Disproportionation of 2,5-dichlorosemiquinone radicals in benzene according to unsteady-state kinetic data for the chain reaction between 2,5-dichlorohydroquinone and N,N′-diphenyl-1,4-benzoquinone diimine

Abstract

A method for determining the rate constant of disproportionation of 2,5-dichlorosemiquinone radicals (k6) from unsteady-state kinetic data for the initiated chain reaction of N,N′-diphenyl-1,4-benzoquinone diimine with 2,5-dichlorohydroquinone have been developed, and two variants of this method are presented. The method is based on the study of the unsteady-state disappearance kinetics of one of the initial reactants (quinone diimine) in its initiated chain reaction with hydroquinone. The unsteadiness of the reaction is due to the presence of semiquinone radicals or initiator radicals accumulated before the start of the reaction at a concentration exceeding the steady-state concentration of semiquinone radicals in the chain reaction. The variants of the method differ in the order of mixing the reactants and initiator, on which the nature and concentration of the radicals accumulated in the system before the reaction depend. In the first variant, a quinone diimine + initiator solution is initially prepared and initiator radicals are accumulated. Hydroquinone is added to this solution (start of the reaction). In the second variant, a hydroquinone + initiator solution is initially prepared and semiquinone radicals from hydroquinone are accumulated. Quinone imine is then added to the solution (start of the reaction). The disproportionation rate constant of semiquinone radicals (k6) is derived from the dependence of the decrease in the quinone imine concentration in a certain short time (∼20 s) after the start of the reaction on the initiation rate. The rate constant k6 in benzene is (7.3 ± 3.7) × 106 l mol−1 s−1 according to the first variant of the method and (5.0 ± 2.2) × 106 l mol−1 s−1 according to the second one.