Electrochemical reduction of halogen containing compounds at a mercury cathode: Chloroacetic, dichloroacetic acids and corresponding ethylesters in dimethylformamide

Abstract

The electrochemical reduction of monochloroacetic ethylester (I) and acid (II), dichloroacetic ethylester (III) and acid (IV) in DMF/0.1 M Et 4 NClO 4 solutions has been investigated. Polarographic experiments, supplemented by controlled potential electrolysis and halogen ion titration of the electrolyzed solutions have been performed. The effect of 3,4-xylenol (HA) as a proton donor also has been studied. The results are: for (I), in the absence of HA, one wave is observed, E 1/2 =−1.87 V, n =1, d[Cl − ]/d[I]=1 mol mol −1 ; in the presence of excess HA, the wave is doubled. For II, the proton donor has no effect on the wave, E 1/2 =−1.96 V ( n =1, d[Cl − ]/d[II]=1 mol mol −1 ). For III and IV, one wave is observed again, E 1/2 =−1.34 V; HA affects only the wave of III, which is doubled (excess HA) and followed by the wave of I. n app and d[Cl − ]/d[III] values are both unity for both III and IV in the electrolysis at E =−1.6 V. An increase of n app and d[Cl − ]/d[III] values is found in electrolysis experiments on III at E >−1.6 V. Thereafter, additional waves are observed. Comparison of the data pertaining to the four compounds examined points towards a mechanism which involves one-electron transfer leading to radical ion (I and III), and subsequent decomposition in neutral radical and Cl − . The radical ion is bypassed in the case of the acids. The results obtained in controlled potential experiments on the dichloroester are interpreted suggesting that its radical ion, besides dissociating, slowly disproportionates in solution.