S. Arias, E. Brillas, Josep Costa
Apr 25, 1990
Citations
0
Influential Citations
4
Citations
Journal
Journal of Electroanalytical Chemistry
Abstract
Abstract The electrochemical oxidation of 2,4-dibromo-6-methylaniline and 4-bromo-2,6-dimethylaniline 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 (CV) and controlled-potential electrolysis. A single oxidation process (Ia) is found for 2,4-dibromo-6-methylaniline up to 5.0 M acid concentration, whereupon it displays a second oxidation process (IIa). The p-benzoquinone 2,3-dibromo-6-methyl-2,5-cyclohexadien-1,4-dione is obtained as the final product of process Ia. For 4-bromo-2,6-dimethylaniline, however, two different oxidation processes are always found, depending on the acid medium. In solutions of acid concentration lower than 3.0 M, the first process (Ib) is followed by process III, corresponding to the oxidation of bromide anions ejected in process Ib to generate bromine. At higher acid concentrations, both processes overlap, the composite process Ib' being observed as well as the second oxidation process (IIb). The final product of processes Ib and Ib' is the dimer 4-[4'-bromo-2',6'-dimethylphenyl)imino]-2,6-dimethyl-2, 5-cyclohexadien-1-one. The CV results show that this type of dimer is also formed in process Ia of 2,4-dibromo-6-methylaniline and then the final p-benzoquinone obtained after exhaustive electrolysis proceeds from its hydrolytic degradation. The electroactive species of processes IIa and IIb is the corresponding protonated dimer. From the voltammetric data of processes Ia, Ib and Ib', the protonated form of each compound is proposed to be initially oxidized in a one-electron reversible step to generate its dication radical, which is subsequently deprotonated. This reaction is the rate-determining step of each process. The head-to-tail coupling of the resulting cation radical, followed by hydrolysis of the diimine derivative formed, leads to the protonated dimer. In processes IIa and IIb, the corresponding protonated dimer undergoes a one-electron reversible oxidation, followed by deprotonation of the resulting species. The rate-determining step of these processes depends on the experimental conditions tested.