Reduction of halogens

The field of reduction is much less well charted than that of oxidation but a substantial literature exists nonetheless and is growing rapidly. Reductions are conveniently classified into (/) those involving and initial electron acceptance by the substrate (possibly followed by rapid protonation) and ( ) those involving electron acceptance concerted with, or followed very rapidly by, homolysis of the substrate the latter includes the important Fenton and silver-persulphate reactions, as well as reductions of halogens, hydrazine and possibly NO3 and NOJ. 

Complex (16), which has a similar structure to Co11 salen, catalyzes the electrocarboxylation of arylmethyl chlorides.274 The enhancement of the catalytic life of (16) as compared to Co-salen may be due to the absence of imino bond in its ligand. The catalytic reduction of halogenated compounds has also been attempted at poly[Mn(salen)]-coated electrodes (M = Ni,253 Co275), which might have potential use for determination of organohalide pollutants.275... 

Several workers have investigated the reduction of halogens ft to a carbon-carbon 7r bond. For example, Fantazier and Poutsma94 have found that a halogen ft to a carbon-carbon triple bond can be reduced easily with a trialkyltin hydride (equation 70). 

Schmal D, van Erkel J, van Duin PJ (1986) Electrochemical reduction of halogenated compounds in process wastewaters, Inst Chem Eng Symp Ser 1986,98 (Electrochem Eng) 281 Chem Abstr 105 158252e... 

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. 

REDUCTION OF HALOGEN DERIVATIVES OF HYDROCARBONS AND BASIC HETEROCYCLES... 

Reductions of halogen, nitro, diazo and azido derivatives of esters resemble closely those of the corresponding derivatives of carboxylic acids (p. 141). 

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],... 

Similar studies [49,50] on the cathodic reduction of halogenated nitro-benzenes revealed the following mechanism ... 

Reaction LXII. Direct Reduction of Halogen Compounds. (J. C. S.,... 

This reaction is the dominant dehalogenation pathway in reduction of halogenated methanes [84] and haloacetic acids [85]. In Fig. 4, this reaction is illustrated for perchloroethene (PCE), where complete dechlorination by this pathway requires multiple hydrogenolysis steps. The relative rates of these steps are a critical concern because they determine whether... 

Hassan SM. Reduction of halogenated hydrocarbons in aqueous media I. involvement of sulfur in iron catalysis. Chemosphere 2000 40 1357-1363. 

Table IV. Polarographic data for the reduction of halogenated uracils and their glycosides...

Chetty, R., Christensen, P. A. and Golding, B. T. (2003) In situ FTIR studies on the electrochemical reduction of halogenated phenols. Chem. Commun. (8), 984-985. 

Schmal, D., van Erkel, J. and van Duin, P. J. (1986) Electrochemical reduction of halogenated compounds in process waster water. Ichem. Symp. Ser. 98, 259-269. 

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