All subjects · Chemistry · WCH14 (Unit 4)

Optical isomerism and chirality

Some molecules exist as two mirror-image forms that cannot be superimposed, like your left and right hands. They behave identically in most reactions but rotate polarised light in opposite directions, which gives evidence for reaction mechanisms.

Chiral centres (spec 15.1, 15.2)

A chiral centre (asymmetric carbon) is a carbon atom bonded to four different groups. A molecule with one chiral centre exists as two enantiomers: non-superimposable mirror images. Draw them in 3D with wedges and dashes, as mirror images of each other. Examples: butan-2-ol, CH3CH(OH)CH2CH3; 2-hydroxypropanoic acid (lactic acid), CH3CH(OH)COOH.

Optical activity (spec 15.3, 15.4)

Each enantiomer rotates the plane of plane-polarised monochromatic light: one clockwise, the other anticlockwise by the same angle. This is optical activity, measured in a polarimeter. A racemic mixture contains equal amounts of the two enantiomers; the rotations cancel, so it is optically inactive.

Evidence for mechanisms (spec 15.5)

SN2: OH- attacks from the side opposite the leaving halogen, so one enantiomer of a halogenoalkane gives a single enantiomer of product with the configuration inverted. The product is optically active.
SN1: the carbocation intermediate is planar, so OH- attacks equally from either side, giving a racemic mixture. The product is optically inactive.
Nucleophilic addition to C=O: the carbonyl group is planar, so CN- attacks from either side equally. With an aldehyde (other than methanal) or an unsymmetrical ketone, a racemic mixture of hydroxynitriles forms.

Worked example

A single enantiomer of 2-bromobutane is hydrolysed. The product is optically active. Which mechanism operated?

SN1 would go through a planar carbocation and give a racemic, inactive product.
An optically active product means one enantiomer formed by attack from one side only.

Answer: SN2, with inversion of configuration

Key idea: Chiral centre: carbon with four different groups. Enantiomers rotate plane-polarised light in opposite directions; a racemic mixture is inactive. SN2 gives an active product; SN1 and addition to C=O give racemic mixtures.

Check yourself

Which compound has a chiral centre?
- Propan-2-ol
- Butan-2-ol
- Butan-1-ol
- 2-methylpropan-2-ol
Show answer
Butan-2-ol. In butan-2-ol, C2 carries H, OH, CH3 and C2H5: four different groups.
What is a racemic mixture?
- A mixture of E and Z isomers
- A single pure enantiomer
- A mixture of structural isomers
- An equal mixture of two enantiomers, which is optically inactive
Show answer
An equal mixture of two enantiomers, which is optically inactive. The equal and opposite rotations cancel.
How do the two enantiomers of a chiral compound differ?
- They have different boiling points
- They rotate plane-polarised light in opposite directions
- They have different molecular formulae
- One reacts with water and the other does not
Show answer
They rotate plane-polarised light in opposite directions. Physical properties such as boiling point are identical; only the direction of rotation differs.
A single enantiomer of 3-bromo-3-methylhexane is hydrolysed by an SN1 mechanism. What is the product?
- A single enantiomer with the same configuration
- A single enantiomer with inverted configuration
- A racemic mixture, which is optically inactive
- An alkene only
Show answer
A racemic mixture, which is optically inactive. The planar carbocation is attacked equally from both faces.
Why is the product of HCN addition to ethanal optically inactive?
- CN- attacks the planar carbonyl group equally from either side, forming a racemic mixture
- The product has no chiral centre
- The reaction goes by SN2
- HCN is optically inactive
Show answer
CN- attacks the planar carbonyl group equally from either side, forming a racemic mixture. 2-hydroxypropanenitrile is chiral, but both enantiomers form equally.
How many chiral centres are there in 2-hydroxypropanoic acid, CH3CH(OH)COOH?
Show answer
1. Only C2 has four different groups: CH3, H, OH and COOH.
A single enantiomer of 2-iodobutane reacts with OH- by SN2. What is true of the product?
- It is optically active, with inverted configuration
- It is a racemic mixture
- It has no chiral centre
- It is an aldehyde
Show answer
It is optically active, with inverted configuration. Backside attack inverts the configuration and gives just one enantiomer.

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Last updated 7 November 2026. Spotted a mistake? Tell us.