Attempts to explain the self-disproportionation observed in the partial sublimation of enantiomerically enriched carboxylic acids

The partial sublimation of two carboxylic acids, the mandelic acid and ibuprofen has been studied. Many (RS) + (S) samples with various enantiomeric excesses (ee) have been slowly and partially sublimed at a low temperature and the sublimates have been condensed before analysis. About 1% of the starting material was sublimed in each experiment. The results are reproducible showing that the sublimation is under control. The ee of sublimates are comparable to the ee of the eutectic but also to those obtained by mixing the sublimates of two apparatuses used to sublime separately the racemate and the enantiomer. Thus, the sublimations of both carboxylic acids could be controlled by the saturated vapor pressure of the components ((RS) and (S)) or, as usually proposed, by the formation of a gas phase with a eutectic composition. In the case of mandelic acid, a definitive answer has been given by the partial sublimation of (S) + (R) solid mixtures where sublimates with a eutectic composition have been obtained and without any indication of the sublimation of a “kinetic conglomerate”. This study paves the way for future investigations on the slow and partial sublimation of enantioenriched compounds to determine how this latter occurs.