Kinetic and Mechanistic Studies of Decarboxylation

Abstract

A kinetic investigation of 2,4- and 2,6-dimethoxybenzoic acid has been undertaken, in an attempt to resolve questions of mechanism which a study of 2,4,6-trimethoxybenzoic acid 9 had been unable to do. The presence of the strongly releasing methoxyl groups in positions ortho and para to the carboxyl group, should facilitate the electrophilic process, as in the case of the trisubstituted acid. Oxocarbonium ion (i.e. ArCO+) formation was not expected to be appreciable for 2,4-dimethoxybenzoic acid, where steric compression of the carboxyl group is considerably reduced by the presence of only one ortho methoxyl group. Although the subject of reaction kinetics in concentrated mineral acids has been generally confused, some order has recently been restored by Bunnett et al,16 in an attempt to classify reaction mechanisms in these media. While their treatment still requires an understanding of why some compounds protonate in accordance with the Hammett acidity function, Ho, and others do not, it was hoped that an application to the kinetic data for the decarboxylation reactions might provide an insight into mechanism. In addition, kinetic carbon-13 and solvent deuterium isotope effects, have been employed in an attempt to identify the rate limiting process (or processes). A mechanism has been proposed for the decarboxylation of the methoxybenzoic acids. A similar kinetic investigation of the decarboxylation of the 2,4-, 2,6- and 2,4,6-hydroxybenzoic acids has been carried out, to compare and contrast differences in mechanism with the corresponding methoxybenzoic acids.