enantiomerspairs

Enantiomers pairs are two stereoisomers that are non-superposable mirror images of each other. They occur in chiral molecules, which lack an internal plane of symmetry. For every chiral molecule, there exists a corresponding enantiomer that is its mirror image and cannot be aligned with it by any rotation.

In achiral environments, enantiomers have identical physical properties such as boiling point, melting point, and density, and they react with achiral reagents in the same way. In chiral environments—such as within biological systems, or when interacting with chiral reagents and catalysts—enantiomers can behave very differently. A common way to distinguish them experimentally is through optical activity: each enantiomer rotates plane-polarized light by equal magnitude but in opposite directions, described by specific rotation values. The configuration around stereogenic centers is often denoted using R/S according to the Cahn–Ingold–Prelog priority rules, while older d/l designations refer to the direction of optical rotation and can be distinct from R/S.

Examples include (S)- and (R)-lactic acid, or the two enantiomers of 2-butanol. A mixture containing equal amounts of both enantiomers is called a racemate or racemic mixture, and it typically displays no net optical rotation. Enantiomeric excess (ee) measures the degree of disproportion between the two enantiomers in a sample.

Separation and preparation methods, such as chiral resolution, asymmetric synthesis, or chiral chromatography, are used to obtain enantiomerically pure compounds. Enantiomer pairs have important implications in fields like chemistry and pharmacology, where the two forms can exhibit different biological activities despite having the same molecular formula and connectivity.