Electrochemical Oxidation of 2,5-Dihydroxybenzoic Acid in the Presence of p-Toluenesulfinic Acid: Formation of a Novel Disulfone Derivative and Experimental–Theoretical Comparison With Other Hydroquinone Derivatives

Abstract

The electrochemical oxidation of 2,5-dihydroxybenzoic acid (2,5-DHBA) in the presence of p-toluenesulfinic acid (TSA) was investigated by cyclic voltammetry and controlled-potential coulometry. The results revealed an ECEC mechanism, leading to the formation of a new disulfone derivative. In contrast, investigation of five other hydroquinone derivatives under identical conditions revealed that their electrochemical oxidation in the presence of the nucleophile follows an EC mechanism. To clarify the origin of this mechanistic difference, density functional theory (DFT) calculations were performed using various functionals in combination with CPCM and SMD solvation models. Comparison of calculated oxidation free energies (ΔG_tot) with experimental voltammetric data identified the M06-2X-D3/6-311++G(d,p)/SMD level as the most reliable method, providing excellent agreement with experiment. The results demonstrated that the total change in Gibbs free energy of the electrochemical oxidation of the species is the key factor governing the reaction pathway. Products whose ΔG_tot are higher than those of the corresponding starting materials do not undergo further oxidation under the experimental conditions, resulting in an EC mechanism. In contrast, species with lower ΔG_tot values are susceptible to further oxidation, leading to an ECEC mechanism.