Sorbitol dehydration in a ZnCl2 molten salt hydrate medium: molecular modeling

Abstract

A molecular modelling study, using standard DFT B3LYP/6-31G^*, was carried out to develop a better understanding of sorbitol dehydration into isosorbide in ZnCl₂ molten salt hydrate medium. Catalysis of sorbitol dehydration by ZnCl₂ most likely starts with complexation of the sugar alcohol functions to Zn, followed by an internal S_N2 mechanism of a secondary alcohol function attacking a primary alcohol function with the Zn-complex acting as a favourable leaving group. The dehydration reactions to 1,4- and 3,6-anhydrosorbitol show a very similar activation barrier in good accordance with experimental results. The same holds for the formation of isosorbide from 1,4- and 3,6-anhydrosorbitol, albeit with a slightly higher activation barrier. The relative level of the activation barriers reflects the increased strain in the sorbitol skeleton in the corresponding transition states. ZnCl₂ turns the dehydration reaction from an endothermic one to an exothermic one by forming a strong complex with the released water. Finally, the ZnCl₂–H₂O system has been compared with HCl–H₂O, which could have been an alternative; it, however, turned out not to be the case.