Impact of meta and para substituents on the electrochemical oxidation of phenols: A comparative study at sp2 and pre-treated sp3 carbon electrodes

This study investigates the electrochemical oxidation of phenol and its meta - and para -substituted derivatives (–CH 3 , –OCH 3 , –COOH, –(CH 2 ) 2 COOH, –Cl, –CF 3 , –NO 2 ) on four different carbon-based electrode surfaces: alumina-polished boron-doped diamond (p-BDD), hydrogen-terminated BDD (H-BDD), anodically oxidized BDD (O-BDD), and basal-plane pyrolytic graphite electrode (bPGE). Cyclic and linear sweep voltammetry were performed in Britton–Robinson buffer at pH 2.0 and 11.0 to evaluate the influence of pH, substituent effects, and electrode surface properties. Oxidation potentials were consistently lower at pH 11.0 due to the formation of phenolate anions, which are more easily oxidized to phenoxy radicals. Hammett correlations between oxidation peak potentials and substituent constants ( σ m and σ p ) revealed strong linear relationships at pH 11.0, particularly on p-BDD, H-BDD, and bPGE, indicating that electron transfer kinetics are predominantly governed by substituent electronic effects under these conditions. In contrast, weaker correlations at pH 2.0 suggest increased influence of surface interactions. Electrode passivation, assessed via voltammetric signal decrease over repeated scans, was most pronounced on H-BDD and bPGE, and was dependent on both pH and substituent type. A global regression model incorporating Hammett substituent constants, pH, and electrode-type descriptors successfully described the oxidation peak potentials for substituted phenols on four carbon-based electrodes. The model confirms that electron-withdrawing substituents increase oxidation potentials, while alkaline conditions significantly facilitate oxidation due to phenolate formation. These results highlight the combined roles of pH, substituent electronic properties, and electrode surface state in controlling electrochemical oxidation of phenolic compounds on carbon-based electrodes.