Electrochemical oxidation of some dihydroxybenzene derivatives in the presence of some Meldrum’s acid derivatives: Mechanistic and thermodynamic study

Electrochemical oxidation dihydroxybenzene derivatives in the absence and in the presence of different nucleophiles were studied by many researchers. It was reported that the mechanisms of the reactions are dependent on the nucleophiles and changes with changing the nucleophile [1,2]. Also, it was reported that the reaction mechanism is dependent on a total change in Gibbs free energy (ΔGtot) of the electrochemical oxidation and species with more positive oxidation potential have larger ΔGtot values [3]. In this work firstly, electrochemical oxidations of some dihydroxybenzene derivatives were studied in aqueous solution at various pHs by the use of cyclic voltammetry. Potential-pH diagram of studied species constructed and oxidation potential of them in pH = 0.0 have been calculated. Also, ΔGtot of the electrochemical oxidation of studied species were calculated using a general thermodynamic cycle (Born- Haber cycle). The theoretical results were calculated at DFT (B3LYP, BP86) levels of theory and different basis sets. The plots of calculated ΔGtot versus Ep0 constructed and it was found that the best correlation is related to the BP86 level of theory and 6–31G (p,d) basis set. Therefore, this level of theory was used to obtain further data. The electrochemical oxidation of catechol, hydroquinone, 3-methoxycatechol, 3-methylecatechol, 2,3-dihydroxybenzoic acid (both anionic and non-anionic forms), 4-methylecatechol and 4-tert-catechol has been studied in the presence of methyl-meldrum’s acid, meldrum’s acid and phenyl-meldrum’s acid as nucleophiles. The calculated results indicate that oxidation potentials of studied species are directly dependent on their ΔGtot. It was found that not only electrochemical oxidation of one electrophile in the presence of different nucleophile has different mechanisms, but also electrochemical oxidations of various electrophiles in the presence of one nucleophile have different mechanism too. The results showed that the electrochemically generated species which are produced during controlled-potential coulometry, based on their ΔGtot, will participate in the subsequent electrochemical and chemical reactions, and reactions proceed in the thermodynamically favored directions.