Resolution of the ethanolamine salt of (±)mandelic acid by using the AS3PC method: Principle, application and results

Abstract

The ethanolamine salt of (′)mandelic acid crystallizes as a stable conglomerate. This study aims at determining if the racemic mixture could be efficiently resolved by preferential crystallization in the azeotropic mixture: Ethanol - Water at ca. 20°C. Despite the optimisation of several pertinent parameters, the efficiency of the process and the optical purity of the crude crops have been found to remain low (e.e. f m a x 5 - 7 % with = 65% e.e. purity (e.e. stands for enantiomeric excess)). In order to determine the origin of this poor entrainment effect precise XRPD measurements (on pure enantiomer and racemic mixture) coupled with DSC analyses have been undertaken: the results clearly show the existence of a partial solid solution whose maximal composition is approximately 36% e.e. at 106°C and probably below 10% e.e. at room temperature. Nevertheless the metastable domains of solid solution and/or the high supersaturation created must be also taken into consideration to explain the poor efficiencies.