10–21. For example, 1 zeptometer (zm) =10–21 meter (m).

zero order Describing a chemical reac-tion in which the rate of reaction is inde-pendent of the concentration of a reactant;i.e.

rate = k[X]0

The concentration of the reactant re-mains constant for a period of time al-though other reactants are beingconsumed. The hydrolysis of 2-bromo-2-methylpropane using aqueous alkali has arate expression,

rate = k[2-bromo-2-methylpropane]i.e. the reaction is zero order with re-

spect to the concentration of the alkali. Therate constant for a zero reaction has theunits mol dm–3 s–1.

zero point energy The energy pos-sessed by the atoms and molecules of a sub-stance at absolute zero (0 K).

zetta- Symbol: Z A prefix denoting 1021.For example, 1 zettameter (Zm) = 1021

meter (m).

Ziegler, Karl (1898–1973) German or-ganic chemist. Karl Ziegler is best knownfor his research into polymers, particularlyZiegler–Natta catalysts. In 1953 Zieglerfound that catalysts consisting oforganometallic compounds mixed withmetals such as titanium polymerize etheneinto a long-chain polymer with usefulproperties such as a high melting point.This method of producing polymers doesnot need high temperatures or pressures.This type of catalysis was developed fur-ther by Giulio NATTA. Ziegler and Nattashared the 1963 Nobel Prize for chemistryfor their work on Ziegler–Natta catalysts.

Ziegler process A method for the man-ufacture of high-density polyethene using acatalyst of titanium(IV]] chloride and tri-ethyl aluminum (Al(C2H5)3) under slightpressure. The mechanism involves forma-tion of titanium alkyls

TiCl3(C2H5)which coordinate the sigma orbitals of tita-nium with the π bond of ethene. The chain-length, and henfce the density, of thepolymer can be controlled. It is named forthe German chemist Karl Ziegler(1896–1973), who introduced it in 1953.See also Natta process.

zwitterion (ampholyte ion) An ion thathas both a positive and negative charge onthe same species. Zwitterions occur when a

Z

zwitterion

230

molecule contains both a basic group andan acidic group; formation of the ion can be regarded as an internal acid-base reaction. For example, amino-ethanoicacid (glycine) has the formula H2N.CH2.-COOH. Under neutral conditions it existsas the zwitterion H3N.CH2.COO–, which

can be formed by transfer of a proton fromthe carboxyl group to the amine group. Atlow pH (acidic conditions) the ion +H3N.-CH2.COOH is formed; at high pH (basicconditions) H2N.CH2.COO– is formed.See also amino acid.

APPENDIXES

In the examples below, the systematic name is given first, followed by the trivial (com-mon) name.

Simple saturated monocarboxylic acids:

methanoic formic HCOOHethanoic acetic CH3COOHpropanoic proprionic C2H5COOHbutanoic butyric C3H7COOHpentanoic valeric C4H9COOHhexanoic caproic C5H11COOHheptanoic enathic C6H13COOHoctanoic caprylic C7H15COOHnonanoic pelargonic C8H17COOHdecanoic capric C9H19COOH

Other simple saturated acids are found in naturally occurring glycerides. They all containeven numbers of carbon atoms:

dodecanoic lauric C11H23COOHtetradecanoic myristic C13H27COOHhexadecanoic palmitic C15H31COOHoctadecanoic stearic C17H35COOHeicosanoic arachidic C19H39COOHdocosanoic behenic C21H43COOHtetracosanoic lignoceric C23H47COOHhexacosanoic cerotic C25H51COOH

Certain important unsaturated monocarboxylic acids occur naturally:

octadec-9-enoic oleic C8H17CH=CHC7H14COOHoctadeca-9,12- dienoic linoleic C5H11CH=CHCH2CH=CHC7H1 4COOHoctadeca-9,12,15- trienoic linolenic C2H5CH=CHCH2CH=CHCH2CH=CH-

C 7H14COOH

There are a number of common saturated dicarboxylic acids:

ethanedioic oxalic HOOCCOOHpropanedioic malonic HOOCCH2COOHbutanedioic succinic HOOCC2H4COOHpentanedioic glutaric HOOCC3H6COOHhexanedioic adipic HOOCC4H8COOH

233

Carboxylic Acids

Appendix I

Examples of unsaturated dicarboxylic acids are:

cis-butenedioic maleic HOOCCH=CHCOOHtrans-butenedioic fumaric HOOCCH=CHCOOH

Certain hydroxy acids occur naturally:

hydroxyethanoic glycolic CH2(OH)COOH2-hydroxypropanoic lactic CH3CH(OH)COOHhydroxybutanedioic malic CH(OH)CH2(COOH)22-hydroxy-propane-1,2,3-tricarboxylic citric (CH2)2C(OH)(COOH)3

Naturally occurring amino carboxylic acids are shown in Appendix II.

Appendix I

234

235

Amino Acids

Appendix II

NH2

CH2

COOH

glutamine glycineNH2

CHC

H2

CH2

O

NH2COOH

NH2

CH

CH3 COOH

NH2

CHC

H2

CH2

CH2

NH

NH

NH2

COOH

alanine arginine

NH2

CHC

H2

SHCOOH

cysteine glutamic acid

NH2

CHC

H2

CH2

COOH

HOOC

NH2

CHC

H2

O

NH2

COOH

asparagine

NH2

CHC

H2

O

OH

COOH

aspartine

Appendix II

236

NH2

CHC

H2

CHCH3

CH3

COOH

NH2

CHC

H2

CH2

CH2

CH2

NH2

COOH

leucine lysine

NH2

CHC

H2

CH2

SCH3 COOH

methionine

NH2

CH

CH2

CHCH

CH

CHCH

COOH

phenylalanine

NH

CH

CH2

CH2

CH2

COOH

NH2

CHC

H2

OHCOOH

proline serine

NH2

CHC

HCH2

O

CH3

CH3

COOH

NH2

CH

O

CH2

N

CHCH

NH

COOH

histidine isoleucine

Amino Acids

237

Appendix II

Amino Acids

CHC

H

NH2

CH3

OH

COOH

NH2

CHC

H2

CHNH

CH

CH

CH

CH

COOH

threonine tryptophan

NH2

CH

CH2

CHCH

CHCH

OH

COOH

NH2

CHC

HCH3

CH3

COOH

tyrosine valine

238

Sugars

Some simple monosaccharides. The β-D-form is shown in each case

Appendix III

O

H

OH

H

HH

OH H

HO

H

OH

arabinose

OHOCH2

CH2OH

OH

HO

OH

βfructose

O

OH

HOCH2

OH

HO OH

galactose

O

OH

HO

HOCH2

OH

OH

glucose

O

OH OH

HOCH2 OH

ribose

O

OH

OH

HO

OH

xylose

239

Nitrogenous Bases and Nucleosides

N

N N

N

NH2

OHOCH2

OH OH

adenosine

NH

NHOCH2 O

H3C

O

O

OH

thymidine

NH

NH

O

O

H3C

thymine

HN

N

N

NH

O

H2N

6543

21 7

89

guanine

HN

N N

N

H2N

HOCH2

OHOH

O

O

guanosine

Appendix IV

N

N N

N

NH2

H

71

3

6

9

adenine

240

N

N

NH2

O

3

1

H

cytosine

O

OH OH

HOCH2N

NH2

O

N

cytidine

HN

NH

O

O

uracil

HN

N

O

O

O

OH OH

HOCH2

uridine

Nitrogenous Bases and Nucleosides

Appendix IV

241

Appendix VThe Chemical Elements

(* indicates the nucleon number of the most stable isotope)

Element Symbol p.n. r.a.m Element Symbol p.n. r.a.m

actinium Ac 89 227*

aluminum Al 13 26.982

americium Am 95 243*

antimony Sb 51 112.76

argon Ar 18 39.948

arsenic As 33 74.92

astatine At 85 210

barium Ba 56 137.327

berkelium Bk 97 247*

beryllium Be 4 9.012

bismuth Bi 83 208.98

bohrium Bh 107 262*

boron B 5 10.811

bromine Br 35 79.904

cadmium Cd 48 112.411

calcium Ca 20 40.078

californium Cf 98 251*

carbon C 6 12.011

cerium Ce 58 140.115

cesium Cs 55 132.905

chlorine Cl 17 35.453

chromium Cr 24 51.996

cobalt Co 27 58.933

copper Cu 29 63.546

curium Cm 96 247*

darmstadtium Ds 110 269*

dubnium Db 105 262*

dysprosium Dy 66 162.50

einsteinium Es 99 252*

erbium Er 68 167.26

europium Eu 63 151.965

fermium Fm 100 257*

fluorine F 9 18.9984

francium Fr 87 223*

gadolinium Gd 64 157.25

gallium Ga 31 69.723

germanium Ge 32 72.61

gold Au 79 196.967

hafnium Hf 72 178.49

hassium Hs 108 265*

helium He 2 4.0026

holmium Ho 67 164.93

hydrogen H 1 1.008

indium In 49 114.82

iodine I 53 126.904

iridium Ir 77 192.217

iron Fe 26 55.845

krypton Kr 36 83.80

lanthanum La 57 138.91

lawrencium Lr 103 262*

lead Pb 82 207.19

lithium Li 3 6.941

lutetium Lu 71 174.967

magnesium Mg 12 24.305

manganese Mn 25 54.938

meitnerium Mt 109 266*

mendelevium Md 101 258*

mercury Hg 80 200.59

molybdenum Mo 42 95.94

neodymium Nd 60 144.24

242

Appendix V

The Chemical ElementsElement Symbol p.n. r.a.m Element Symbol p.n. r.a.m

neon Ne 10 20.179

neptunium Np 93 237.048

nickel Ni 28 58.69

niobium Nb 41 92.91

nitrogen N 7 14.0067

nobelium No 102 259*

osmium Os 76 190.23

oxygen O 8 15.9994

palladium Pd 46 106.42

phosphorus P 15 30.9738

platinum Pt 78 195.08

plutonium Pu 94 244*

polonium Po 84 209*

potassium K 19 39.098

praseodymium Pr 59 140.91

promethium Pm 61 145*

protactinium Pa 91 231.036

radium Ra 88 226.025

radon Rn 86 222*

rhenium Re 75 186.21

rhodium Rh 45 102.91

rubidium Rb 37 85.47

ruthenium Ru 44 101.07

rutherfordium Rf 104 261*

samarium Sm 62 150.36

scandium Sc 21 44.956

seaborgium Sg 106 263*

selenium Se 34 78.96

silicon Si 14 28.086

silver Ag 47 107.868

sodium Na 11 22.9898

strontium Sr 38 87.62

sulfur S 16 32.066

tantalum Ta 73 180.948

technetium Tc 43 99*

tellurium Te 52 127.60

terbium Tb 65 158.925

thallium Tl 81 204.38

thorium Th 90 232.038

thulium Tm 69 168.934

tin Sn 50 118.71

titanium Ti 22 47.867

tungsten W 74 183.84

uranium U 92 238.03

vanadium V 23 50.94

xenon Xe 54 131.29

ytterbium Yb 70 173.04

yttrium Y 39 88.906

zinc Zn 30 65.39

zirconium Zr 40 91.22

243

Appendix VI

1

1

H2

He

1 2 3 4 5 6

3

Li

4

Be

5

B6

C7

N8

O9

F1

0 Ne

11 N

a1

2 Mg

13 A

l1

4 Si1

5

P1

6

S1

7 Cl

18 A

r

19 K

20 C

a2

1 Sc2

2 Ti

23 V

24 C

r2

5 Mn

26 F

e2

7 Co

28 N

i2

9 Cu

30 Z

n3

1 Ga

32 G

e3

3 As

34 Se

35 B

r3

6 Kr

37 R

b3

8 Sr3

9 Y4

0 Zr

41 N

b4

2 Mo

43 T

c4

4 Ru

45 R

h4

6 Pd

47 A

g4

8 Cd

49 In

50 Sn

51 Sb

52 T

e5

3

I5

4 Xe

55 C

s5

6 Ba

57-7

1

La-

Lu

72 H

f7

3 Ta

74 W

75 R

e7

6 Os

77 Ir

78 P

t7

9 Au

80 H

g8

1 Tl

82 P

b8

3 Bi

84 P

o8

5 At

86 R

n

87 F

r8

8 Ra

89-1

03

Ac-

Lr

10

4 Rf

10

5 Db

10

6 Sg1

07 Bh

10

8 Hs

10

9 Mt

11

0 Ds

11

1

Uu

u1

12

Uu

b1

13

11

4

Uu

q1

15

11

6

Uu

h

23

45

67

89

10

11

12

13

14

15

16

17

18

Peri

odic

Tab

le o

f th

e E

lem

ents

- gi

vin

g gr

ou

p,

ato

mic

nu

mb

er,

and

ch

emic

al s

ymb

ol

Lan

than

ides

Act

inid

es

Mo

der

n f

orm

Eu

rop

ean

con

ven

tio

nN

. A

mer

ican

con

ven

tio

n

Th

e ab

ove

is

the

mo

der

n r

eco

mm

end

ed f

orm

of

the

tab

le u

sin

g 1

8gr

ou

ps.

Old

er g

rou

p d

esig

nat

ion

s ar

e sh

ow

n b

elo

w.

Period

6 7

57 L

a

89 A

c

58 C

e

90 T

h

59 P

r

91 P

a

60 N

d

92 U

61 P

m

93 N

p

62 Sm

94 P

u

63 E

u

95 A

m

64 G

d

96 C

m

65 T

b

97 B

k

66 D

y

98 C

f

67 H

o

99 E

s

68 E

r

10

0 Fm

69 T

m

10

1 Md

70 Y

b

10

2 No

71 L

u

10

3 Lr

12

34

56

78

91

01

11

21

31

41

51

61

71

8

IAII

AII

IAIV

AV

AV

IAV

IIA

VII

I (o

r V

IIIA

)IB

IIB

IIIB

IVB

VB

VIB

VII

B0

(o

r V

IIIB

)

IAII

AII

IBIV

BV

BV

IBV

IIB

VII

I (o

r V

IIIB

)IB

IIB

IIIA

IVA

VA

VIA

VII

AV

IIIA

(or

0)

7

244

The Greek Alphabet

A α alphaB β betaΓ γ gamma∆ δ deltaE ε epsilonZ ζ zetaH η etaΘ θ thetaI ι iotaK κ kappaΛ λ lambdaM µ mu

N ν nuΞ ξ xiO ο omikronΠ π piP ρ rhoΣ σ sigmaT τ tauΥ υ upsilonΦ φ phiX χ chiΨ ψ psiΩ ω omega

Appendix VII

245

Fundamental Constants

Appendix VIII

speed of light c 2.997 924 58 × 108 m s–1

permeability of free space µo 4π × 10–7

= 1.256 637 0614 × 10–6 H m–1

permittivity of free space ε0=µ0–1c–2 8.854 187 817 × 10–12 F m–1

charge of electron or proton e ±1.602 177 33 × 10–19 Crest mass of electron me 9.109 39 × 10–31 kgrest mass of proton mp 1.672 62 × 10–27 kgrest mass of neutron mn 1.674 92 × 10–27 kgelectron charge-to-mass ratio e/m 1.758 820 × 1011 C kg–1

electron radius re 2.817 939 × 10–15 mPlanck constant h 6.626 075 × 10–34 J sBoltzmann constant k 1.380 658 × 10–23 J K–1

Faraday constant F 9.648 531 × 104 C mol–1

246

Chemical society webpages include:

American Chemical Society www.chemistry.org/

Royal Society of Chemistry www.rsc.org/

The International Union of Pure and www.iupac.orgApplied Chemistry

Information on nomenclature is available at:

Queen Mary College, London www.chem.qmul.ac.uk/iupac/

Advanced Chemistry Development, Inc www.acdlabs.com/iupac/nomenclature/

An extensive set of organic chemistry links can be found at:

WWW Virtual Library, Chemistry Section www.liv.ac.uk/Chemistry/Links/

Webpages

Appendix IX

There are a number of comprehensive texts covering organic chemistry. These include:

Carey, Francis & Sundberg, Richard J. Advanced Organic Chemistry: Structure andMechanism. 4th ed. New York: Plenum, 2000

Carey, Francis & Sundberg, Richard J. Advanced Organic Chemistry: Reactions. 4th ed.New York: Plenum, 2000

Clayden, Jonathon; Greeves, Nick; Warren, Stuart; & Wothers, Peter Organic Chemistry.Oxford, U.K.: Oxford University Press, 2000

McMurray, John Organic Chemistry. 6th ed. Pacific Grove, Calif.: Brooks/Cole, 2004

March, Jerry Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. 4thed. New York: Wiley, 1992

Volhardt, Peter K. & Schore, Neil E. Organic Chemistry: Structure and Function. 4th ed.New York: W. H. Freeman, 2003

More specialized books are:

Eliel, Ernest L.; Wilen, Samuel H.; & Doyle, Michael P. Basic Organic Stereochemistry.New York: Wiley, 2001

Gilchrist, Thomas L. Heterocyclic Chemistry. 3rd ed. Harlow, U.K.: Longman, 1997

Isaacs, Neil S. Physical Organic Chemistry. Harlow, U.K.: Longman, 1987

Sykes, Peter Guidebook to Mechanism in Organic Chemistry. 6th ed. Harlow, U.K.:Longman, 1996

An advanced text on biochemistry is:

Nelson, David L. & Cox, Michael M. Lehninger Principles of Biochemistry. 3rd ed. NewYork: Worth Publishers, 2000

247

Bibliography