p-Phenylenediamine electrochemical oxidation revisited: An insight into the mechanism and kinetics in acid medium
Ángela Fernández-Merino, Rafael Del Caño, Miriam Chávez, Guadalupe Sánchez-Obrero, Rafael Madueño, Manuel Blázquez, Teresa Pineda
Abstract
The present study is focused on the electrochemical oxidation of p-phenylenediamine in aqueous acid solution. A general view of the process in a wider pH range is first presented to highlight the unique features taking place in the acid medium. The oxidation process in this pH range involves two electrons and three or two protons to yield the diimine quinone species. A quasi-reversible electron transfer with rate constants of 6x10-4 and 6.4x10-4 cm/s are determined by cyclic voltammetry and rotating disk voltammetry, respectively. The presence of a second reduction peak that is strongly dependent on the scan rate and the solution pH indicates the occurrence of a chemical reaction in which the oxidation product is involved. The analysis of the cyclic voltammetric curves indicates that the subsequent chemical reaction generates an electroactive species that can be reduced within the potential range of the voltammetric scan. Controlled potential electrolysis allows us to determine the characteristics of the oxidation process as well as the nature of the chemical reaction. A diimine quinone hydrolysis reaction involving both imine groups, accounting for the 60 % conversion to benzoquinone, together with the reaction of the parent diamine with the diimine quinone product to yield the semiquinone free radical are proposed to take place immediately after the oxidation reaction. The presence of other coupling reaction yielding dimer or trimer species such as the Brandowski’s base cannot be discarded.