Enantiomer

In chemistry two stereoisomers are said to be enantiomers if they are mirror images of each other. Much as a left and right hand are different but one is the mirror image of the other, two stereoisomers are enantiomers if they are different but each can be superposed on the mirror image of the other.

Enantiomers rotate plane polarized light by equal amounts but in opposite directions. 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- vs. L-
• (+)- vs. (−)-
• (R)- vs. (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.
• enantiomeric excess or ee is a measure for how much of one enantiomer is present compared to the other. For example in a sample with 40% ee in R, the remaining 60% is racemic with 30% of R and 30% of S so that the total amount of R is 70%.

External link

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