Acid-catalyzed transesterification and esterification in methanol: a theoretical cluster-continuum investigation of the mechanisms and free energy barriers

Abstract

The mechanism of the acid-catalyzed transesterification reaction was widely studied by experimental methods. However, theoretical studies with reliable calculation of the free energy barrier for solution phase reaction are scarce. In this report, we have done a theoretical investigation (using MP4, X3LYP, CPCM and SMD methods) of the acid-catalyzed transesterification of ethyl acetate and esterification of acetic acid in methanol solution, two prototypical reactions important for biodiesel production via acid catalysis. We have found three mechanisms of the AAC2 type and one mechanism of the AAL2 type. Only one AAC2 mechanism is kinetically viable and involves initial formation of a complex with protonated methanol hydrogen bonded to the ester (acid) carbonyl oxygen. This reaction pathway takes place through several intermediates and transition states, and the highest free energy barriers were estimated as being 23.1 and 22.4 kcal mol−1 for transesterification and esterification reactions, respectively. This last value is in good agreement with the experimentally determined free energy barrier of 19.0 kcal mol−1. The present results point out the acid-catalyzed transesterification and esterification reactions take place through the classical AAC2 mechanism in methanol solution.