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Which of the following molecules can exhibit optical isomerism?

1. CH3-CH(OH)-COOH (lactic acid)
2. CH3-CH2-CH2-OH (1-propanol)
3. CH3-CO-CH3 (acetone)
4. CH3-CH=C=CH2 (buta-1,2-diene)

Correct answer: CH3-CH(OH)-COOH (lactic acid)

Solution

Optical isomerism arises when a molecule lacks a plane of symmetry. CH3-CH(OH)-COOH has a chiral carbon (C-2 bears H, OH, CH3, COOH — four different substituents), so it is optically active. 1-propanol has no chiral centre. Acetone has a plane of symmetry. Buta-1,2-diene (CH3-CH=C=CH2) is an allene; for an allene to be chiral, the two terminal groups on each sp-carbon must be different. Here the terminal carbons bear H,H and H,CH3 — the CH3 end has two different groups (H and CH3) but the other end has two identical H atoms, so the molecule is NOT chiral. Therefore only lactic acid shows optical isomerism.

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