Enantiomeres

Enantiomeres are a pair of stereoisomers that are non-superimposable mirror images of each other. They occur in chiral molecules, which lack an internal plane of symmetry. Although enantiomeres share most physical properties in an achiral environment, they interact differently with polarized light and with other chiral substances, such as enzymes and receptors, leading to distinct biological activities.

Each enantiomer can be assigned a absolute configuration using the Cahn–Ingold–Prelog priority rules, typically labeled (R) or (S). A molecule with a single stereogenic center has exactly two enantiomeres, while molecules with multiple stereocenters can have two enantiomeric families, provided every stereocenter is inverted. The two enantiomers are mirror images; diastereomers are other stereoisomers that are not mirror images. A meso compound is achiral despite having stereocenters due to an internal plane of symmetry, and it does not have an enantiomer.

In physical terms, enantiomeres have identical melting and boiling points and densities in an achiral setting; they differ in optical rotation, each rotating plane-polarized light by the same magnitude but in opposite directions. The sign and magnitude depend on concentration, solvent, and temperature.

In practical terms, enantiomeres are crucial in synthesis and pharmacology: many biological systems are enantioselective, so enantiomers can have different potency, metabolism, or toxicity. Separation and production of specific enantiomers—through asymmetric synthesis, resolution of racemates, or chiral chromatography—are common tasks in chemistry and drug development. Examples include lactic acid enantiomers and alcohols like 2-butanol.