Chirality and optical isomerismEdexcel A-Level Chemistry: Revision notes

A chiral centre (asymmetric carbon atom) is a carbon atom bonded to four different atoms or groups. A molecule with a single chiral centre exists as two forms called optical isomers, or enantiomers. This type of stereoisomerism is optical isomerism.

Examples: butan-2-ol, CH₃CH(OH)CH₂CH₃, has a chiral carbon (H, OH, CH₃, C₂H₅); lactic acid, CH₃CH(OH)COOH, has a chiral carbon (H, OH, CH₃, COOH). Propan-2-ol is not chiral because its central carbon has two identical CH₃ groups.

The two enantiomers of a chiral molecule are non-superimposable mirror images, like left and right hands. They cannot be rotated to look identical.

On a diagram, show the chiral carbon with two bonds in the plane of the paper, one wedge (towards the viewer) and one hashed bond (away), then draw the mirror image across a vertical line.

Enantiomers have the same boiling point, density and chemical reactions with non-chiral reagents. They differ in how they rotate plane-polarised light, and in their reactions with other chiral molecules, such as enzymes in the body.

Plane-polarised light vibrates in a single plane. When it passes through a solution of a single enantiomer, the plane is rotated. This property is optical activity.

Optical activity is the ability of a single optical isomer to rotate the plane of plane-polarised monochromatic light (light of a single wavelength). It is measured with a polarimeter.

• One enantiomer rotates the plane clockwise (+), and the other rotates it anticlockwise (−).
• The two enantiomers rotate the plane by equal angles in opposite directions.
• The size of rotation depends on concentration, path length, temperature and wavelength.

A racemic mixture (racemate) contains equal amounts of the two enantiomers. The rotations are equal and opposite, so they cancel and the mixture is not optically active (net rotation 0°).

If the amounts are unequal, the mixture is optically active, and the net rotation is proportional to the excess of one enantiomer. For example, a solution with 75% (+) and 25% (−) of an isomer that gives +6.0° when pure has rotation 0.75 × 6.0 − 0.25 × 6.0 = +3.0°.

A racemic mixture forms when a reaction creates a chiral centre from a planar reactant or intermediate, because attack from either side is equally likely.

Optical activity of the reactant and product shows how a halogenoalkane reacts with a nucleophile such as OH⁻.

• SN1 (tertiary halogenoalkane, rate = k[RX]): the slow step forms a planar carbocation. The nucleophile attacks it from either side with equal probability, so a racemic mixture forms. A single optically active enantiomer therefore gives a product that is optically inactive.
• SN2 (primary or secondary, rate = k[RX][Nu⁻]): the nucleophile attacks from the side opposite the leaving group in one step, so the configuration is inverted. A single optically active enantiomer therefore gives a single enantiomer of the product, which is optically active.

Combine this with the rate equation for the strongest evidence.