Enantiomer

In chemistry two stereoisomers are said to be enantiomers if one can be superimposed on the mirror image of the other, and vice versa. A simple analogy would be that your left and right shoes are enantiomers of each other. Two molecules that are made up of the exact same atoms, having exactly the same neighbors, and differing only in their spatial orientation are said to be stereoisomers. A test for enantiomers can be stated thus: Do the molecules possess mirror planes of symmetry? That is, is it possible to find a plane that cuts through the molecule such that the two halves are mirror images of each other? It has to bisect all of the chiral centers.

An enantiomer of an optically active isomer rotates plane polarized light in an equal but opposite direction of the original isomer. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of plane polarized light of zero. A more in-depth explanation of this is in the footnotes for optical isomerism.

Enantiomers will have the opposite prefixes of each other:

• D- becomes L-
• dextro becomes levo
• (+)- becomes (-)-
• R- becomes S-

Stereo chemistry of enantiomers is of great importance nowadays. Food and Drug Administration (FDA) of the United States of America, recently recommended that drug molecules having stereocentres should be given to the patients only in the active enantiomeric form and not as a racemic mixture.

Definitions:

• Any non-racemic chiral substance is called scalemic
• A chiral substance is enantiopure or homochiral when only one of two possible enantiomers is present.
• A chiral substance is enantioenriched or heterochiral when an excess of one enantiomer is present but not to the exclusion of the other.

External link

• Course notes for a lecturer's introduction to stereochemistry and enantiomers.
• infelicitous stereochemical nomenclatures for stereochemical nomenclature