Electroreduction of Organic Compounds, 34 [1]. Cathodic Dehalogenation of Chloroarenes with Electron-Donating Substituents

Abstract

The electrochemical reduction of chlorinated arenes with electron-donating substituents, i. e. chlorotoluenes, -anisoles and -phenols, is studied. Preparative electrolyses are run in various solventsupporting electrolytes under potentiostatic and galvanostatic conditions at lead or carbon cathodes. A partial and mostly regioselective hydrodechlorination of compounds with two or more chloro substituents is possible under suitable conditions. The replacement of one single chloro substituent, in particular in a para-position, is difficult. Highly toxic and persistent oligochloro derivatives are thus transformed into less problematic compounds with a low degree of chlorination. The chlorine content of real-life materials such as extracts of soil contaminated with chlorinated phenols and Nitrofen® can also be significantly decreased by electroreduction.

Active-alkali metal-promoted reductive cleavage of chlorinated phenols

“…Moreover, the recovery of a major amount of 4-chlorphenol (1a) in the reduction of 2,4,6-TCP (1f) with DSS (3b) (Table 1, entry 14) is worth noting. This last result points out the relatively low reducibility of a C-Cl bond para to the oxygen atom, as already observed in the cleavage of isomeric mono-and poly-chlorinated phenols under electrochemical reaction conditions [11].…”

“…Accordingly, exhaustive dehalogenation of these compounds was obtained with an excess of different SET reducing agents (Table 1, entries 9-14). In some instances, significant amounts of either 3,3 0 -dihydroxybiphenyl (5b) or 2,2 0 -dihydroxybiphenyl (5a) were recovered in the reduction with aromatic radical anions of 2,3,5-TCP (1e) and 2,4,6-TCP (1f), respectively (Table 1, entries [11][12][13]. Moreover, the recovery of a major amount of 4-chlorphenol (1a) in the reduction of 2,4,6-TCP (1f) with DSS (3b) (Table 1, entry 14) is worth noting.…”

“…Due to their relative toxicities [3], as well as to the possible formation of even more toxic by-products during incineration processes [4,5], several methodologies have been proposed for their degradation including, inter alia, photochemical treatment [6][7][8], electrochemical reduction/ oxidation [9][10][11], catalytic hydrodechlorination [12][13][14], reduction with zerovalent mono-or bi-metallic systems in protic solvents [15][16][17], Fenton-like oxidation reactions [18][19][20], or biological degradation [21][22][23].…”

Electroreductive Remediation of Halogenated Environmental Pollutants

“…Although we have touched on the electrochemical reductions of chlorofluorocarbons, pesticides, flame retardants, disinfection byproducts, and other halogenated pollutants, some of these families are too large to permit exhaustive coverage. Furthermore, there are some well-recognized and historically significant pollutants that are absent from this review, including polychlorinated biphenyls, , dioxins, , chlorinated phenols, and chlorobenzoic acids. Thus, there remain many compounds and concepts that need to be investigated much more intensely for the electrochemical remediation of environmental pollutants.…”