High-Pressure Crystallization and Thermodynamic Stability Study on the Resolution of High-Density Enantiomers from Low-Density Racemates.

High-pressure recrystallization could be the cheapest clean method of resolving enantiomers from the racemates defying Wallach's rule. We have investigated the effect of pressure on sodium tartrate monohydrate (NaTa·H 2 O), a notorious exception from Wallach's rule: both racemic polymorphs α-dl-NaTa·H 2 O and β-dl-NaTa·H 2 O are less dense than the enantiomers. According to the mobile-equilibrium principle, such high-density enantiomorphs should spontaneously separate under high pressures. The pressure dependence of the Gibbs free energy explains the preferential crystallization of mixed enantiomers of NaTa·H 2 O.