Wallach's rule enforced by pressure in mandelic acid

Abstract

At normal conditions, mandelic acid (MA) is exempt from Wallach's rule, as enantiomers l-MA and d-MA are denser than racemate dl-MA. However, the enantiomers are less compressible than the racemate, dl-MA, which additionally is anomalously compressed at the phase transition at 0.65 GPa and becomes more dense than the enantiomers. Our results exclude the possibility of resolving racemic dl-MA into enantiomers due to their higher density in a spontaneous crystallization induced by pressure up to 2.6 GPa at least. Also, the l-MA crystal undergoes an isostructural phase transition at 1.52 GPa, allowing a tighter packing of molecules mimicking a racemic arrangement. It is a general thermodynamic rule that pressure-induced phase transitions in racemates and enantiomers have the opposite effect for their density relations and act toward or against Wallach's rule, respectively. © 2014 American Chemical Society.