CCEA ADVANCED SUBSIDIARY

CHEMISTRY

MODULE 4

4.5 Isomerism

Isomerism will incorporate the examples in Module 2.

Isomerism (greek isos, equal; meros, parts) occurs when there are two or more compounds (called isomers) with the same molecular formula but different arrangements of their atoms. Isomers have different physical and chemical properties but the differences may be great or small depending on the type of isomerism.

There are two main classes of isomerism;

(i) structural isomerism

(ii) stereoisomerism

which are themselves sub-divided.

ISOMERISM

structural isomerism stereoisomerism

chain positional functional group geometric optical

isomerism isomerism isomerism isomerism isomerism

Structural isomerism

Structural isomers are molecules with the same molecular formula but with different structural arrangements of the atoms.

· Chain isomers

This occurs where isomers have different arrangements of the carbon chain. There is only one alkane corresponding to each of the formulae CH₄, C₂H₆ and C₃H₈.

For butane C₄H₁₀ two arrangements are possible.

H H H H H H H

│ │ │ │ │ │ │

H—C—C—C—C—H H—C——C——C—H

│ │ │ │ │ │ │

H H H H H H—C—H H

│

butane methylpropane

b.pt. 273K b.pt. 261K

As the number of carbon atoms increases the number of possible isomers increases rapidly.

Formula	Number of structural isomers
C₅H₁₂	3
C₆H₁₄	5
C₇H₁₆	9
C₁₀H₂₂	75
C₂₀H₄₂	366,319
C₃₀H₆₂	Over 4000 million

· Positional isomers

This occurs when isomers have the same carbon skeleton but the functional group is in different positions in the molecule.

H H H H H H

│ │ │ │ │ │

H—C—C—C—OH H—C C C—H

│ │ │ │ │ │

H H H H OH H

propan-1-ol propan-2-ol

Cl Cl Cl

│ Cl │ │

Cl │

1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene

Isomerism between compounds containing different functional groups, e.g. CH₃COCH₃ and CH₃CH₂CHO.

· Function group isomers

Compounds have the same molecular formula but different functional groups.

e.g. C₃H₈O

CH₃CH₂CH₂OH CH₃OCH₃

propan-1-ol methoxyethane

(an alcohol) (an ether)

C₅H₁₀O

C₄H₉C=O C₂H₅C=O

│ │

H C₂H₅

pentanal pentan-3-one

(an aldehyde) (a ketone)

C₂H₄O₂

H—C=O CH₃—C=O

│ │

OCH₃ OH

methylmethanoate ethanoic acid

(an ester) (a carboxylic acid)

Exercise 1

(a) Draw and name the five structural isomers of C₆H₁₄.

(b) Write the structural formulae of all the alcohols of molecular formula C₄H₁₀O. Name the isomers.

Stereoisomerism

Stereoisomers have identical molecular formulae, and the atoms are linked together in the same order, but have different relative positions in space.

The two types of stereoisomerism are

(i) Geometric (or cis-trans ) isomerism

(ii) Optical isomerism

· Geometric isomers

Geometric or cis-trans isomers exist because the π bond of the C=C bond prevents free rotation .

H H Cl H

C=C C=C

Cl Cl H Cl

cis-1,2-dichloroethane trans-1,2-dichloroethane

H H HOOC H

C=C C=C

HOOC COOH H COOH

cis-butenedioic acid trans-butenedioic acid

Cis-trans isomerism can also occur in inorganic complexes about a single bond with square planar or octahedral structures.

e.g. diaminedichloro platinum (II)

Cl NH₃ Cl NH₃

Pt Pt

NH₃ Cl Cl NH₃

trans cis

(non-polar) (polar)

tetraaminedichloro cobalt (II)

Cl NH₃

H₃N NH₃ H₃N NH₃

Co Co

NH₃ NH₃ NH₃ Cl

Cl Cl

trans isomer cis isomer

Chiral (optical) isomerism in compounds containing a single asymmetric centre.

Candidates are expected to know that a compound prepared in the lab which contains a chiral centre may be a mixture of the optical isomers, but resolution of the isomers is not required, nor is the term ’racemic mixture’.

· Optical isomers

This type of isomerism occurs when a molecule has one or more asymmetric carbon atoms i.e. a carbon atom linked to four different atoms or groups.

The optical isomers have a chiral centre (Greek: cheir, the hand) and like a left and right handed glove are mirror images of each other.

a * = chiral centre

│