racemate

A racemate, or racemic mixture, is a chemical mixture containing equal amounts of two enantiomers of a chiral molecule. Enantiomers are mirror-image forms that are non-superimposable; they typically have identical physical properties except for the direction in which they rotate plane-polarized light and how they interact with other chiral environments. In a racemate, the two enantiomers rotate plane-polarized light in opposite directions by the same magnitude, so the net optical rotation is zero, making the mixture optically inactive.

The term racemate comes from the older term racemic acid, used in studies of tartaric acid. Historically, Louis Pasteur demonstrated that the two enantiomeric forms could crystallize separately, pioneering the concept of chirality in chemistry. Racemates can arise from non-stereoselective synthesis or from racemization of a single enantiomer under certain conditions.

Many processes aim to produce enantiomerically enriched products rather than racemates, because biological systems distinguish between enantiomers. The degree of enrichment is measured as enantiomeric excess (ee) or enantiopurity. When racemates must be separated, methods include formation of diastereomeric salts with a chiral auxiliary, chiral chromatography, or enzymatic or chemical resolution.

In pharmacology and agrochemistry, the distinction is important because the enantiomers of a chiral compound can have different biological activities, pharmacokinetics, or toxicity. Some drugs are used as racemates; others are administered as a single enantiomer to improve safety and efficacy.