A Priori Assessment of the Maximum Possible Entrainment Effect Attainable during Preferential Crystallization. The Case of the Simultaneous Resolution of (±)-Ephedrine and (±)-Mandelic Acid

Abstract

Abstract The strong link observed between the detection of unstable or metastable racemic compounds in the systems studied and the maximum entrainment effect attainable during preferential crystallization (PC) (assessed by the maximum value of the enantiomeric excess of the mother solution at the end of PC (eefmax)) is confirmed by the results obtained with the simultaneous resolution of enantiomorphous p and p′ salts of (±)-ephedrine and (±)-mandelic acid (eefmax = 10.6% with no racemic compound detected). The detection of such a racemic compound indicates that strong heterochiral interactions at one or more (hkl) crystal–mother liquor interfaces are favorable to the 2D nucleation of the counter enantiomer as soon as its local supersaturation at these interfaces (β*hkl) attains a threshold above which the desorption of a docked counter-enantiomer molecule is no longer probable. Therefore, the mode and the rate of stirring must be optimized in order to keep β*hkl as close as possible to the overall supersaturation of the counter enantiomer β*bulk, while minimizing the damaging effects on the particles. The crystal structures of an unstable racemic compound and of a new polymorphic form of 1-phenylethylammonium hydratropate salts are reported and contribute to the understanding of the limited entrainment effect (eefmax = 5.2%).