Racemisation

In chemistry, racemization is the conversion of an enantiomerically pure mixture (one where only one enantiomer is present) into a mixture where more than one of the enantiomers are present. If the racemization results in a mixture where the D and L enantiomers are present in equal quantities, the resulting sample is described as a racemic mixture, a racemate, or racemic. Racemization can proceed through a number of different mechanisms, and it has particular significance in pharmacology as different enantiomer may have different pharmaceutical effects.

Chiral molecules have two forms (at each point of asymmetry), which differ in their optical characteristics: The levorotatory form (the (−)-form) will rotate the plane of polarization of a beam of light to the left, whereas the dextrorotatory form (the (+)-form) will rotate the plane of polarization of a beam of light to the right. The two forms, which are non-superimposable when rotated in 3-dimensional space, are said to be enantiomers. The notation is not to be confused with D and L naming of molecules which refers to the similarity in structure to D-glyceraldehyde and L-glyceraldehyde. Also, (R)-(+) and (S)-(−) refer to the chemical structure of the molecule based on Cahn–Ingold–Prelog priority rules of naming rather than rotation of light.

Racemization occurs when one pure form of an enantiomer is converted into equal proportion of both enantiomers, forming a racemate. When there are both equal numbers of dextrorotating and levorotating molecules, the net optical rotation of a racemate is zero. Enantiomers should also be distinguished from diastereomers which are a type of stereoisomer that have different molecular structures around a stereocenter and are not mirror images.

Racemate may have different physical properties from either of the pure enantiomers because of the differential intermolecular interactions (see Biological Significance section). The change from a pure enantiomer to a racemate can change its density, melting point, solubility, heat of fusion, refractive index, and its various spectra. Crystallization of a racemate can result in separate (+) and (−) forms, or a single racemic compound.

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