chapter one alkanes and cycloalkanesscbaghdad.edu.iq/files/lectures/bio/2019/chem.pdf4- wurtz...

Chapter One Alkanes and Cycloalkanes

Oday H. Raoof Al-Jelawi Assis. Prof., PhD Organic Chemistry

Dep. Of chemistry/College of science/ University of Baghdad/ Iraq

Families of Organic Compounds • Organic compounds can be grouped into families by

their common structural features • We shall survey the nature of the compounds in a

tour of the families in this course • This chapter deals with alkanes, compounds that

contain only carbons and hydrogens, all connected exclusively by single bonds

3.1 Functional Groups

• Functional group - collection of atoms at a site within a molecule with a common bonding pattern

• The group reacts in a typical way, generally independent of the rest of the molecule

• For example, the double bonds in simple and complex alkenes react with bromine in the same way (See Figure 3.1)

Survey of Functional Groups

• Table 3.1 lists a wide variety of functional groups that you should recognize

• As you learn about them in each chapter it will be easier to recognize them

• The functional groups affect the reactions, structure, and physical properties of every compound in which they occur

Types of Functional Groups: Multiple Carbon–Carbon Bonds

• Alkenes have a C-C double bond

• Alkynes have a C-C triple bond

• Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring

Functional Groups with Carbon Singly Bonded to an Electronegative Atom

• Alkyl halide: C bonded to halogen (C-X) • Alcohol: C bonded O of a hydroxyl group (COH) • Ether: Two C’s bonded to the same O (COC) • Amine: C bonded to N (CN) • Thiol: C bonded to SH group (CSH) • Sulfide: Two C’s bonded to same S (CSC) • Bonds are polar, with partial positive charge on C (δ+)

and partial negative charge (δ−) on electronegative atom

Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)

• Aldehyde: one hydrogen bonded to C=O • Ketone: two C’s bonded to the C=O • Carboxylic acid: OH bonded to the C=O • Ester: C-O bonded to the C=O • Amide: C-N bonded to the C=O • Acid chloride: Cl bonded to the C=O • Carbonyl C has partial positive charge (δ+) • Carbonyl O has partial negative charge (δ-).

3.2 Alkanes and Alkane Isomers • Alkanes: Compounds with C-C single bonds and C-H

bonds only (no functional groups) • Connecting carbons can lead to large or small molecules • The formula for an alkane with no rings in it must be

CnH2n+2 where the number of C’s is n • Alkanes are saturated with hydrogen (no more can be

added • They are also called aliphatic compounds

Different Ways to Write Butane

Alkane Isomers • CH4 = methane, C2H6 = ethane, C3H8= propane • The molecular formula of an alkane with more than

three carbons can give more than one structural isomer – C4H10

–C5H12

Constitutional Isomers • Isomers that differ in how their atoms are arranged in

chains are called constitutional isomers • Compounds other than alkanes can be constitutional

isomers of one another • They must have the same molecular formula to be

isomers

Names of Small Hydrocarbons No. of Carbons Formula Name (CnH2n+2)

1 Methane CH4 2 Ethane C2H6 3 Propane C3H8 4 Butane C4H10 5 Pentane C5H12 6 Hexane C6H14 7 Heptane C7H16 8 Octane C8H18 9 Nonane C9H20 10 Decane C10H22

Names of Larger Hydrocarbons No. of Carbons Formula Name (CnH2n+2)

11 Undecane C12H26

12 Dodecane C12H26 13 Tridecane C13H28

14 Tetradecane C14H30

15 Pentadecane C15H32

16 Hexadecane C16H34

17 Heptadecane C17H36

18 Octadecane C18H38

19 Nonadecane C19H40

20 Isocane C20H42

3.3 Alkyl Groups • Alkyl group – remove one H from an alkane (a part of a

structure) • General abbreviation “R” (for Radical, an incomplete

species or the “rest” of the molecule) • Name: replace -ane ending of alkane with -yl ending

– CH3 is “methyl” (from methane) – CH2CH3 is “ethyl” from ethane

• See Table 3.4 for a list

Types of Alkyl groups • Classified by the connection site (See Figure 3.3)

– a carbon at the end of a chain (primary alkyl group) – a carbon in the middle of a chain (secondary alkyl

group) – a carbon with three carbons attached to it (tertiary

alkyl group)

3.4 Naming Alkanes • Compounds are given systematic names by a process that

uses – Prefix-Parent-Suffix

• Follows specific rules – Named as longest possible chain – Carbons in that chain are numbered in sequence – substituents are numbered at their point of attachment – Compound name is one word (German style) – Complex substituents are named as compounds would be

• See specific examples in text

3.5 Properties of Alkanes • Called paraffins (low affinity compounds) because they

do not react as most chemicals • They will burn in a flame, producing carbon dioxide,

water, and heat • They react with Cl2 in the presence of light to replace H’s

with Cl’s (not controlled)

Physical Properties • Boiling points and melting points increase as size of

alkane increases • Forces between molecules (temporary dipoles,

dispersion) are weak

4- wurtz reaction This method prepare symmetrical alkane

and yield even longest alkanes R-X + 2Na R Na + NaX R Na + R-X R-R + NaX

Ex: 2CH3CH2X + 2Na CH3CH2CH2CH3+2NaX

3.6 Cycloalkanes • Cycloalkanes are alkanes that have carbon atoms that

form a ring (called alicyclic compounds) • Simple cycloalkanes rings of CH2 units, (CH2)n, or

CnH2n • Structure is shown as a regular polygon with the number

of vertices equal to the number of C’s (a projection of the actual structure)

cyclopropane cyclohexane cyclopentane

cyclobutane

Complex Cycloalkanes • Naturally occurring materials contain cycloalkane

structures • Examples: chrysanthemic acid (cyclopropane),

prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane)

Properties of Cycloalkanes • Melting points are affected by the shapes and the way

that crystals pack so they do not change uniformly

3.7 Naming Cycloalkanes • Count the number of carbon atoms in the ring and the number in the

largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane

• For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible.

• Number the substituents and write the name • See text for more details and examples

H3C

CH3CH3C2H5 CH3

H3C

Give the IUPAC names or common name for the following:

Give the structures corresponding to the following names :

(1) / 1-Tert.butyl-2-methylcyclopentane

(2) / 1,1-Dimethylcyclobutane(3) / 1-Ethyl-4-isopropylcyclohexane

3.8 Cis-Trans Isomerism in Cycloalkanes

• Rotation about C-C bonds in cycloalkanes is limited by the ring structure

• Rings have two “faces” and substituents are labeled as to their relative facial positions

• There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)

Stereoisomers • Compounds with atoms connected in the same order but which

differ in three-dimensional orientation, are stereoisomers • The terms “cis” and “trans” should be used to specify stereoisomeric

ring structures • Recall that constitutional isomers have atoms connected in

different order

Preparation This preparation is an extension of wurtz reaction by using

[alkyl di halide (not adjacent) with metal ex. 2Na or Zn]

Home work Prepare the following 1- cyclopropane from 1,3-dibromo propane 2- cyclo hexane from 1,6-dichloro hexane 3- 1,4- dimethyl cyclobutane from suitable alkyldihalide

Reaction

Chapter Alkenes

Structure, Preparation and Reaction for biology



2

Alkene - Hydrocarbon With Carbon-Carbon Double Bond

• Also called an olefin but alkene is better • Includes many naturally occurring materials

– Flavors, fragrances, vitamins • An orange pigment responsible for the color of carrots, β-

carotene is an important dietary source of vitamin A and is thought to offer some protection against certain types of cancer

• Important industrial products These are feedstock's for industrial processes, Ethylene and propylene, the simplest alkenes, are the two most important organic chemicals produced industrially.

• Ethylene, propylene, and butene are synthesized industrially by steam cracking of light (C2–C8) alkanes

3

• Compounds derived industrially from ethylene and propylene

4

5

Nomenclature (Naming Alkenes)

• Alkenes are named using a series of rules similar to those for alkanes with the suffix -ene used instead of -ane to identify the functional group.

There are three steps. Step 1 Name the parent hydrocarbon. Find the longest carbon chain

containing the double bond, and name the compound accordingly, using the suffix -ene:

Step 2 Number the carbon atoms in the chain. Begin at the end nearer the double bond or, if the double bond is equidistant from the two ends, begin at the end nearer the first branch point. This rule ensures that the double bond carbons receive the lowest possible numbers.

6

Step 3 Write the full name. Number the substituents according to their positions in the chain, and list them alphabetically. Indicate the position of the double bond by giving the number of the first alkene carbon and placing that number directly before the parent name. If more than one double bond is present, indicate the position of each and use one of the suffixes -diene, -triene, and so on.

7

We should also note that IUPAC changed their naming recommendations in 1993 to place the locant indicating the position of the double bond immediately before the -ene suffix rather than before the parent name: but-2-ene rather than 2-butene, for instance.

Cycloalkenes • are named similarly, but because there is no chain end to begin from,

we number the cycloalkene so that the double bond is between C1 and C2 and the first substituent has as low a number as possible. It’s not necessary to indicate the position of the double bond in the name because it’s always between C1 and C2. As with open-chain alkenes, newer but not yet widely accepted naming rules place the locant immediately before the suffix in a diene.

8

9

Many Alkenes Are Known by Common Names

11

CH3

12

Electronic Structure of Alkenes

• Carbon atoms in a double bond are sp2-hybridized – Three equivalent orbitals at 120º separation in plane – Fourth orbital is atomic p orbital

• Combination of electrons in two sp2 orbitals of two atoms forms σ bond between them

• Additive interaction of p orbitals creates a π bonding orbital – Subtractive interaction creates a π anti-bonding orbital

• Occupied π orbital prevents rotation about σ-bond • Rotation prevented by π bond - high barrier, about 268 kJ/mole

in ethylene

13

Cis-Trans Isomerism in Alkenes

• The presence of a carbon-carbon double can create two possible structures – cis isomer - two similar

groups on same side of the double bond

– trans isomer similar groups on opposite sides

• Each carbon must have two different groups for these isomers to occur

Cis or Trans?

16

C CCH3

H

CH2CH3

HC C

CH3 CH3

CH2CH2CH3H

H.W./ Draw the isomers for the following: (1)- 5-chloropent-2-ene

(2)- 2-pentene

18

Sequence Rules: The E,Z Designation

• Neither compound is clearly “cis” or “trans” – Substituents on C1 are

different than those on C2

– We need to define “similarity” in a precise way to distinguish the two stereoisomers

• Cis, trans nomenclature only works for disubstituted double bonds

21

Develop a System for Comparison of Priority of Substituents

• Assume a valuation system – If Br has a higher “value”

than Cl – If CH3 is higher than H

• Then, in A, the higher value groups are on opposite sides

• In B, they are on the same side – Requires a universally

accepted “valuation”

22

E,Z Stereochemical Nomenclature

• Priority rules of Cahn, Ingold, and Prelog

• Compare where higher priority group is with respect to bond and designate as prefix

• E -entgegen, opposite sides

• Z - zusammen, together on the same side

23

Ranking Priorities: Cahn-Ingold-Prelog Rules

• Must rank atoms that are connected at comparison point • Higher atomic number gets higher priority

– Br > Cl > O > N > C > H

In this case,The higher priority groups are opposite: (E )-2-bromo-2-chloro-propene

24

Alkene Stability

• Cis alkenes are less stable than trans alkenes • Compare heat given off on hydrogenation: ∆Ho

• Less stable isomer is higher in energy – And gives off more heat – tetrasubstituted > trisubstituted > disubstituted >

monosusbtituted – hyperconjugation stabilizes alkyl

Prepartion of Alkenes

problems

30

Reactions of alkenes

31

32

Elimination Reactions

34

Addition Reactions

Chapter Alkynes

Structure, Preparation and Reaction for biology



2

Alkyne is a hydrocarbon that contains a carbon–carbon triple bond. Acetylene(HCΞCH), the simplest alkyne, was once widely used in industry as the starting material for the preparation of acetaldehyde, acetic acid, vinyl chloride

Naming Alkynes Alkyne nomenclature follows the general rules for hydrocarbons

discussed in (Alkane and Alkene) The suffix -yne is used, and the position of the triple bond is indicated by

giving the number of the first alkyne carbon in the chain. Numbering the main chain begins at the end nearer the triple bond so

that the triple bond receives as low a number as possible. Compounds with more than one triple bond are called diynes, triynes

enynes Numbering of an enyne chain starts from the end nearer the first

multiple bond. whether double or triple. When there is a choice in numbering,

double bonds receive lower numbers than triple bonds. For example:

3

5

Physical properties of alkynes Being compounds of low polarity, the alkynes have physical properties

that are essentially the same as those of the alkanes and alkenes. They are insoluble in water but quite soluble in the usual organic

solvents of low polarity: ligroin, ether, benzene, carbon tetrachloride. Their boiling points increase with increasing carbon number

Industrial source of acetylene

Preparation of alkynes

6

7

Reaction of sodium acetylides with alkyl halides permits conversion of smaller alkynes into larger ones. Practically, the reaction is limited to the use of primary halides because of the great tendency for secondary and tertiary halides to undergo a side reaction, elimination (formation alkene).

8

Reaction of alkynes The chemistry of the carbon-carbon triple bond like alkenes, alkynes

undergo electrophilic addition, and for the same reason: availability of the loosely held π electrons.

alkynes undergo certain reactions that are due to the acidity of a hydrogen atom held by triply-bonded carbon.

13

Addition of water to acetylene to form acetaldehyde All other higher alkynes yield or form ketones

14

Alkynes found in to types 1-terminal alkyne 2- internal or (medial) alkyne A mixture of both possible ketones results when an unsymmetrically

substituted internal alkyne (RC Ξ CR′) is hydrated. The reaction is therefore most useful when applied to a terminal

alkyne (RC Ξ CH) because only a methyl ketone is formed.

(5) Reaction of terminal alkynes

This reaction used to distinguish (Identification) between terminal alkynes and internal alkynes

Terminal alkynes (reaction) while internal alkyne (no reaction)

18

This reaction used to prepare longest alkynes from lowest alkynes

19

Why are terminal alkynes more acidic than alkenes or alkanes? The simplest explanation involves the hybridization of the negatively charged carbon atom. An acetylide anion has an sp-hybridized carbon, so the negative charge resides in an orbital that has 50% “s character.” A vinylic anion has an sp2-hybridized carbon with 33% s character, and an alkyl anion (sp3) has only 25% s character. Because (s ) orbitals are nearer the positive nucleus and lower in energy than p orbitals, the negative charge is stabilized to a greater extent in an orbital with higher s character

20

Acetylide anion can react with electrophiles, such as alkyl halides (must be primary), in a process that replaces the halide and yields a new alkyne product.

The alkylation reaction is limited to the use of primary alkyl halide because acetylide ions are sufficiently strong bases to cause elimination instead of substitution when they react with secondary and tertiary alkyl halides.

Chapter Benzene and Aromaticity



Today, the association of aromaticity with fragrance has long been lost, and we now use the word aromatic to refer to the class of compounds that contain six-membered benzene-like rings with three double bonds. Many naturally occurring compounds are aromatic in part, including steroids such as estrone and well-known pharmaceuticals such as the cholesterol-lowering drug atorvastatin, marketed as Lipitor. Benzene itself causes a depressed white blood cell count (leukopenia) on prolonged exposure and should not be used as a laboratory solvent.

2

4

Aromatic Compounds • Aromatic was used to described some fragrant compounds in

early 19th century – Not correct: later they are grouped by chemical behavior

(unsaturated compounds that undergo substitution rather than addition)

• Current: distinguished from aliphatic compounds by electronic configuration

5

Sources of Aromatic Hydrocarbons • From high temperature distillation of coal tar • Heating petroleum at high temperature and pressure over a

catalyst

6

Naming Aromatic Compounds • Many common names (toluene = methylbenzene; aniline =

aminobenzene) • Monosubstituted benzenes systematic names as hydrocarbons

with –benzene – C6H5Br = bromobenzene – C6H5NO2 = nitrobenzene, and C6H5CH2CH2CH3 is

propylbenzene

7

Common Names

9

The Phenyl Group

• When a benzene ring is a substituent, the term phenyl is used (for C6H5

) – You may also see “Ph” or “φ” in place of “C6H5”

• “Benzyl” refers to “C6H5CH2”

10

Disubstituted Benzenes

• Relative positions on a benzene ring – or tho- (o) on adjacent carbons (1,2) – meta- (m) separated by one carbon (1,3) – para- (p) separated by two carbons (1,4)

• Describes reaction patterns (“occurs at the para position”)

11

CH3

CH3

CH3H3C CH3

H3C

ortho-Xylene meta-Xylene para-Xylene

12

Naming Benzenes With More Than Two Substituents

• Choose numbers to get lowest possible values • List substituents alphabetically with hyphenated numbers • Common names, such as “toluene” can serve as root name

Aromaticity characters • The Criteria for Aromaticity:

Four structural criteria must be satisfied for a compound to be aromatic.

[1] A molecule must be cyclic. [2] A molecule must be planar. [3] A molecule must be completely conjugated. [4] A molecule must satisfy Hückel’s rule, which requiresa

particular number of π electrons. 4n+ 2 π electrons (n is 0,1,2,3,4)

15

NOTES π = 2e Pair of electron: a) inside ring add (2e) b) outside ring not add any thing Charges: a) Nagative charge add (2e) b)positive charge not add any thing Free radical: add (1e)

16

[1] A molecule must be cyclic.

To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms.

22

Aromaticity of the Cyclopentadienyl Anion • 1,3-Cyclopentadiene

contains conjugated double bonds joined by a CH2 that blocks delocalization

• Removal of H+ at the CH2 produces a cyclic 6-electron system, which is stable

• Removal of H- or H• generate nonaromatic 4 and 5 electron systems

• Relatively acidic (pKa = 16) because the anion is stable

Polycyclic Compounds

25

• Aromatic compounds can have rings that share a set of carbon atoms (fused rings)

• Compounds from fused benzene or aromatic heterocycle rings are themselves aromatic

26

Structure and Stability of Benzene • Benzene reacts with slowly with Br2 to give bromobenzene

(where Br replaces H) • This is substitution rather than the rapid addition reaction

common to compounds with C=C, suggesting that in benzene there is a higher barrier

27

Benzene’s Unusual Structure

• All its C-C bonds are the same length: 139 pm — between single (154 pm) and double (134 pm) bonds

• Electron density in all six C-C bonds is identical • Structure is planar, hexagonal • C–C–C bond angles 120° • Each C is sp2 and has a p orbital perpendicular to the plane of

the six-membered ring

28

Drawing Benzene and Its Derivatives • The two benzene resonance forms can be represented by a

single structure with a circle in the center to indicate the equivalence of the carbon–carbon bonds

• This does indicate the number of π electrons in the ring but reminds us of the delocalized structure

• We shall use one of the resonance structures to represent benzene for ease in keeping track of bonding changes in reactions

29

Bond distances and Bond Angles of Benzene

• If benzene is 1,3,5-cyclohexatriene as Kekulé proposed, what should its chemistry be? Alkenes, dienes, cyclcoalkenes, etc. typically give addition reactions with electrophiles. But benzene doesn’t undergo the reactions typical of unsaturated hydrocarbons!

30

32

Reactions of Benzene

36

Reactions of Benzene

37

Substituent Effects in Aromatic Rings • Substituents can cause a compound to be (much) more or

(much) less reactive than benzene • Substituents affect the orientation of the reaction – the

positional relationship is controlled – ortho- and para-directing activators, ortho- and para-

directing deactivators, and meta-directing deactivators

38

Activators/ Deactivators

• Arrange the following aromatic organic compound towards electrophilic aromatic substitution.

1- Nitro benzene, acetophenone, bromobenzene, benzonitrile, benzaldehyde 2- anisole, aniline, phenol, benzene, chlorobenzene, benzoic acid 3- N-methylaniline, N,N-dimethyl aniline, toluene, benzenesulphonic acid

47

chapter one alkanes and cycloalkanesscbaghdad.edu.iq/files/lectures/bio/2019/chem.pdf4- wurtz...

Documents