Oscillating Crystallization in Solution between (+)- and (−)-5-Ethyl-5-methylhydantoin under the Influence of Stirring

Although the title compound crystallizes as a stable conglomerate without any detectable solid solution, particles in the shape of single crystals grown from the racemic aqueous solution without stirring contain almost no enantiomeric excess. From stereoselective dissolution experiments carried out in a solution saturated with a single enantiomer, it is shown that the formation of these particles results from the epitaxial association of macroscopic homochiral lamellar fragments parallel to the {101} faces. This alternated 2D nucleation and growth process is shown to constitute an oscillating crystallization mechanism controlled by diffusion only. This is confirmed by the implementation of a gentle stirring of the mother liquor during the crystallization which led to crystals having a high enantiomeric excess. Molecular modeling investigations indicate that the epitaxial region can be described at a molecular level. The structure of two racemic compounds could be generated from this epitaxial zone.