Dechlorination reductive

An alternative use of added readily degradable substrates is to drive the local environment toward anaerobiosis so that reactions such as reductive dechlorinations or reductive removal of nitro-groups are promoted. 

Pentafluorobenzene. Pentafluoroben2ene has been prepared by several routes multistage saturation—rearomati2ation process based on fluorination of ben2ene with cobalt trifluoride reductive dechlorination of chloropentafluoroben2ene with 10% pabadium-on-carbon in 82% yield (226,227) and oxidation of penta uorophenylbydra2ine in aqueous copper sulfate at 80°C in 77% yield (228). Its ioni2ation potential is 9.37 V. One measure of toxicity is LD q = 710 mg/kg (oral, mouse) (127). 

DDT is slowly converted in vivo by reductive dechlorination to DDD and by further dechlorinations to 4,4 -dichlorodiphenylacetic acid [83-05-6] (DDA), the predominant excretory metaboUte. Anaerobically, it may form 4,4 -dichlorodiphenyiacetonitrile [20968-04-1] (DDCN). However, most DDT that enters the environment is sequestered as DDE, which is ubiquitously present in the body Hpids of invertebrate and vertebrate animals. In humans. 

Reductive reactions typically occur in anaerobic environments where there is an abundant supply of electron donors. Electron donors are typically of microbial origin, eg, porphyrins or cysteine, which sometimes leads to confusion regarding the nature, ie, chemical vs enzymatic, of the reductive reaction. By definition, all reductive reactions which are not enzymatically catalyzed are chemical. The most significant chemical reductive reaction is reductive dechlorination. 

Reductive dechlorination of chlorinated aUphatic hydrocarbons, eg, lindane (11) (eq. 17) is extremely facile and occurs almost exclusively via chemical mechanisms, although microorganisms are typically the source of electron donors (30). 

The reductive dechlorination of chlorinated aromatics is more compHcated in that the initial dechlorination of more highly chlorinated compounds may be either chemical or enzymatic, eg, PGP, whereas the dechlorination of less chlorinated compounds or dechlorinated products is typically enzymatic. For example, the first dechlorination of 2,4-dichlorophenol (ortho position) can occur either chemically or enzymatically the second dechlorination (para position) is enzymatic (eq. 10). 

Bowmer and Tonelli [161] have also studied the thermal characteristics of the whole range of ethylene-vinyl chloride copolymers prepared by partial reductive dechlorination of PVC using tri-n-butyltin-hydride. Naqvi [162] has substantiated further his explanations for the thermal stability characteristics of ethylene-vinyl chloride copolymers reported by Braun et al. [159] using the results of Bowmer and Tonelli [161] as a basis. 

The refractory nature of some pollutants, notably, persistent polyhalogenated compounds, has raised problems of bioremediation of contaminated sites (e.g., sediments and dumping sites). There has been interest in the identification, or the production by genetic manipulation, of strains of microorganisms that can metabolically degrade recalcitrant molecules. For example, there are bacterial strains that can reductively dechlorinate PCBs under anaerobic conditions. 

Under anaerobic conditions, p,p -DDT is converted to p,p -DDD by reductive dechlorination, a biotransfonnation that occurs postmortem in vertebrate tissues such as liver and muscle and in certain anaerobic microorganisms (Walker and Jefferies 1978). Reductive dechlorination is carried out by reduced iron porphyrins. It is carried out by cytochrome P450 of vertebrate liver microsomes when supplied with NADPH in the absence of oxygen (Walker 1969 Walker and Jefferies 1978). Reductive dechlorination by hepatic microsomal cytochrome P450 can account for the relatively rapid conversion of p,p -DDT to p,p -DDD in avian liver immediately after death, and mirrors the reductive dechlorination of other organochlorine substrates (e.g., CCI4 and halothane) under anaerobic conditions. It is uncertain to what extent, if at all, the reductive dechlorination of DDT occurs in vivo in vertebrates (Walker 1974). 

Certain anaerobic bacteria can reductively dechlorinate PCBs in sediments (EHC 140). Higher chlorinated PCBs are degraded more rapidly than lower chlorinated ones, which is in contrast to the trend for oxidative metabolism described earlier. Genetically engineered strains of bacteria have been developed to degrade PCBs in bioremediation programs. 

Walker, C.H. (1969). Reductive dechlorination of p,p -DDT by pigeon liver microsomes. Life Science , 111-115. 

Walker, C.H. and Jefferies, D.J. (1978). The post mortem reductive dechlorination of p,p -DDT in avian tissues. Pesticide Biochemistry and Physiology 9, 203-210. 

Ifi-Ditritiodihenzo- -dioxin, prepared by reductive dechlorination of 1,6-dichlorodihenzo-p-dioxin with tritium gas, is chlorinated in chloroform solution, containing catalytic amounts of iodine and ferric chloride, to produce 1,6-ditritio-2,3,7,8-tefrachlorodibenzo-p-dioxin. 

Chlorinated dibenzo ip-dioxins are contaminants of phenol-based pesticides and may enter the environment where they are subject to the action of sunlight. Rate measurements showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is more rapidly photolyzed in methanol than octachlorodi-benzo-p-dioxin. Initially TCDD yields 2,3,7-trichlorodiben-zo-p-dioxin, and subsequent reductive dechlorination is accompanied by ring fission. Pure dibenzo-p-dioxin gave polymeric material and some 2,2 -dihydroxybiphenyl on irradiation. Riboflavin-sensitized photolysis of the potential precursors of dioxins, 2,4-dichlorophenol and 2,4,5-trichloro-phenol, in water gave no detectable dioxins. The products identified were chlorinated phenoxyphenols and dihydroxy-biphenyls. In contrast, aqueous alkaline solutions of purified pentachlorophenol gave traces of octachlorodibenzo-p-dioxin on irradiation. 

Freedman DL, Gossett JM. 1989. Biological reductive dechlorination of tetrachloroethylene and trichloroethylene to ethylene under methanogenic conditions. Appl Environ Microbiol 55 2144-2151. 

Maymo-Gatell X, Chien Y-t, Gosset JM, et al. 1997. Isolation of a bacterium that reductively dechlorinates tetrachloroethene to ethene. Science 276 1568-1571. 

It is well established that important photochemical reactions are mediated by humic material in the aquatic environment (Zepp et al. 1981a,b), and that these are particularly signihcant for hydrophobic contaminants. Partial reductive dechlorination of the persistent insecticide mirex associated with... 

Bosma TNP, FHM Cottaar, MS Posthumus, CJ Teunis, A van Veidhuizen, G Schraa, AJB Zehnder (1994) Comparison of reductive dechlorination of hexachloro-l,3-butadiene in Rhine sediments and model systems with hydroxocobalamin. Environ Sci Technol 28 1124-1128. 

---