Electrocatalytic Reduction of 2,4-Dichlorophenoxyacetic Acid (2,4-D) at Silver Cathodes in Dimethylformamide Containing Tetraalkylammonium Salts

In this study, the direct reductions of 2,4-D and a monochlorinated analogue (4-chlorophenoxyacetic acid) have been investigated in dimethylformamide (DMF) containing several different supporting electrolytes [tetramethylammonium tetrafluoroborate (TMABF 4 ), tetraethylammonium tetrafluoroborate (TEABF 4 ), and tetra- n -butylammonium tetrafluoroborate (TBABF 4 )]. 2,4-Dichlorophenoxyacetic acid (2,4-D) is a widely utilized pesticide despite environmental concerns. It was a major component of Agent Orange, dispersed throughout Southeast Asia, and continues to be applied worldwide. Although there is some debate about the relationship between 2,4-D and various cancers and lymphomas, a 2015 review of evidence led the World Health Organization to declare 2,4-D as a possible carcinogen. 1 These concerns led to our interest in the mechanisms pertaining to electroreductive cleavage of its carbon–chlorine bonds. Literature documents well the catalytic effect of silver cathodes on the electroreduction of halogenated organic compounds, 2 which often results in more complete dechlorination of polychlorinated substrates and a positive shift in reduction potentials compared with reductions at other electrodes. Previous work in our laboratory utilized silver cathodes to dechlorinate pesticides such as dichlorodiphenyltrichloroethane (DDT), 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan), and (1 r ,2 R ,3 S ,4 r ,5 R ,6 S )-1,2,3,4,5,6-hexachlorocyclohexane (lindane). 3 Extensive research has focused on the electrochemical oxidation of 2,4-D, 4 and a study of the electrochemical reduction of 2,4-D at various carbon materials has been reported. 5 Cyclic voltammograms for reduction of 2,4-D in DMF containing tetra- n -butylammonium tetrafluoroborate (TBABF 4 ) at silver and glassy carbon electrodes are shown in Figure 1. When glassy carbon is used as the working electrode, cathodic peaks are seen at –1.68, –1.96, and –2.24 V vs. a Cd(Hg) reference electrode. 6 Three peaks at approximately –0.75, –1.55, and –1.83 V result from the reduction of 2,4-D at a silver cathode. This shift, to less negative peak potentials, illustrates the catalytic nature of the silver surface. Cyclic voltammograms of 2,4-D at these cathodes are similar regardless of the identity of the supporting electrolyte; however, the onset of solvent–electrolyte breakdown occurs at more negative potentials with an increased length of the alkyl chain of the ammonium cation. In a