racemic mixture

Meaning: A racemic mixture (racemate), written (±), is an equal 50:50 mixture of two enantiomers. Each enantiomer rotates plane-polarized light by the same amount in opposite directions, so the observed rotation is 0° and the mixture is optically inactive. It forms when a new stereocenter arises from achiral reactants with no chiral influence, such as an achiral nucleophile adding to a planar carbonyl, or SN1 attack on a planar carbocation (often close to 50:50). Because enantiomers share the same physical properties in achiral conditions, separating them (resolution) needs a chiral agent that converts them to diastereomers.

Equation: ee = |R − S| / (R + S) × 100%, where R and S are the moles of each enantiomer; ee (enantiomeric excess) is dimensionless (percent). A racemic mixture has ee = 0%.

On the MCAT: A passage shows SN1 or addition to a ketone creating a stereocenter and asks for the product's stereochemistry or optical activity. The answer is a racemic mixture with zero rotation. Contrast SN2, which inverts the stereocenter and gives a single enantiomer from a single enantiomer.

Common confusion: Racemic and meso both give zero rotation, but a meso compound is one molecule with an internal mirror plane, while a racemate is two different molecules whose rotations cancel. Zero rotation does not mean the molecules are achiral.