Electrochemical oxidation of o-phenylenediamine and 1,3 dihydrospiro[benzo[d]imidazole-2,1 '-cyclohexane]. A comprehensive study and introducing a novel case of CE mechanism

摘要

Electrochemical behavior of o-phenylenediamine (PDA) and 1,3-dihydrospiro[benzo[d]imidazole-2,1'-cyclohexane] (DBI) was extensively studied in water and water/ethanol mixture using different voltammetric techniques. Our data showed that the oxidation of PDA is highly dependent on pH, follows a complex pattern and participate in following chemical reactions such as polymerization. Unlike acidic and neutral solutions, in highly alkaline solutions (pH >= 11), however, PDA shows a simple reversible redox system. Contrary to PDA, the presence of the cyclohexyl group in the structure of DPI makes its oxidation pattern less complex than that of PDA and causes the molecule less susceptible to participate in the following chemical reactions. Our results showed that DBI in aqueous solutions is unstable and undergoes acid catalyzed hydrolysis to give PDA. The instability of DPI in acidic solutions is so high that it turns completely into PDA in the time scale of the voltammetric experiments. Different from acidic media, in alkaline solutions (pH >= 9.0), the hydrolysis rate is slow, so that DPI shows a reversible redox couple. The kinetic of DPI hydrolysis using differential pulse voltammetry method was studied and the apparent hydrolysis rate constants (K-h(obs)) were found by assuming the pseudo-first order rate kinetics. In addition, in this work, adsorption activity, diffusion coefficient and pK(a) values of DPI and PDA species were determined and the Pourbaix diagrams for these compounds were constructed. The most important part of this paper is devoted to introducing a rare type of mechanism in electrochemical reactions. In this way, the rarely studied mechanism has been introduced in relation to the reaction of PDA with cyclohexanone and the formation of DPI. (C) 2020 Elsevier Ltd. All rights reserved.