Oxidation of 2,4-dibromo-6-nitroaniline in aqueous sulphuric acid solutions on a platinum electrode

Abstract

The electrochemical oxidation of 2,4-dibromo-6-nitroaniline in aqueous sulphuric acid solutions at concentrations c_acid > 9.0 mol dm^–3 on a platinum electrode has been studied by the use of a rotating-disc electrode, cyclic voltammetry and controlled-potential electrolysis. A single oxidation process is found at c_acid < 13.0 mol dm^–3. At higher acid concentrations a second oxidation process is also observed. The first process behaves as a first-order ECE (electrochemical–chemical–electrochemical) reaction mechanism and the second as an irreversible two-electron process controlled by diffusion. Voltammetric data, as well as the change in colour of solutions observed with rising acid concentration, show the existence of two initial electroactive forms for the first process: species (I) at c_acid⩽ 13.0 mol dm^–3 and species (II) at c_acid > 11.0 mol dm^–3. Both species are present in solution over the acid concentration range 11.0–13.0 mol dm^–3. Species (I) has only the amino group protonated, whereas species (II) has both the amino and nitro groups protonated. Voltammetric results allow one to establish that both species follow analogous decomposition pathways to produce the corresponding p-benzoquinone. Each electroactive form is initially oxidized in a reversible one-electron step to generate its radical cation, which is subsequently deprotonated, and this irreversible reaction is the rate-determining step. The resulting radical undergoes a reversible one-electron oxidation to form a dication. Hydrolysis of this dication gives the p-benzoquinonimine derivative, which is further hydrolysed to the corresponding p-benzoquinone. The loss of one proton from the benzene ring of the initially electrogenerated radical cation from species (II) produces the electroactive species of the second process. The initial irreversible two-electron charge transfer of this species is the rate-determining step of the second process.