Sonoelectrochemical Reduction of Tetrabromobisphenol-a at Silver Cathodes in Dimethylformamide

Abstract

First reported in the early 1930s, sonoelectrochemistry is an analytical technique that seeks to incorporate ultrasound into electrochemical experiments. Compton and co-workers reviewed the numerous advantages of sonoelectrochemistry over “silent” experiments without ultrasound, including increased mass transport and electrode surface roughening.1 Presently, there is a lack of research employing sonoelectrochemistry to study environmental remediation, a topic of particular interest in our group. As a beginning inquiry into the various factors that optimize sonoelectrochemical reduction, a comparative study into the reduction of the flame retardant 3,3',5,5'-tetrabromobisphenol-A (TBBPA) was undertaken with the aid of cyclic voltammetry, controlled-potential (bulk) electrolysis, and sonoelectrochemistry. Cyclic voltammograms of TBBPA at silver cathodes in dimethylformamide containing 0.10 M tetramethylammonium tetrafluoroborate (TMABF4) show three irreversible cathodic peaks with peak potentials of –0.50 V, –0.70 V, and –1.34 V versus a cadmium-saturated mercury amalgam reference electrode (Cd(Hg)). However, bulk electrolyses of TBBPA at –1.50 V versus Cd(Hg) under the same conditions show minimal debromination as a result of competitive reduction pathways between the carbon–bromine bonds and the phenolic hydrogens within the molecule. To improve debromination, two alternate pathways are explored: the methylation of TBBPA (MeTBBPA) to prevent O–H reduction and the introduction of ultrasound to electrochemical analysis. Cyclic voltammograms of MeTBBPA show two irreversible cathodic peaks (–0.70 V and –0.90 V versus Cd(Hg)) and bulk electrolyses at –1.50 V versus Cd(Hg) show complete debromination in TMABF4–DMF. Similarly, sonoelectrochemical bulk electrolysis experiments at –1.50 V vs. Cd(Hg) show a substantial increase in debromination compared to electrolyses under “silent” conditions. 1 R. G. Compton, J. C. Eklund, F. Marken, Electroanalysis 1997, 9, 509–522.