Halogen organic compounds, reduction

Mazur DJ, Weinberg NL (1987) Methods for the electrochemical reduction of halogenated organic compounds, US, US 4,702,804 Chem Abstr 108 (1988) 175949s... 

In this part of the chapter, the discussion is focused on the direct cathodic reduction of halogenated organic compounds, although the last section will address the increasingly active area of catalytic reductions of carbon-halogen bonds. 

In this section, we discuss processes in which cobalt-containing catalysts are employed for a variety of applications such as the reductions of molecular oxygen, carbon dioxide, and halogenated organic compounds as well as the oxidation of hydrazine. 

In a fashion similar to that of cobalt salen, distinct redox chemistry can be observed for each of the two cobalt centers of (28). Coulometric studies showed that the complex can undergo a one-electron reduction as well as a one-electron oxidation of each cobalt center and that there is no interaction between those cobalt centers. Thus, the two metal centers can be used as separate catalytic sites for the reduction of halogenated organic compounds. In the same article [147] is a hst of literature citations of work done over the past 20 years by the group of Hisaeda on the catalytic behavior of electroreduced vitamin B12 derivatives. 

The reductions of halogenated organic compounds (RX) involve the cleavage of carbon-halogen bonds [62]. Depending on the solvent, supporting electrolyte, electrode material and potential, it is possible to electrogenerate either alkyl radicals (R ) or carbanions (R ), which then can lead to the fonnation of dimers (R-R), alkanes (RH) and olefins [R(-H)] ... 

Studying the electrochemical reduction of halogenated organic compounds has practical importance, especially related to organic syntheses [62], Moreover, the reductive cleavage of the C-X bond is applicable as a method to convert hazardous chlorinated compound for example, polychlorinated biphenyls (PCBs) to biphenyl by reducing in DMF at -2.8 V vs SCE [63],... 

Numerous studies [295-300] have shown that the rate of electron transfer between an electrogenerated radical anion (mediator) and a halogenated organic compound (substrate) increases as the difference between the standard potentials for reduction of the precursor of the mediator and for reduction of the substrate decreases. 

In the following sections, examples of catalytic reductions of halogenated organic compounds by electrogenerated nickel, cobalt, and various other species are mentioned. 

Some papers have appeared that deal with the use of electrodes whose surfaces are modified with materials suitable for the catalytic reduction of halogenated organic compounds. Kerr and coworkers [408] employed a platinum electrode coated with poly-/7-nitrostyrene for the catalytic reduction of l,2-dibromo-l,2-diphenylethane. Catalytic reduction of 1,2-dibromo-l,2-diphenylethane, 1,2-dibromophenylethane, and 1,2-dibromopropane has been achieved with an electrode coated with covalently immobilized cobalt(II) or copper(II) tetraphenylporphyrin [409]. Carbon electrodes modified with /nc50-tetra(/7-aminophenyl)porphyrinatoiron(III) can be used for the catalytic reduction of benzyl bromide, triphenylmethyl bromide, and hexachloroethane when the surface-bound porphyrin is in the Fe(T) state [410]. Metal phthalocyanine-containing films on pyrolytic graphite have been utilized for the catalytic reduction of P anj -1,2-dibromocyclohexane and trichloroacetic acid [411], and copper and nickel phthalocyanines adsorbed onto carbon promote the catalytic reduction of 1,2-dibromobutane, n-<7/ 5-l,2-dibromocyclohexane, and trichloroacetic acid in bicontinuous microemulsions [412]. When carbon electrodes coated with anodically polymerized films of nickel(Il) salen are cathodically polarized to generate nickel(I) sites, it is possible to carry out the catalytic reduction of iodoethane and 2-iodopropane [29] and the reductive intramolecular cyclizations of 1,3-dibromopropane and of 1,4-dibromo- and 1,4-diiodobutane [413]. A volume edited by Murray [414] contains a valuable set of review chapters by experts in the field of chemically modified electrodes. 

Zero-valent iron (ZVI) barriers are a specific application of reductive precipitation that recently have been utilized for removing low concentrations of some halogenated organic compounds from waste or run-off streams. Interestingly enough, ZVI, which functions at ambient temperature and near neutral pH values, has been found to decompose chlorinated compounds by a reductive mechanism in the absence of air and by an oxidative one in its presence." In the latter case, the ZVI actually catalyzes the formation of the hydroxyl radical, a very potent oxidant. Attention has been placed recently on the use of nano-sized ZVI for in-situ remediation of soils and groundwater." ... 

The electrochemical reduction of halogenated organic compounds (RX) in organic solvents is well known [7, 8]. The accepted general electrochemical mechanism starts with the transfer of one electron from the working electrode, followed by a fast elimination of the halide anion to form a radical intermediate R . This radical can either be reduced further to the carbanion R , which generally occurs at potentials more positive with respect to the reduction of the parent molecule RX, or it can couple to produce a dimeric product as a consequence of an RRC process. Therefore, the mechanism and the stmcture of the obtained products depend on the starting material and conditions selected for the electrochemical reduction. 

Another possible application of electrochemical reactors for urine treatment is the removal of micropollutants such as pharmaceutical residues. Depending on the characteristics of the micropollutants, direct oxidation at the anode [33] or reduction at the cathode (e.g., for the removal of halogenated organic compounds such as trihalo-methanes, [34]) can be a suitable process. 

Xu, J., Dozier, A., and Bhattacharyya, D. (2005). Synthesis of nanoscale bimetallic particles in polyelectrolyte membrane matrix for reductive transformation of halogenated organic compounds. J. Nanopart. Res. 7, 449-467. 

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