Cocrystallization out of the blue: dl-mandelic acid/ethyl-dl-mandelate cocrystal

• Mandelic acid dissolved in ethanol forms dl-mandelic acid/ethyl-dl-mandelate cocrystal.
• Similar compounds were tested.
• dl-tropic acid and dl-phenyllactic acid showed no cocrystallization phenomenon.

This work focuses on a peculiar behavior of racemic mandelic acid in ethanol solution. Dissolution of racemic mandelic acid in ethanol followed by evaporation to dryness results in a dl-mandelic acid/ethyl-dl-mandelate cocrystal. This behavior indicates that racemic mandelic acid tends not only to transform into an ester in ethanol, but also to cocrystallize with untransformed acid molecules. Cocrystal formation for mandelic acid in ethanol was found to be reproducible under various conditions. dl-tropic acid and dl-phenyllactic acid that contain similar functional groups and that were tested as well, on the other hand, showed no cocrystal formation: dl-phenyllactic acid partly converted into an ester, whereas dl-tropic acid mostly recrystallized.