Electrochemical Oxidation of Dibenzothiophene in Acetonitrile and Acetonitrile-Water Mixtures
It is known that refractory organosulphur compounds such as dibenzothiophene (DBT) can be oxidized to dibenzothiophenesulfoxide (DBTO) and dibenzothiophenesulfone (DBTO2), that can be removed by physical methods such as extraction and adsorption using polar solvents. Therefore, in this work the electrochemical oxidation of a model sulfur aromatic compound such as dibenzotiophene (DBT) was studied in acetonitrile in the presence of low ( < 0.1 M ) and high ( > 1 M) content of water on carbon electrodes. This study is focused to obtain mechanistic information about the products formed.
Cyclic voltammetry of DBT in acetonitrile in the presence of < 0.1 M water on glassy carbon electrode showed three chemically irreversible anodic peaks. The first and second peaks involve a global transfer of one electron which allows respectively the formation of sulfoxide and sulfone derivatives as well as the release of one equivalent of protons, according to the global proposed reaction.
DBT – e- + H2O = DBTO + H+ + 1/2H2
When water was added to the acetonitrile solution ( >1 M ), the voltammetric pattern was maintained, however the global mechanism giving rise to the sulfoxide and sulfone derivatives was modified to two electrons with the release of two equivalents of protons.
DBT – 2e- + H2O = DBTO + 2H+
The proton formation was confirmed by cyclic voltammetry using acetate ions as proton probe while the products dibenzothiophene sulfoxide and dibenzothiophene sulfone were prepared by constant potential electrolysis and characterized by HPLC-MS-TOF and 1H and 13C NMR experiments. The analysis of the variation of the peak potential and half-peak width with the scan rate was used to establish the relevant role of water on the reaction mechanism, which changes from an ECCCC to an ECCEC pathway when water is present in excess
According to the above mentioned results, different oxidation products can be formed depending on the electrolysis conditions and for this reason, this work deals with the anodic oxidation process of DBT in acetonitrile to determine how the intermediates and the composition of the electrolyte solution govern the formation of the oxygenated products. Cyclic voltammetry, sampled current chronoamperometry, coulometry, preparative electrolysis, chromatography coupled with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy were used as analytical techniques
The authors acknowledge CONACYT for financial support through the project 221548. E.M.A also acknowledges B.R. Díaz and G. Cuellar for their assistance in some electrochemical, NMR and MS analysis respectively.
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