29.4.2—Terms: optically active and racemic mixture

An optically active sample rotates plane-polarised light. A racemic mixture contains equal amounts of both enantiomers, whose opposite rotations cancel, so the mixture shows no net rotation.

Optical activity describes the sample, not simply the presence of a chiral carbon. A sample enriched in one enantiomer can be active; a perfectly 1:1 mixture is racemic.

A pure (+) enantiomer and pure (−) enantiomer rotate light by equal magnitudes in opposite directions. Mixing equal amounts gives zero observed rotation.

No net rotation does not prove that no chiral molecules are present; it may indicate cancellation in a racemate.