Theoretical study of the mechanism of thermal decarboxylation of salicylic and p-aminobenzoic acids; models for aqueous solution

Abstract

The mechanism of decarboxylation of salicylic and p-aminobenzoic acids in aqueous solution has been investigated by ab initio methods using the STO-3G basis set and fully optimized geometries. Single-point 3–21G energy calculations were then performed to improve the results. The mechanism suggested is a pseudo-unimolecular concerted decomposition of the free acid by interaction with a chain of water molecules, the α-protonated form of the carboxylate anion representing the activated complex. In comparison with unsubstituted benzoic acid, the presence of ortho- and para-electron-releasing groups increases the electron density at the carbon atom next to the carboxy group, hence facilitating the approach of the electrophilic proton, and decreasing the activation energy.