Electrochemical oxidation of 2,4-dichloroaniline in aqueous sulphuric acid medium at a platinum electrode

Abstract The electrochemical oxidation of 2,4-dichloroaniline in aqueous sulphuric acid solutions at concentrations higher than 1.0 M , at a platinum electrode, has been studied by rotating disk electrode, cyclic voltammetry and controlled-potential electrolysis. In solutions of acid concentration lower than 5.0 M , only one single oxidation process (I) is found. At higher acid concentrations, two other oxidation processes (II andIII) are observed. These processes shift with increasing acid concentration to less positive potentials, process II overlapping with process I at acid concentrations higher than 8.0 M and process III with the composite process I + II at acid concentrations higher than 12.0 M . Processes I and III are controled either by diffusion, or by adsorption, depending on the medium used, whereas processes I + II and II are always diffusion controlled. The electroactive species of process I, as well as of process I + II in the solutions where it appears, is always the protonated form (Ar N + H 3 ) of 2,4-dichloroaniline. This species is oxidized in a two-electron step to form the (Ar-NH 3 ) cation. Deprotonation of this cation and subsequent hydrolysis of the resulting species gives the corresponding p -benzoquinoneimine, which is further hydrolysed to chloro- p -benzoquinone. Two additional redox pairs observed in cyclic voltammograms are attributed to the reduction equilibrium of the two last compounds. The chloride anion ejected in the chloro- p -benzoquinone formation is oxidized to the chloro radical in process II. The loss of two protons from the benzenic ring of the (Ar-NH 3 ) 3+ cation produces the electroactive species of process III, which is bielectronically oxidized. Under diffusion control, processes I, I + II and III are irreversible and the corresponding initial two-electron transfer is the rate-determining step.