Mechanism of the reaction between N,N′-diphenyl-1,4-benzoquinone diimine and thiophenol in n-propanol

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We studied the kinetics of the reaction between N,N′-diphenyl-1,4-benzoquinone diimine and thiophenol in n-propanol at 343 K and compared the results with the same data for the chlorobenzene medium. The replacement of chlorobenzene with n-propyl alcohol increases the reaction rate by one order of magnitude. The order of the reaction with respect to thiophenol was determined to be 1.0 in both solvents, and the order of the reaction with respect to quinone diimine decreases from 1.5 to 1.2 on passing from chlorobenzene to n-propanol. In contrast to the reaction in chlorobenzene, where the radical chain mechanism is realized, the reaction in n-propanol simultaneously proceeds via a radical chain channel and a heterolytic channel. In the absence of an initiator and at equal reactant concentrations of ∼10−4 mol/L, the reaction rates in both pathways are commensurable. The chain reaction is characterized by short chains whose length is a few units. Chain generation in the absence of an initiator occurs by the direct bimolecular reaction between the reactants, whose rate constant in n-propanol is one order of magnitude higher than that in chlorobenzene, which is explained by the high polarity of the transition state in this endothermic homolytic reaction. The reaction mechanism was proposed, and, for this mechanism, the rate constants of elementary steps were determined.