Mixed crystals pseudo-racemic

The inactive substance is a pseudo-racemic mixed crystal [solid solution). 

In cases where the active components can form mixed crystals or solid solutions, the freezing-point curve will exhibit one of the forms given in Figs. 43 and 48. The inactive mixed crystal containing 50 per cent, of the dextro and laevo compound, is known as a pseudo-racemic mixed crystal. So far, only curves of the types I. and II. have been obtained. 

In the case of the carvoximes solid solutions are also formed, but the equilibrium curve in this case exhibits a maximum (Fig. 58). At this maximum point the composition of the solid and of the liquid solution is the same. Since the curve must be symmetrical, this maximum point must occur in the case of the solution containing 50 per cent, of each component, which will therefore be inactive. Further, this inactive solid solution will melt and solidify at the same temperature, and behave, therefore, like a chemical compound (p. 121). The melting-point of the active compounds is 72° that of the inactive pseudo-racemic mixed crystal is 91-4°... 

Pseudo-racemates are solid solutions (mixed crystals) of equimolecular proportion of the (+) and (—) enantiomer. Quasi-racemates are racemic solid solutions involving two very similar chiral substances. 

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