Asymmetric transformation of the second kind

If one of two equilibrating diastereomers crystallizes out of solution, shifting the equilibrium in one direction, the process is referred to as an asymmetric transformation of the second kind lUPAC Compendium of Chemical Technology, 2nd Edition, Mc-Naught, A. D., Wilkinson, A., Eds. Blackwell Science, 1997. 

Asymmetric transformation The conversion of a mixture (usually 1 1) of stereoisomers into a single stereoisomer or a mixture in which one isomer predominates. An asymmetric transformation of the first kind involves such a conversion without separation of the stereoisomers. An asymmetric transformation of the second kind also involves separation, such as an equilibration accompanied by selective crystallization of one stereoisomer [76]. The terms first- and second-order asymmetric transformations to describe these processes are inappropriate. See also stereoconvergent. 

The same equilibria are attained if to a solution of racemate in an initially achiral medium (equal concentrations of d and I species) there is added another chiral species d or L. The equilibrium is then displaced in favour of one or other of the constituents of the racemic mixture. This process has recently been termed enantiomerization, although examples of optically labile systems in equilibria sensitive to the presence of other chiral molecules or ions have long been recognized. A typical example of what was earlier termed an asymmetric transformation of the first kind (no second-phase involved) is that of Read and McMath (1925) in which solutions in dry acetone of (—) or ( ) chlorobromomethanesulphonic acid d-l ) together with (—)-hydroxyhydrindamine (l+) showed a change of optical rotation interpreted in terms of an equilibrium... 

Selective Asymmetric transformation precipitation I of the second kind... 

Whenever x is greater than or less than 0.5, a rotation will be observed. Although the Pfeiffer and the Harris explanations appear to be identical, Harris contends, in opposition to Pfeiffer but in agreement with the present authors, that an A-B association of some kind is essential to the development of the Pfeiffer-type activity. The term first-order asymmetric transformation is used for a system in which no optically active phase other than that originally introduced can be isolated. If such a new phase is isolatable, the transition is called second order. Turner and Harris have used the former term (first order) to describe the observations of Pfeiffer, Dwyer, and others. ... 

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