In chemistry, an enantiomer ( /ɨˈnæntɪ.ɵmər/ ə-NAN-tee-ə-mər; from the Greek ἐνάντιος, opposite, and μέρος, part or portion) is one of two stereoisomers that are mirror images of each other that are non-superposable (not identical), much as one's left and right hands are the same except for opposite orientation.
Organic compounds that contain a chiral carbon usually have two non-superimposable structures. These two structures are mirror images of each other and are, thus, commonly called enantiomorphs (enantio = opposite ; morph = form) Hence, optical isomerism (which occurs due to these same mirror-image properties) is now commonly referred to as enantiomerism.
Enantiopure compounds refer to samples having, within the limits of detection, molecules of only one chirality.
Enantiomers have, when present in a symmetric environment, identical chemical and physical properties except for their ability to rotate plane-polarized light (+/−) by equal amounts but in opposite directions (although the polarized light can be considered an asymmetric medium). A mixture of equal parts of an optically active isomer and its enantiomer is termed racemic and has zero net rotation of plane-polarized light because the positive rotation of each (+) form is exactly counteracted by the negative rotation of a (−) one.
Enantiomers of each other often show different chemical reactions with other substances that are also enantiomers. Since many molecules in the bodies of living beings are enantiomers themselves, there is often a marked difference in the effects of two enantiomers on living beings. In drugs, for example, often only one of a drug's enantiomers is responsible for the desired physiologic effects, while the other enantiomer is less active, inactive, or sometimes even responsible for adverse effects (unwanted side-effects).
Owing to this discovery, drugs composed of only one enantiomer ("enantiopure") can be developed to enhance the pharmacological efficacy and sometimes do away with some side effects. An example of this kind of drug is eszopiclone (Lunesta), which is enantiopure and therefore is given in doses that are exactly 1/2 of the older, racemic mixture called zopiclone. In the case of eszopiclone, the S enantiomer is responsible for all the desired effects, though the other enantiomer seems to be inactive; while an individual must take 2 mg of zopiclone to get the same therapeutic benefit as they would receive from 1 mg of eszopiclone, that appears to be the only difference between the two drugs.
Read more about Enantiomer: Criterion of Enantiomerism, Examples, Enantioselective Preparations, Enantiopure Medications, Quasi-enantiomers