Electrocatalytic Reduction of 2,4-Dichlorophenoxyacetic Acid (2,4-D) at Silver Cathodes in Dimethylformamide Containing Tetraalkylammonium Salts
Authors: McGuire Caitlyn M. , Mali Sonali , Peters Dennis G 
Published: February 2020

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 (TMABF4), tetraethylammonium tetrafluoroborate (TEABF4), and tetra-n-butylammonium tetrafluoroborate (TBABF4)].  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 (1r,2R,3S,4r,5R,6S)-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 (TBABF4) 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 addition to cyclic voltammetry, bulk electrolyses of 2,4-D have been conducted at silver mesh cathodes.  Catholytes from bulk electrolyses conducted at –1.65 V were analyzed via gas chromatography–mass spectrometry (GC–MS) to identify products.  It was discovered that, when TMABF4 is used as the supporting electrolyte, the major product is the methyl ester of 4-chlorophenoxyacetic acid; use of TEABF4 results in the major product being the corresponding ethyl ester.  When 2,4-D is reduced in the presence of TBABF4, major products are a mixture of 4-chlorophenoxyacetic acid and its butyl ester. 

     A chemical standard of 4-chlorophenoxyacetic acid has been obtained for investigation of the chemical mechanism for esterification, and for use as a quantitation standard.  Gas chromatography will be utilized to quantitate the products with respect to an internal standard.

References

1.  Loomis, D.; Guyton, K.; Grosse, Y.; Ghissasi, F.E.; Bouvard, V.; Benbrahim-Tallaa, L.; Guha, N.; Mattock, H.; Straif, K. Lancet, 2015, 16, 891–892.

2.  Isse, A.A.; Falciola, L.; Mussini, P.R.; Gennaro, A. Chem. Commun. 2006, 344–346.

3. Peters, D.G.; McGuire, C.M.; Pasciak, E.M.; Peverly, A.A.; Strawsine, L.M.; Wagoner, E.R.; Barnes, J.T. J. Mex. Chem. Soc
. 2014, 58, 287–302.

4. Brillas, E.; Boye, B.; Sirés,I.; Garrido, J.A.; Rodríguez, R.M.; Arias, C.; Carbot, P.-L.; Comninellis, C. Electrochem. Acta
2004, 49, 4487–4496.

5. Tsyganok, A.; Otsuka, K.; Yamanaka, I.; Plekhanov, V.; Kulikov, S. Chem. Lett. 1996, 261–262.

6.  This reference consists of a cadmium-saturated mercury amalgam in contact with DMF saturated with both CdCl2 and NaCl; this electrode has a potential of –0.76 V vs. SCE at 25°C.

Figure 1