Enantiomers

Enantiomers are a pair of stereoisomers that are non-superimposable mirror images of each other. A molecule with at least one chiral center, typically a carbon atom bonded to four different substituents, can exist as two enantiomers. In such cases, the two forms have opposite configurations at every chiral center and are often labeled as R and S according to CIP rules, or as D and L in reference to glyceraldehyde, though the latter nomenclature can be context-dependent.

Enantiomers share many physical properties in achiral environments, such as molecular weight, melting point, and boiling point. They differ in how they interact with plane-polarized light, rotating it in equal magnitude but opposite directions. They also interact differently with other chiral substances, including enzymes, receptors, and tastes or smells, which can lead to markedly different biological activities and sensory properties. Carvone, for example, has one enantiomer with a spearmint scent and the other with a caraway scent.

Racemates or racemic mixtures contain equal amounts of both enantiomers and are often optically inactive. Enantiomeric excess (ee) measures the predominance of one enantiomer in a mixture and is a key parameter in evaluating enantioselectivity in synthesis or resolution processes. Separation of enantiomers, known as resolution, and enantioselective synthesis using chiral catalysts or resolving agents are common strategies in chemistry and drug development.

Diastereomers are a different class of stereoisomers that are not mirror images of each other and typically have more distinct physical properties. Enantiomers thus form a fundamental concept in stereochemistry, with important implications for chemistry, biology, and pharmacology.