High-pressure kinetics of the reactions of p-benzoquinone with aliphatic amines in aprotic solvents

Abstract

The rates of substitution reaction between p-benzoquinone and dibutylamine and of the electron-transfer reaction between chloranil and tripropylamine were examined spectrophotometrically, under pressure, in dichloromethane, 1,2-dichloroethane, chloroform, and acetonitrile. For the substitution to produce 2-dibutylamino-p-benzoquinone the reaction rate obeyed a third-order kinetic equation: first order with respect to p-benzoquinone and second order with respect to dibutylamine. The activation volume was as large as ca.–60 cm³ mol^–1 in the solvents used, and a negative activation energy of –31 kJ mol^–1 was observed in acetonitrile. A reaction scheme has been proposed where an electron-transfer reaction to form the p-benzoquinone anion radical occurs prior to the rate-determining step, which is the second attack by the amine. By taking into account the result that the electron transfer from chloranil to tripropylamine accompanied the activation volume of ca.–30 to –45 cm³ mol^–1, the reaction volume of this process was estimated to be ca.–45 cm³ mol^–1. Thus, in the p-benzoquinone–dibutylamine system we may reasonably assign the activation volume of –10 to –15 cm³ mol^–1 to the second bimolecular attack by the amine.