Study of the electrochemical oxidation of 4,6-dimethyldibenzothiophene on a BDD electrode employing different techniques

Abstract

The electrochemical oxidation of 4,6-dimethyldibenzothiophene (4,6-DMDBT) at low concentrations on a BDD anode was investigated in a monophasic acetonitrile (93.5% v/v)–water (6.5% v/v, 0.01 M LiClO4) solution. Two oxidation steps related to the sequential formation of sulfoxide and sulfone derivatives were identified. Kinetic parameters such as the electron transfer coefficient α, the number of electrons nα involved in the rate-determining step, the total number of electrons n, the reaction rate constant k0 and the diffusion coefficient D of 4,6-DMDBT for the first transformation were determined by cyclic voltammetry, differential pulse voltammetry (DPV), square wave voltammetry and bulk electrolysis under potentiostatic conditions. The process was bielectronic with α = 0.57, nα = 1, k0 = 7.46 × 10−6 cm s−1 and D = 2.30 × 10−6 cm2 s−1. DPV was the most sensitive electroanalytical technique. Using 27 mg L−1 of 4,6-DMTDB, DPV allowed determining a conversion of 91% to sulfoxide after 60 min of electrolysis in a BDD/BDD cell at an anodic potential of 1.50 V, with an apparent rate constant of 0.034 min−1. The electrochemical characterization was corroborated via gas chromatography–mass spectrometry and ultra-high performance liquid chromatography coupled to electrospray ionization and quadrupole time-of-flight mass spectrometry, confirming the formation of the sulfoxide in the first step and the sulfone in the second one as main products, alongside a minor proportion of dimers.