The conversion of a racemate into a pure enantiomer or into a mixture in which one enantiomer is present in excess, or of a diastereoisomeric mixture into a single diastereoisomer or into a mixture in which one diastereoisomer predominates. This is sometimes called deracemization. If the two enantiomers of a chiral substrate A are freely interconvertible and if an equal amount or excess of a non-racemizing second enantiomerically pure chemical species, say (R
)-B, is added to a solution of racemic A, then the resulting equilibrium mixture of adducts A·B
will, in general, contain unequal amounts of the diastereoisomers (R)-A·(R)-B
The result of this equilibration is called asymmetric transformation of the first kind. If, in such a system, the two diastereoisomeric adducts differ considerably in solubility so that only one of them, say (R)-A·(R)-B
, crystallizes from the solution, then the equilibration of diastereoisomers in solution and concurrent crystallization will continue so that all (or most) of the substrate A can be isolated as the crystalline diastereoisomer (R)-A·(R)-B.
Such a 'crystallization-induced asymmetric transformation' is called an asymmetric transformation of the second kind.
See also: stereoconvergence
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2200 (https://doi.org/10.1351/pac199668122193)