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(S)-(+)-lactic acid (left) and (R)-(–)-lactic acid (right) are nonsuperposable mirror images of each other

In chemistry, an enantiomer ({{#invoke:IPAc-en|main}} ə-NAN-tee-ə-mər; from gre ἐνάντιος (enantíos), meaning "opposite", and μέρος (méros), meaning "part") 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 being reversed along one axis (the hands cannot be made to appear identical simply by reorientation).<ref>IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "enantiomer".</ref> Organic compounds that contain a chiral carbon usually have two non-superposable structures. These two structures are mirror images of each other and are, thus, commonly called enantiomorphs (enantio = opposite ; morph = form), hence this structural property is now commonly referred to as enantiomerism.

Enantiopure compounds refer to samples having, within the limits of detection, molecules of only one chirality.<ref>IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "enantiomerically pure (enantiopure)".</ref>

When present in a symmetric environment, enantiomers have 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 biological molecules are enantiomers themselves, there is sometimes a marked difference in the effects of two enantiomers on biological organisms. 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. 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.

Enantiomer sections
Intro  Naming conventions   Criterion of enantiomerism    Examples   Enantioselective preparations  Enantiopure medications  Quasi-enantiomers  History of enantiomers  See also  References  External links  

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