On thermodynamics of electron, proton and PCET processes of catechol, hydroquinone and resorcinol – Consequences for redox properties of polyphenolic compounds

The hydroxyl group acidities represent a characteristic feature of p-hydroquinone, catechol and resorcinol in their electric neutral and charged states. The transformation of dihydroxybenzenes to semiquinone followed by semiquinone to quinone forms can proceed through different proton-coupled electron transfer steps as demonstrated by the Density functional theory calculations combined with solvent continuum model for water environment. These steps were also analysed using the Pourbaix diagram (electrochemical potential vs. pH) diagrams, constructed from the theoretically predicted pKa values, and theoretically estimated electrochemical standard potentials. The obtained results enable the identification of alternative reaction pathways of the radical scavenging abilities of dihydroxybenzenes or polyphenols occurring in redox reactions and biological processes.