Enantiomer

From Freepedia

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, enantiomers are stereoisomers whose molecules are nonsuperposable mirror images of each other.

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. 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.

In a non-symmetric environment such as in a biological environment enantiomers may react at different speed with other substances. This is the basis for Chiral synthesis.

There are several conventions used for naming chiral compounds, all displayed as a prefix before the chemical name of the substance:

• (+)- vs. (−)-
• D- vs. L-
• (R)- vs. (S)-

The (+)- vs. (−)- convention is based on the substances ability to rotate polarized light. The other two conventions are based on the actual geometry of each enantiomer.

In nature, many chiral substances are only produced in one optical form, while (most) man-made chiral substances are racemic mixtures.

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