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.
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