Racemic

In chemistry, a racemate is a mixture of equal amounts of left- and right-handed stereoisomers of a chiral molecule. Because the two isomers rotate plane-polarised light (they are optically active) in opposite directions, a racemic mixture does not rotate plane-polarised light. The first known racemic mixture was 'racemic acid', which Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.

There are three types of racemates - racemic modifications:

• Racemic mixture or better racemic conglomerate

It's the mechanical mixture of enantiomerically pure crystals of one enantiomer and its opposite. Molecules (in the crystal structure) have higner affinity to the same enantiomer, than to the opposite enantiomer. The melting point of the racemic conglomerate is always lower than the pure enantiomer. By addition of small amount of one enantiomer to the racemic compound, the melting point increases.

• Racemic compound or just racemate

Molecules have higner affinity to the opposite enantiomer, than to the same enantiomer. By addition of small amount of one enantiomer to the racemic compound, the melting point decreases. But the pure enantiomer can have higher or lower melting point than the racemic compound.

• Racemic solid solution or pseudoracemate

Unlike the racemic compound or conglomerate, there is no big difference in affinity between the same and opposite enantiomers. By addition of small amount of one enantiomer to the racemic compound, the melting point changes just little bit, or not at all.

The reagents of, and the reactions that produce, racemic mixtures are said to be "not stereospecific" or "not stereoselective", for their indecision in a particular stereoisomerism.

Related topics

• Optical isomerism
• Chirality