enantio

Enantio- is a combining form derived from the Greek enantios meaning opposite. In chemistry and related disciplines, it is used to describe relationships and processes involving enantiomers, the non-superimposable mirror-image stereoisomers of a chiral molecule. The term appears in concepts such as enantioselectivity, enantioselective synthesis, and enantiomer, as well as in the broader idea of enantiomeric excess.

Enantiomers have the same molecular formula and connectivity, and in many physical properties they are indistinguishable in an achiral environment. They differ in how they interact with polarized light and with other chiral substances, and they may exhibit different biological activities. Each enantiomer can be assigned an absolute configuration, commonly denoted as (R) or (S) according to the Cahn–Ingold–Prelog priority rules.

The prefix enantio- coexists with related terms such as enantioselective, enantioselectivity, and enantiomeric excess (ee), which measures the excess of one enantiomer over the other in a mixture. Techniques to obtain or analyze enantiomerically enriched samples include chiral resolution, enantioselective synthesis, and separation methods such as chiral chromatography, or resolution via diastereomeric salt formation.

Enantioselectivity is especially important in pharmaceuticals, agrochemicals, and flavors and fragrances, because the two enantiomers of a compound can interact very differently with biological targets. A historical example highlighting the importance of enantiomers is thalidomide, where different enantiomers produced distinct pharmacological effects.