Benzoquinone imines. Part IV. Mechanism and kinetics of the formation of Bandrowski's base

Abstract

The kinetics of the formation of Bandrowski's base, by oxidation of p-phenylenediamine with ferricyanide in aqueous solution at pH > 8, indicate that the rate-controlling step involves electrophilic attack by protonated p-benzoquinone di-imine on neutral p-phenylenediamine. The resulting intermediate, 2,4′,5-triaminodiphenylamine, suffers rapid attack at C(4) by a second molecule of protonated di-imine to give the reduced form of Bandrowski's base. The latter is oxidised by a third molecule of di-imine, thereby regenerating a molecule of p-phenylenediamine, which can thus be considered a catalyst in the conversion of the di-imine into Bandrowski's base. At pH < 8 the kinetics of the reaction are complicated by the formation of significant amounts of p-benzosemiquinone di-imine radicals at the expense of the primary reactants. At low concentrations, the radicals are relatively stable and disproportionate as the formation of Bandrowski's base proceeds.