The kinetics of unsymmetrical quinone–hydroquinone redox reactions

Abstract

The kinetics of the chloranil–hydroquinone redox reaction have been studied, as a function of solvent and pH. The reaction was followed from the absorption spectra of the reactants, charge-transfer complex and chloranil radical-anion, and by the e.s.r. spectrum of the latter. In dioxan–water mixtures the rate of the reaction increases markedly with increasing water concentration, and it is acid- and base-catalysed. With an excess of hydroquinone the decay of chloranil, charge-transfer complex, and, in the later stages, chloranil radical-anion is first-order. With equimolar reactants the decay of chloranil is second-order and the decay of the complex is of order 3/2. Irradiation in the region of the charge-transfer band did not accelerate the reaction. A kinetic scheme involving two stages of electron-transfer is in agreement with all the experimental observations. In the alkaline region the initial stage is the transfer of an electron from hydroquinone negative ion to chloranil. In the acid region it is probably the transfer of an electron from hydroquinone to the protonated form of chloranil. There is no evidence for electron-transfer between the two neutral molecules. The second stage involves reactions between both like and unlike radicals. It is argued that the normal charge-transfer complex does not lie along the reaction path, although an ionised form of this complex may do so.