Influence of solvent and the operating conditions on the crystallization of racemic mandelic acid

The crystallization of racemic mandelic acid is studied in unseeded batch cooling experiments in four solvent systems. Apart from the thermodynamically stable racemic compound, a metastable modification of the racemic compound is also found to initially form upon nucleation. The metastable racemate is characterized by thermal analysis (MDSC), FTIR spectroscopy, and X-ray powder diffraction (XRPD). The experiments show that the type of crystal modification that is formed upon nucleation is strongly influenced by the operating conditions such as filtration, cooling rate, and stirring rate, as well as by the solvent. The solvent effect can be related to the characteristics of the crystal structures of the two modifications, and to the hydrogen bonding properties of the solvent. The crystal structure of the metastable modification of the racemic compound is very similar to that of the pure enantiomer, which explains the similarity of the FTIR spectra, the XRPD diffraction pattern, and the thermodynamic stability.