Processes and phenomena of purity decrease during the optical resolution of dl-threonine by preferential crystallization

In order to elucidate the phenomena of purity decrease in the later stages of the batchwise preferential crystallization process resolving racemic mixtures of dl-Threonine (dl-Thr), changes both in the purity and surface structures of threonine crystals were measured and observed. It was found that the onset of purity decrease was strongly related to the consumption of the supersaturation of the crystallizing enantiomer (l-Thr) and to the surface morphology changes of the seed crystals. Based on the observations the mechanism of purity decrease was concluded to be the surface nucleation of the unseeded enantiomer (d-Thr) during the resolution, phenomenon responsible for the start of the deposition of d-Thr. The relative rates of deposition of l- and d-Thr were found to determine the start of the purity decrease.