Thermodynamics of semiquinone disproportionation in aqueous buffer

The thermodynamic parameters, Kdisp, ΔH° and ΔS°, controlling the disproportionation of semiquinones derived from 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 2-methylbenzoquinone (MBQ), menadione (MNQ), naphthazarin (NZQ) and quinizarin (QNZ), have been determined. Smaller disproportionation constants, Kdisp, are observed upon addition of a fused benzene ring to the semiquinone structure. Negative enthalpies and positive entropies of disproportionation govern the disproportionation equilibria. Addition of OH groups to the 5 and 8 positions in NQ˙– displaces the disproportionation equilibrium to the semiquinone probably due to intramolecular hydrogen bonding.