Isomers are compounds that share the same molecular formula but that possess different structures.

There are two types of isomerism:

• structural isomerism
• stereoisomerism

These two groups can be subdivided:

• structural isomerism:

? positional isomerism

? chain isomerism

? functional isomerism

• stereoisomerism:

? geometrical isomerism

? optical isomerism

Positional isomerism: the functional group is located at the end of the molecule. For example, amides, carboylate salts, and acyl chlorides.

Chain isomerism: all the molecules you covered in A2 show chain isomerism. For example, butyl ethanoate.

Functional isomerism: in most cases members of different homologous series share the same general formula. For example:

• carboxylic acids and esters share the general formula C_nH_2nO₂
• primary, secondary and tertiary amines share the general formula C_nH_2n+N
• alcohols and esters share the general formulaC_nH_2n+2O
• alkenes and cycloalkanes share the general formulaC_nH_2n
• carbonyls, alkenols and cycloalcohols share the general formulaC_nH_2nO

Geometrical isomerism: as you covered in AS Chemistry, this occurs when both carbon atoms forming the double bond belong to two different groups. In a double bond, the second bond is a ?-bond, caused by an overlap of two p-orbitals side-on. Rotation is restricted around the ?-bond as there is an overlap in two places. This is the reason why cis and trans isomers are not able to interconvert and, therefore, are different.

Optical isomerism

As you did not cover optical isomerism in AS Chemistry it will be explained in more detail here.

A molecule which consists of a carbon atom attached to four different groups is said to be chiral. A chiral molecule is asymmetrical and so cannot be imposed onto its own mirror image.

Two mirror images which are non-superimposable are called optical isomers or enantiomers.

Distinguishing between different enantiomers

Optical isomers are basically identical in terms of their physical and chemical properties. There is, in fact, only one physical method that can be used to distinguish them.

Light which has been filtered into a 2D plane is called plane-polarised light.

When plane-polarised light passes through a liquid which contains a chiral molecule, the plane of the light is rotated. The direction of rotation cannot be predicted and is detected using a polarimeter. A substance capable of this is called optically active. Molecules which are not chiral will not cause this rotation.

Two optical isomers will rotate the plane-polarised light in opposite directions and this is how they can be distinguished from one another.

• The isomer rotating the plane-polarised clockwise has the prefix (+) or D-.
• The isomer rotating the plane-polarised anticlockwise has the prefix (-) or L-.

The importance of optimal isomers in biochemistry

In the majority of reactions, optical isomers show the same chemical properties. However, a molecule must have a specific orientation of groups for particular biochemical processes. A lot of enzymes and drugs are chiral which means that only one of the optical isomers is able to interact with the body’s target molecule. Therefore, different optical isomers can posses different biochemical effects.

Racemates

In general, optical isomers are found in equal quantities within a mixture. If plane-polarised light is used on a mixture of two isomers of equal amounts (an equimolar mixture) there will be no overall rotation making the mixture not optically active. These mixtures are known as racemic mixtures or racemates.

Therefore, optical activity is only possible when one isomer is present in a greater quantity to the other. If you know the mechanism for a reaction, it is possible to predict whether a racemate or a single enantiomer will be produced.

• If an addition reaction is used to produce a chiral substance then the product will always be a racemate. This is because the attacking electrophile or nucleophile can attack the planar molecule either from below or from above.
• If a substitution reaction is used to produce a chiral substance and a singlt enantiomer is the starting molecule then a single enantiomer will be produced. This is because the attacking species is only able to attack from one side.