Racemate

A racemate consists of equal numbers of two molecules called enantiomers, which are made up of the same atoms and have much the same physical properties, but differ in the three-dimensional shape in which the atoms are bonded together. Such molecules are called chiral because, like a pair of hands, they are mirror images which cannot be completely supraimposed on each other. They are conventionally designated (+) and (-). It has been well known for many years that, despite their similarities, the two enantiomers may bind to different proteins and produce different biological and therapeutic effects. The most notorious example was thalidomide, which consisted of a (+) enantiomer which was effective to prevent morning sickness in pregnant women but also, unknown to the consumers, a (-) enantiomer which was teratogenic and caused severe birth defects.

The resolution of a racemate by separation into its enantiomers is not a straightforward matter. Because they have the same boiling point, they cannot be separated by conventional fractional distillation. For similar reasons, fractional crystallisation may not work. There are various indirect methods of resolving the problem.