Isomerism: optical

A molecule shows optical isomerism when it contains a chiral centre — a carbon bonded to four different atoms or groups. Such a molecule is non-superimposable on its mirror image, giving a pair of enantiomers.

The two enantiomers are chemically identical in most ways, but each rotates the plane of plane-polarised light by the same angle in opposite directions (one clockwise, one anticlockwise) — they are optically active.

A racemic mixture is an equimolar (50:50) mixture of the two enantiomers. Because their equal-and-opposite rotations cancel, a racemate is optically inactive (no net rotation).

For a molecule with n chiral centres (each with different substituents), the number of optical isomers is 2ⁿ.

The optical outcome of a substitution tells you which mechanism operated.

SN1 (tertiary halogenoalkanes): the C–X bond breaks first, giving a planar carbocation. The nucleophile then attacks from either side with equal probability, producing both enantiomers — a racemic mixture.

SN2 (primary halogenoalkanes): the nucleophile attacks from the side opposite to the leaving group in one step. This inverts the configuration (like an umbrella turning inside out), giving a single product with the opposite arrangement.

So if a single enantiomer of starting material gives a racemate, the reaction went via a planar carbocation (SN1); if it gives an inverted single product, it went by SN2. Optical activity is therefore a powerful probe of mechanism.

The same 'planar intermediate → racemate' idea applies to nucleophilic addition to a carbonyl. When HCN adds to an aldehyde (e.g. ethanal), the carbonyl carbon is sp² and planar, so CN⁻ attacks from either face equally.

The product (a hydroxynitrile, e.g. 2-hydroxypropanenitrile) has a new chiral centre, formed as both enantiomers in equal amounts — a racemic mixture.

This is a recurring theme: whenever a new chiral centre is created by attack on a planar group (a carbocation or a carbonyl), the product is a racemate.