The Role of Solvents in the Dimerization of the 2,3-Dichloro-5,6-dicyano-p-benzoquinone Anion Radical

Abstract

The reversible dimerization of the anion radical of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) has been studied in water, acetonitrile (MeCN), and other organic solvents. The analysis of the thermodynamic parameters of dimerization equilibria reveals that a large negative entropy term plays a primary role in determining the equilibrium constants. The temperature-jump measurement shows that the forward rate constant in water containing 5.8 M urea is nearly a diffusion controlled one, (1±0.5)×10⁸ M⁻¹ s⁻¹ at −6°C. In MeCN-water mixtures, the backward reaction rate increases with the square of the MeCN concentration. This result suggests that a dimer is effectively decomposed by the successive attacks of the two MeCN molecules.