Tetrachloromethane reduction

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

Amines can act as reductants for excited states of other electron acceptors, too. Again, two possibilities exist (1) amine and acceptor form a CT (2) the complex formation takes place either with the excited state of the amine or with that of the electron acceptor (see discussions of ketone-amine combinations). Examples for the former principle are such combinations as DMA-nitrobenzene [105], triphenylamine-tetracyano ethylene, 4-nitrophenole, 4-aminochlorobenzene [106], tributylamine-tetrachloromethane [107], DMA-substituted chloroacetic adds [108, 109],... 

Picardal F., Arnold R. G., and Huey B. B. (1995) Effects of electron donor and acceptor conditions on reductive dehalogenation of tetrachloromethane by Shewanella putre-faciens 200. Appl. Environ. Microb. 61(1), 8—12. 

When a mixture of tetrachloromethane and benzaldehyde in DMF was treated, at room temperature, with a catalytic amount of lead(ll) bromide and a shght excess of aluminum as a stoichiometric reductant the coupled product was obtained in good yield (Scheme 13.69) [86]. Subsequent reductive 1,2-elimination of trichloromethyl carbinol by means of the Pb/Al bimetal system could be readily achieved by changing the reaction media. The mechanism of the Pb/Al bimetal redox system presumably involves lead(O) reduction of polyhalomethane to provide an organolead complex which then reacts wifh an aldehyde to give the couphng product. Regenerahon of lead(O) by reduchon of lead(ll) with aluminum metal would complete the catalyhc cycle. 

An additional aspect of these dehalogenations that elucidates the role of vitamin B12 is provided by experiments with Shewanella alga strain BrY (Workman et al. 1997). This organism carries out reduction of Fe(III) and Co(III) during growth with lactate and H2, and was used to reduce vitamin B12a anaerobically in the presence of an electron donor. The biologically reduced vitamin B12 was then able to transform tetrachloromethane to CO. 

An attractive pathway with a lot of potential uses the transition metal mediated reaction of organic halides with carbon monoxide. Suitable substrates are organic halides capable of oxidative addition to low-valent transition metal compounds. Insertion of carbon monoxide and reductive elimination of an acid halide will complete the catalytic cycle. In tins way it was shown tiiat allyl chloride yields butenoic acid chloride in >80% yield accor g to equation 22)P As well as palladium, rhodium and iridium also act catalytically. It is of no surprise that allylic halides, benzylic halides and aryl halides in particular are readily converted to acid halides. Simple aliphatic halid undergo the oxidative addition step more slowly and, if they cany hydrogen atoms on an sf hybridized C atom in the -position to the halogen atom, may give alkenes via 3-hydrogen elimination. Alkenes can also be converted to acid halides widi carbon monoxide in the presence of transition metal catalysts in solvents such as methylene chloride or tetrachloromethane. ... 

Fig. 5. Redox scales for some major elements in groundwater (as in Fig. 1), and for model contaminants representing forms of hazardous waste other than radionuclides. The scale at right shows the potentials associated with the mobility of chromium, and the reductive dechlorination of tetrachloromethane to methane (Vogel et al. 1987) and oxidation of methane to C02(g).

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