Optical isomerism as a form of stereoisomerism. The chiral centre and its four different groups. Optical isomers (enantiomers) as non-superimposable mirror images. The effect of enantiomers on plane-polarised light and the meaning of a racemic mixture.

What this dot point is asking

AQA wants you to define optical isomerism, identify a chiral centre, explain why enantiomers are non-superimposable mirror images, describe their effect on plane-polarised light, and explain how a racemic mixture forms.

Chirality and enantiomers

For example, in 2-hydroxypropanoic acid (lactic acid), $CH_3CH(OH)COOH$, the central carbon carries $CH_3$, $OH$, $COOH$ and $H$, all different, so it is chiral. To spot a chiral centre quickly, look for a carbon with four different attached groups; a carbon with any two identical groups (such as two hydrogens) is not chiral. A molecule can have more than one chiral centre, and most naturally occurring amino acids and sugars are single enantiomers, which matters because biological systems are themselves chiral. This is why optical isomerism is so important in pharmacy: the two enantiomers of a drug can have very different effects in the body, since enzyme active sites and receptors are stereospecific and bind only one of the pair. AQA expects you to connect chirality to this drug-action context.

Effect on plane-polarised light

Enantiomers are optically active: a solution of one enantiomer rotates the plane of plane-polarised light. One enantiomer rotates it clockwise (+), the other anticlockwise (-), by the same angle. They are otherwise identical in physical properties such as melting point and boiling point, and in most chemical reactions, because these do not depend on the handedness of the molecule. The angle of rotation is measured with a polarimeter and depends on the concentration and path length, so it is the direction and the equal-but-opposite magnitudes that identify a pair of enantiomers.

Why racemates form

When a reaction passes through a planar (trigonal) intermediate or transition state, such as the carbocation in $S_N1$ substitution or the carbonyl carbon in nucleophilic addition, the reagent can attack from either face with equal probability. This produces equal amounts of both enantiomers, giving a racemic mixture. The same logic applies to the $S_N1$ hydrolysis of a tertiary halogenoalkane, where the flat carbocation intermediate is attacked from both faces. Recognising when an intermediate is planar is the key to predicting whether a product will be a single enantiomer or a racemate.

Try this

Q1. State the requirement for a carbon atom to be a chiral centre. [1 mark]

• Cue. It must be bonded to four different atoms or groups.

Q2. Explain why a racemic mixture does not rotate plane-polarised light. [2 marks]

• Cue. It contains equal amounts of both enantiomers, which rotate light by equal but opposite amounts, so the effects cancel.