Racemic mixture

In chemistry, a racemic mixture, or racemate /reɪˈsimeɪt/, is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was racemic acid, which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid. A sample with only a single enantiomer is an enantiomerically pure, enantiopure or homochiral compound.

From racemic acid found in grapes; from Latin racemus, meaning a bunch of grapes.

A racemic mixture is denoted by the prefix (±)- or dl- (for sugars the prefix dl- may be used), indicating an equal (1:1) mixture of dextro and levo isomers. Also the prefix rac- (or racem-) or the symbols RS and SR (all in italic letters) are used.

If the ratio is not 1:1 (or is not known), the prefix (+)/(−), d/l- or d/l- (with a slash) is used instead.

The usage of d and l is strongly discouraged by IUPAC.

A racemate is optically inactive, meaning that there is no net rotation of plane-polarized light. Although the two enantiomers rotate plane-polarized light in opposite directions, the rotations cancel because they are present in equal amounts.

In contrast to the two pure enantiomers, which have identical physical properties except for the direction of rotation of plane-polarized light, a racemate sometimes has different properties from either of the pure enantiomers. Different melting points are most common, but different solubilities and boiling points are also possible.

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