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Optical isomerism; chiral centre and optical activity

Short answer

Optical isomerism involves isomers that rotate plane-polarised light differently due to chirality. A chiral centre alone does not guarantee optical activity; a molecule may have a plane of symmetry (meso, internal compensation) or be a racemic mixture where rotations cancel externally.

What is optical isomerism? Comment why presence of chiral centre is not sufficient for the molecule to be optically active.

Optical isomerism

It is a type of stereoisomerism in which isomers have the same structural formula but differ in the direction in which they rotate plane-polarised light; they are mirror images that cannot be superimposed (enantiomers). It arises from chirality, as in lactic acid CH₃CH(OH)COOH, which has a carbon bonded to four different groups.

A chiral centre is not sufficient

The real condition for optical activity is that the molecule as a whole is chiral, i.e. it has no plane or centre of symmetry. A chiral centre alone does not guarantee this:

  1. Meso compounds: meso-tartaric acid has two chiral centres, but an internal plane of symmetry makes it achiral and inactive (internal compensation).
  2. Racemic mixtures: each molecule is chiral, but the equal and opposite rotations of the d and l forms cancel (external compensation), so the mixture is inactive.
  3. Centre of symmetry: a molecule with chiral centres that also has a centre of symmetry (as in some meso compounds) is achiral and therefore inactive.

So optical activity requires a chiral molecule, not just a chiral centre.

✓ Answer checked by Claude Opus 5.5 (high effort)

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