racémic

Racémic, often written racemic in English, refers to a substance that contains equal amounts of two enantiomers, the non-superimposable mirror images of a chiral molecule. Such a mixture is optically inactive because the optical rotations of the two enantiomers cancel each other when the mixture is exposed to polarized light.

In crystallography and pharmaceutical contexts, racemism can describe different forms. A racemate is the mixture itself; in some substances the racemate crystallizes as a racemic compound, where both enantiomers are integrated in the crystal lattice, while others form a conglomerate, where crystals of each enantiomer occur separately as enantiopure crystals. Although enantiomers have identical physical properties in an achiral environment, they can interact differently with chiral surroundings, leading to differences in biological activity or metabolism.

Racemization is the process by which a pure enantiomer converts into a racemate. Separation of enantiomers, or resolution, is a major area of chemistry and can be achieved by methods such as forming diastereomeric salts, chiral chromatography, or selective crystallization.

Racemic mixtures are important in medicine and industry because one or both enantiomers may contribute to efficacy or safety. The concept has its historical roots in Louis Pasteur’s work on tartrate crystals, which established the link between chirality and optical activity and led to the terminology of racemic mixtures.