Electrochemical reduction of 4-thienylquinazoline in dimethylformamide*

Abstract: The electroreduction of 4-(2-thienyl)quinazoline (4-TQ) in acetonitrile solutions gave a single two-electron well-defined polarographic wave. The well-defined wave was found to be diffusion-controlled and irreversible on the basis of the usual criteria. Based on the controlled-potential electrolysis and cyclic voltammetric experiments, a disproportionation reaction mechanism is suggested for 4-TQ reduction in acetonitrile.

Abstract: The electrochemical behaviour of 4(2′-thienyl)quinazoline (4-TQ) was studied in acid solutions of varying ionic strengths using polarography, cyclic voltammetry and controlled-potential electrolysis. 4-TQ is reduced in two single electron steps to give the dihydro-4-TQ product. The two polarographic waves were found to be diffusion-controlled, the first wave being reversible and the second wave irreversible. On the basis of the results obtained, a mechanism is suggested for 4-TQ reduction in which a dimerization reaction goes in parallel with the electron-transfer step.

Abstract: was developed for solving the integral equations obtained from the boundary value problems, and extensive data were calculated which permit construction of stationary electrode polarograms from theory. Correlations of kinetic and experimental parameters made it possible to develop diagnostic criteria so that unknown systems can be characterized by studying the variation of peak current, half-peak potential, or ratio of anodic to cathodic peak currents as a function of rate of voltage scan.

Abstract: A general mechanism involving two successive charge-transfer steps coupled with acid-base reactions is considered. It is shown that ECE reaction paths as well as disproportionation reactions must be taken into account in the general case. According to the value of the experimental parameters two limiting behaviours are to be considered (i) ECE mechanism without any kinetic interference of disproportionation reactions (ii) apparent disproportionation of the first charge-transfer product, the rate constant being dependent on the acidity of the medium. Theoretical relationships allowing the kinetic analysis of such reaction schemes by linear sweep voltammetry are established. Diagnostic criteria are developed whereby variation of peak current and peak potential with sweep rate and initial concentration can be used to characterize the mechanism. Rate-constant determination is discussed.