Isomerism: optical

Chemistry A2 · An introduction to A Level organic chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A carbon atom joined to four different atoms or groups is a chiral centre. A molecule with one chiral centre exists as two enantiomers: mirror images that cannot be superimposed. A molecule can have more than one chiral centre.

Enantiomers have the same melting point, boiling point, solubility and reactions with ordinary reagents. They differ in two ways only. They rotate plane polarised light by the same angle in opposite directions, and they can act differently in living things, because enzymes and receptors are chiral.

A racemic mixture has equal amounts of the two enantiomers, so the rotations cancel. For drugs this matters: one enantiomer may work while the other is inactive or harmful.

Rules to remember

Optically active: the substance rotates the plane of plane polarised light.
One enantiomer rotates the plane clockwise (+), the other anticlockwise (−), by the same angle at the same concentration.
Racemic mixture: 50% of each enantiomer; optically inactive because the two rotations cancel.
Making a chiral compound from non-chiral reagents with no chiral catalyst gives a racemic mixture.
Enantiomers cannot be separated by distillation or simple crystallisation: their physical properties are identical.
Drug synthesis: separate the racemic mixture into pure enantiomers (difficult and costly), or use a chiral catalyst to make only one enantiomer.
A single enantiomer drug can mean a lower dose and fewer side effects.

Common mistake

Saying a racemic mixture is not optically active because its molecules are not chiral. The molecules are chiral; the mixture has no effect because the two equal and opposite rotations cancel.

Worked example

A solution of the pure (+) enantiomer of a compound rotates plane polarised light by +40°. A mixture of the two enantiomers, at the same total concentration and under the same conditions, gives a rotation of +10°. What percentage of the mixture is the (+) enantiomer?

The rotation is 10 ÷ 40 = 0.25 of the pure value, so the (+) enantiomer is in excess by 25% of the mixture.
The other 75% is racemic and cancels: 37.5% (+) and 37.5% (−).
Total (+) = 25 + 37.5 = 62.5%.
Check: (0.625 − 0.375) × 40° = 0.25 × 40° = +10°.

Answer: 62.5% (+) enantiomer and 37.5% (−) enantiomer

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Watch: Live Class - AS Organic Chemistry Part 7 - Optical Isomers, Chiral Centre

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