organic chem 2011

8/2/2019 Organic Chem 2011

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Organic molecules

Organic chemistry is defined as the chemistry ofcompounds that consist primarily ofcarbonandhydrogen.

Atoms ofnitrogen, oxygen and the halogens are

also sometimes part of organic molecules.


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The chemistry of carbon

Carbon atoms have the ability to form strongcovalent bonds with other carbon atoms and also

with atoms of other non-metals.

Carbon atoms bond with other carbon atoms toform chains or ring structures. These chains can bethousands of atoms long.

Catenation the ability of carbon atoms to bondwith themselves to form chain or ring structures.


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Important features of carbonAllotropes of carbon exist. Graphite and diamond are

two examples.

Allotropes are different crystalline structures of the same

element.Diamond Graphite


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Carbon has avalency of 4. This means it can form 4

bonds and has 4 valence electrons (4 electrons in theoutermost energy level).

Carbon will form a maximum ofFOURbonds withother elements.


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A carbon atom in the ground state

1s

2s

2p

1s 2s2pElectron configuration:


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A carbon atom in the excited state

In the excited state, valence electrons are unpaired

i.e. the 2s electron moves to higher 2p orbital.

1s

2s

2p

C

1s 2s2pElectron configuration:

CouperStructure


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The hydrocarbons Hydrocarbons organic molecules that are made up of

carbon and hydrogen atoms only.

Saturated hydrocarbons hydrocarbons which have allfour bonds to the carbon atoms in its structure singularlyoccupied.

Alkanes CHH

H

H CH

H

H

C

H

H

H


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The Hydrocarbons

Unsaturated hydrocarbons organic molecules whichhave multiple bonds (double or triple) to the carbonatoms within its structure.

AlkenesCH

H

C

H

H

Alkynes CH C H


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Homologous series

Homologous series a family of organic molecules whichare identified by the same functional group and obey thesame general formula.


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Functional group

Functional group a bond, atom or group of atoms inthe organic molecule which identifies to whichhomologous series that molecule belongs and isresponsible for the chemical properties of that molecule.

C C

Functional group s of hydrocarbons:

C C C C

Alkanes Alkenes Alky


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Naming the hydrocarbons

Number the longest continuouschain of carbon atoms from the endnearest the functional group. Thisdetermines the prefix of the name.

No of C

atoms

Prefix

1 Meth

2 eth -

3 prop -

4 but -

5 pent -6 hex -

7 hept -

8 oct -

Alkanes (single bonds) end in -ane.

Alkenes (double bond) end in -ene.

Alkynes (triple bond) end in yne.

The functional group determinesthe suffix:


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Types of formulae

Structural formula the structure of an organic moleculeshowing all the bonds to all the atoms in the molecule.

CH

H

H

C

H

H

C

H

H

C

H

H

H

Butane

CH

H

H

C

H

C

H

C

H

H

H

But-2-ene

1 2 3 4 12 3 4


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Types of formulae

Semi (condensed) structural formula the structure ofan organic molecule that only shows the most importantbonds in the molecule.

C H C H C H C H 3 2 2 3 C H C C C H3 3

H H

Butane But-2-ene

C H C C C H3 3

But-2-yne


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Types of formulae Molecular formula only indicates the number and

types of atoms present in the organic compound.

C3H8 C5H10 C3H4

General formula:

Alkanes

CnH2n+2

Alkenes

CnH2n

Alkynes

CnH2n -2

Draw the structural formulae for these molecules and name them.


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AlkanesAlkanes are non-polar molecules containing onlycarbon

and hydrogen atoms. They are hydrocarbonswith singlebondsbetween the carbon atoms.

They are saturated. This makes the alkanes quiteunreactive.

As the size of the molecule increases, the strength of thevan derWaals forces increases hence boiling pointsincrease. Methane to butane are gases followed by liquids.The larger molecules (more than 12 carbon atoms) are solids

at room temperature.


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Name these alkanes and write down the molecular formula

for each molecule.

methane

CH4

ethane

C2H6

propane

C3H8


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Complete Exercise 1


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Alkenes The alkenes contain at least one double bond

between two carbon atoms. This makes themunsaturated hydrocarbons.

The simplest alkene is ethene (ethylene).

Ethene is an important commercial organic chemicaland is used to make the plastic polythene(polyethylene).

H C C H

HH


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H2C = CHCH2 CH31 2 3 4

But - 1 - ene


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Alkynes The alkynes contain at least one triple bond between

two carbon atoms.

This makes them unsaturated hydrocarbons.

The simplest alkyne is ethyne (acetylene).

The combustion of ethyne and oxygen releases amassive amount of heat. (3 000 0 C). It is used in theoxyacetylene torch to cut metals.


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HC CCH2 CH31 2 3 4

But-1-yne


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Branched organic moleculesAlkyl substituent a carbon based side chain

which is attached to the longest continuous carbonchain in an organic molecule.

Alkyl group structure Alkyl nameCH3 - methyl

CH3-CH2- ethylCH3-CH2-CH2- propyl


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Rules for naming organic compounds

(IUPAC rules)1. Identify the functional group of the molecule this

determines the ending of the name.

C C C C

H

H

C H3

C H3

C H2C H

3

ClBr

H

H

Alkene


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2. Find the longest continuous carbon chain that

contains the functional group, and allocate its prefixaccording to the number of carbon atoms in the chain(see table for prefixes).

C C C C

H

HC H

3

C H3

C H2

C H3

ClBrH

H

6 carbon atoms = hexene.


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3.Number the carbon atoms in the chain. Number

them so that the functional group is on the carbonof lowest possible number. Double and triple bondstake preference over side chains.

1C C C C

H

H

C H3

C H3

C H2C H

3

ClBr

H

H

2 43

6

5

hex -2-ene.


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4.Name the alkyl group according to the number of

carbon atoms it has and give it a number accordingto the carbon atom it is attached to.

Alkyl groups must be named in alphabetical order.

1C C C C

H

H

C H3

C H3

C H2C H

3

ClBr

H

H

2 43

6

5

4,4-di-methyl-hex-2-ene


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5.If a halogen atom is attached to the carbon chain, it is

treated as an alkyl group.The prefixes: fluoro-, chloro-, bromo -, and iodoare used. They must be named in alphabetical order.

1C C C C

H

H

C H3

C H3

C H2C H

3

ClBr

H

H

2 43

6

5

1-bromo-2-chloro-4,4-di-methyl-hex-2-ene


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6.If there is more than one alkyl group of the

same kind, use the Greek prefixes di, tri andtetra to indicate this.

1C C C C

Br

C H3

H

H

ClBr

Cl

Cl

2 43 5

1,3-di-bromo-1,1,2-tri-chloro-pent-2-ene


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Branched organic molecules

CH3 CHCH2 CH3

CH3

1 2 3 4

2methyl-butane


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CH3 CHCH CH31 2 3 4

CH3 CH3

2,3di-methyl-butane


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1 2 3 4

CH3

3-ethyl-3-methyl-pentane

CH2

CH2

CH3

CH3CH2

CH3

C5


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1 2 3 4

H2C = CHCH CH3

CH3

3methyl-but-1 -ene


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1 2

H2C = CHC CH3

CH3

3,3di-methyl-but-1-ene

CH33 4


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1 2 3 4

HC CCH CH3CH3

3-methyl-but-1-yne


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Draw the semi structural formula for each of the following:

(a) 2,3-di-bromo-butane

(b) 4,5-di-methyl-hex-2-ene

(c) 4-bromo-4-chloro-3-ethyl-pent-1-yne

(d) 2-bromo-3,3,4-tri-chloro-pentane

Exercise 2


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Dienes

These alkene molecules contain two double bondssituated throughout the longest continuous carbon chain.

Examples

(a)1 2 3 4 5

CH3H2C = C CH = CHCH3

2methylpenta1,3-diene


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2 4 53

H2C = CCH2

CH3

CH = CH

CH36

1

(b)

2methylhexa1,4-diene

CH3H2C = C = C

CH3

1 2 3 4(c)

3methylbuta1,2-diene


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Diynes These alkyne molecules contain two triple bonds

situated throughout the longest continuous carbon chain.

Example

1 2 3 4 5

CHHC CCH C

CH3

3methylpenta1,4-diyne


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Isomerism Examine the two structural formulae shown below. Both

are structural formulae with molecular formulae C6H14

CH C C H3

C H2

C H3

C H3

3CH CH C H

2C H

2

C H3

C H33

Isomer s organic molecules which has the same

molecular formula but different structural formulae.

2,2-di-methyl-butane 2-methyl-pentane These two molecules are said to be ISOMERS.


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Draw the isomers ofC5H12


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Alkyl halides (Haloalkanes)

Functional group: XC

where X = F, Cl, Br, I

Exercise 3:

Draw the structural formula for:

(a) chloromethane(b) 1,1,1-tri-chloro-ethane

(c) 2-bromo-2-chloro-1,1,1-trifluoro-ethane

(d) 3-bromo-2-chloro-2- methyl-butane


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Complete Exercise 4


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Alcohols

Ethanol is used widely as a solvent in paints, glues,perfumes, aftershaves and any other householdproducts.

The strong hydrogen bonds in alcohols result inalcohols having higher melting and boiling pointsthan hydrocarbons of similar size.

Functional group: OHC Hydroxyl group


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Alcohols

Alcohol molecules have a non-polar hydrocarbonend and a polarOH section.

Non -polarend

Polar end

Alcohols are solvents for polar and non-polar solutes.


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Alcohols

Alcohols are named using the ending ol.Number the longest chain beginning at the endnearest the hydroxyl group. (OH)

ethanol propan-1-ol 2-methyl-propan-1-ol


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Alcohols

Propan-2-ol Butan-2-ol Pentan-3-ol


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Alcohols

CH2 CH CH2

OHOH OH

Propan 1,2,3, - triol

Glycerine/Glycerol

2-methyl-propan-2-ol 2-methyl-butan-2-ol


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AlcoholsAlcohols are oxidized to carboxylic acidswhen

treated with strong oxidizing agents.


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Carboxylic acids

The first member of the acid series is methanoic acid(CHOOH). This acid can be found in stinging nettlesand ants.

CHO

OH

Functional group: Carboxyl group C

O

OH

The names end in oic acid. They are relatively weakacids.


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Carboxylic acids

Ethanoic acid is the sour component of vinegar.

CCH3

O

OH


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Carboxylic acids

Butanoic acid gives the unpleasant smell to rancidbutter.

CCH2

O

OHCH2CH3


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Carboxylic acids

3,3-di-methyl-butanoic acid

NomenclatureNumber the

longest carbonchain beginningat the carboxyl

group. (COOH)

3CH3 CCH2 C - OH4 1

CH3 O

CH32


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Esters

Esters are the compounds which are largely responsiblefor the flavours and scents of fruits and flowers. Esters areused as food flavourings and in perfumes and used as

solvents and adhesives.

Functional group: C

O

O C


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Ester formation (esterification)

CH3 CH2 CH2 C - OH

O

butanoic acid

+

propan-1-ol

HSO(Concentrated)

+ H2O

Acts as a catalyst and

dehydrating agent

Oxygen link(bridge)

propan-1-olbutanoic acid +

HSO(Concentrated)

propyl butanoate + water

CH3 CH2 CH2 C -

O

O CH2 CH2 CH3

carboxylic acid +alcohol ester

+water

CH2O CH2 CH3H


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ethanolethanoic

acid

+ ethyl ethanoate + water

NB. Use the oxygen atom as a divider when naming themolecule.


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Name the following

CH3 CH2 C -

O

O CH2 CH2 CH3 CH3

butyl propanoate

CH3 O -

O

C CH2 CH2 CH3 CH3

methyl pentanoate

butan-1-ol

+propanoic acid

methanol+

pentanoic acid

REACTANTS


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Draw the products

+ CC

O

OHH

H

H

HSO(Concentrated)

CH

H

OH

H

CH

H

O

H

CC

H

H

O

Hmethyl ethanoate

methanol ethanoic acid

+ H2O


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Draw the semi-structural formula for each of

the following esters:

(a) pentyl ethanoate

(b) ethyl propanoate

(c) propyl butanoate


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Complete Exercise 5


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Aldehydes

Functional group: HCO

carbonyl group

The simplest aldehyde is methanal (Formaldehyde). In

aldehydes, the C=O on an outer carbon is polar.

HC

O

H

NomenclatureAldehydes are named by attaching the suffix -al.The longest chain is numbered beginning at the endnearest to the carbonyl group.


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AldehydesH- C = OH

Methanal

CH3

- C = OH

Ethanal

CH2

- C = OH

- CH3

Propanal

CH2

- C = OH

- CH

CH3

- CH3

3methyl butanal

CH

- C = OH

- CH2- CH - CH2 - CH3

CH3

CH3

2,4-di-methyl hexanal

1.

5.

4.

3.

2.


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KetonesFunctional group: C

O

carbonyl group

In ketones, the C=O (carbonyl group) on an inner carbon ispolar.

CH3C

O

CH3

In a ketone, the carbon atom in the carbonyl group is bonded toa carbon on both sides. The simplest ketone is propanone(acetone).


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KetonesNomenclature

Ketones ( R CO R) are named by attaching the suffix one.

The longest chain is numbered beginning at the end nearest tothe carbonyl group. The C in the carbonyl group has the lowestnumber.


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Ketones

- C -

O

CH3CH3

propanoneCH3

- C -

O

CH2CH2CH3 CH2 CH3

hexan-3-one

C -

O

CH2CHCH3 CH2 CH3

2methyl hexan-3-one

CH3

- C -

O

CH2CHCH3 CH CH3

CH3

2,5-di-methyl-hexan-3-one

1. 3.

2. 4.


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Intermolecular forces

Intermolecular forces exist between molecules in thesolid, liquid or gas phase.

The strength of these forces decreases from solids toliquids to gases.

Higher melting and boiling points indicate strongerintermolecular forceswithin the substance.

The strength of the intermolecular forces increases withincreasing size of the molecule.


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Van der Waals forces These areweak forces between particles carrying

small positive and negative charges.

They are non-directional and act over distanceswhich are large in comparison to the size of the

molecule.


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Van der Waals forces

1. Dipole-dipole:

Attraction between the end of onepolar

molecule and the end of another.

Van der Waals force

Aldehydes and ketones form polar molecules


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Van der Waals force2. Temporary dipoles:

Non-polar molecules can form induced (temporary) dipoleswhen the nucleus of one molecule attracts the electron cloudof another.

Van der Waals force

These arevery weak forces but theyincrease in strengthas the length of the chain increases because they actover a larger surface area.

Hydrocarbons and some alkyl halides form non-polar

molecules.


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Hydrogen bondsThese intermolecular forces occur between molecules thatcontain hydrogen bonded to a small highly electronegativeatom such as N; O; or F.

The H end of one molecule attracts the end ofanother molecule.

Hydrogen bonds are stronger than v.d. Waals forces and act

over shorter distances.

Hydrogen bond force


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Hydrogen bondsAlcohols and carboxylic acids can form hydrogenbonds between the molecules.


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Physical properties Molecules in solids and liquids are held together by

intermolecular forces.

To move the molecules apart requires energy; the strongerthe I.M. forces the greater the energy required.


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Trends in physical properties Boiling and melting points increase as the strength ofthe I.M. forces increase.

Molecules which form hydrogen bondswill have higherm.p and b.p than those withv.d Waals forces.

Molecules with longer chainswill have higher m.p and

b.p than those with shorter chains.


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Trends in physical propertiesVapour pressure is the pressure which the vapourabove a liquid exerts on the surface of the liquid.

Substances withweak I.M. forceswillvapourizeeasily.

Ahigh vapour pressure indicatesweak I.M. forces.

Alkyl halidesvapourize readily so they are used aspropellants in aerosol sprays.


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Trends in physical propertiesViscosity. This is an indication of how runny aliquid is. A liquid with low viscosity flows easily.

As the length of the carbon chain increases, theI.M. forces become stronger and liquids becomemore viscous.

Liquids in which the molecules can form hydrogenbonds will have a higher viscosity.


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Solvent properties For the molecules of one substance to mix withmolecules of another, the I.M. forces between themolecules must be of the same kind and strength.

We say a solid dissolves in a liquid or that two liquidsare miscible.

Alkyl halides form non-polar molecules and thismakes them important solvents for fats and oilswhich also have non-polar molecules.


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Solvent properties The hydrocarbons form non-polar molecules andtherefore theydo not dissolve inwater which is heldtogether byhydrogen bonds.

Simple alcohols mix readily withwater because oftheir ability to form hydrogen bonds.


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Combustion reactionsAlkanes, alkenes and alkynes burn in oxygen andform carbon dioxide andwater.

The reaction is exothermic and a great deal of energyis released.

Alkanes

Propane is the gas used in bunsen burners in thelaboratory. Propane burns as follows:

C3H8 + 5O2 3CO2 + 4H2O + energy


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Combustion reactions If propane burns in limited oxygen then carbon(black) is produced:

C3H8 + 2O2 3C + 4H2O


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Combustion reactionsAlkenes Ethene burns with a blue flame.

C2H4 + 3O2 2CO2 + 2H2O + energy


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Combustion reactionsAlkynes Ethyne is used, along with oxygen, in an oxy-acetylene

torch, which is used to cut metals.

2C2H2 + 5O2 4CO2 + 2H2O + energy


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Combustion reactionsAlcohols burn in oxygen to form carbon dioxide andwater.

Ethanol

C2H5 OH + 3O2 2CO2 + 3H2O + energy


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Addition reactionsAddition reactions take place when a double or triplebond is broken and new molecular fragments areadded to both ends of the bond with nothing being

taken away.

If we break the double bond in ethene, each carbonwill have one electron to bond to a new atom.

CH

H

C

H

H CH

H

C

H

H


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Addition reactions

We can then add the following:

CH

H

C

H

H

Hydrogen H2

CH

H

Cl

C

H

Cl

H

A halogen Cl2

CH

H

H

C

H

H

H

ethane 1,2- di-chloro-ethane


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Addition reactions

CH

H

C

H

H

We can then add the following:A hydrogen halide HCl

CH

H

H

C

H

Cl

H

1-chloro-ethane

Water H2O

CH

H

H

C

H

OH

H

ethanol


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Addition reactions Hydrogenation CH2 = CH2 + H2 CH3 CH3 ethene ethane

Halogenation (chlorination)

CH2 = CH2 + Cl2 CH2Cl CH2Cl

alkene alkyl halide

Hydrohalogenation

CH2 = CH2 + HCl CH3 CH2Cl

alkene alkyl halide


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Addition reaction Hydration (addition of water) CH2 = CH2 + H2O CH3 CH2- OH

alkene alcohol


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Test for a double bond To test for a double bond, react the compound withbromine.

The red-brown colour of the bromine rapidlydisappears if a double bond is present, due to theaddition reaction that takes place.

CH2 = CH2 + Br2 CH2Br CH2Br

red-brown colourless


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Elimination reactions

Elimination reactions take place when atoms ormolecular fragments are removed from adjacentatoms in a molecule leaving a double bondwithnothing being added. This is the exact opposite of

addition.

Eliminating hydrogen

CH3 - CH3 CH2 = CH2 + H2

Eliminating a halogen

CH2Cl CH2Cl CH2 = CH2 + Cl2


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Elimination reactions Eliminating hydrogen halide

CH3 CH2Cl CH2 = CH2 + HCl

Eliminating water

CH3 CH2OH CH2 = CH2 + H2O

S b tit ti ti


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Substitution reactions Substitution reactions take place when one atom is

replaced by another. Example 1: (Alkane becomes alkyl halide)

Halogens such as chlorine will substitute into an alkanemolecule in the presence of sunlight. The chlorine uses the

sunlight to separate into chlorine atoms.

One chlorine atom bonds with a hydrogen atom and theother chlorine atom takes its place.

Cl Cl + energy 2Cl + CH

H

H

H HCl + CCl

H

H

H


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Substitution reactions CH4 + Cl2 CH3Cl + HCl


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Substitution reactions Example 2: (Alcohol becomes alkyl halide) Cl replaces OH in a molecule of ethanol.

H Cl + CH

H

H

C

H

H

OH CH

H

H

C

H

H

Cl + H2O

HCl + C2H5OH C2H5Cl + H2O


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Substitution reactions Example 3: (Hydrolysis) Bases (or water) with haloalkanes produce alcohols.

KOH + CH

H

H

C

H

H

Br CH

H

H

C

H

H

OH + KBr

KOH + C2H5Br C2H5OH + KBr


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Complete Exercise 6


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Plastics and Polymers

Plastics are made by joining small molecules which

have C= Cbonds into very long chains which haveC C bonds.

The small molecules are known as monomers.

The long chains are called polymers.

The process is known as addition polymerisation.


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Definitions Macromolecules very large molecules having

molecular weights that may be several millions ofatomic mass units.

Monomers small similar molecules which can join

to make a chain.


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Polymers long chainsof monomers which are

covalently bondedtogether.


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Polymerization the process by which monomersjoin to make a chain.

1 Addition polymerization


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1. Addition polymerization There are three steps in the production of the chain:

1. Initiation: The reaction is initiated by a free radical,which is a molecular fragment with an unpairedelectron.

The free radical adds to the double bond of themonomer and creates another free radical.

R

R + C C

H

HH

H

C C

H

HH

H

R

The pi bond of the carbon carbon double bond is broken.


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2. Propagation: The new free radical adds to thedouble bond of another monomer molecule and

creates another free radical with a longer chain.

Successive additions of this kind happen very fast anda chain may grow to 1000 units or more within asecond.

C C

H

HH

H

R + C CH

HH

H

C C

H

HH

H

R C C

H

HH

H


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3. Termination: This is the step which terminates thechain.

The formation of the chain will end if:

(a)two radicals join together

C CH

HH

HR C C

H

HH

HC CH

HH

HRC C

H

HH

H+

C C

H

HH

H

R C C

H

HH

H

C C

H

HH

H

RC C

H

HH

H


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The formation of the chain will end if:

(b) one radical removes a hydrogen atom fromanother radical forming an alkane and an alkene.

C C

H

HH

H

R C C

H

HH

H

C C

H

HH

H

RC C

H

HH

H

+

C C

H

HH

H

R C C

H

HH

H

H C C

H

HH

RC C

H

HH

H

+


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Complete Exercise 7

2 C d i l i i


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2. Condensation polymerization

These polymers are made from two differentmonomers which have reactive groups at bothends of their molecules.

OHCH2HO CH2

a dialcohol

Long chain of

carbon atoms.

C

O

OHC

O

HO

a dicarboxylic acid

A molecule with two OH groups.

A molecule with two COOH groups.


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The monomers join to each other in an alternatingstructure and eliminate a small molecule, such as

H2O.

O HHO

C

O

OHC

O

HO

+

+ H2OHO O

ester

C

O

OHC

O


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O C

O

C

O

O

n

a polyester

M ki l


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Making a polyester Example:

OHCH2HO CH2 +

ethan-1,2-diol

C

O

OHC

O

HO CH2

propan-1,3-dicarboxylic acid

- O - CH2 - CH2 - O -- C - CH2 - C -

O O

- O - CH2 - CH2 - O -- C - CH2 - C -

O O

a polyester


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Draw two repeating units of the polyester when thefollowing monomers are used:

HO - CH2 - OH + HO - C - (CH2)2 - C - OH=

=

O O

- O - CH2 O - C - (CH2)2 - C= =

O O

- O - CH2 O - C - (CH2)2 - C -= =

O O

H2O molecules are eliminated


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Complete Exercise 8


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Plastics Plastics are man made macromolecules.

Properties of plastics

They are polymers of carbon.

They are light because the atoms from which they aremade are small.

They are strong because the molecules are long. Thelonger the molecules the stronger the intermolecularforces.


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They are unreactive, this means that they are difficult

to dispose of because they will not rot away they arenot biodegradeable.

They are electrical and thermal insulators.

Some plastics catch fire easily and they may give outpoisonous gases.

They are divided into two groups calledthermoplastics and thermosets.


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Thermoplastics These plastics contain long thin moleculeswith

tangled chains. The forces between the chains areweak Van der Waals forces.

This means that the chains can move easily over eachother on heating, flexing and stretching.

Thermoplastics melt on heating and when cool theyset into a new shape.


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Thermosets They contain a network of large molecules with

strong cross-linkswhich form when the plastic ismade.

This means that on stretching the molecules cannotslide over each other.

They will char or break down at high temperatures,rather than melt.


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They are rigid and will break rather than bend.

They are moulded into shape when they are beingmade, because the shape cannot be changed later.

organic chem 2011

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