Dechlorination aromatic compounds

Tetrahalobenzynes, however, react with a variety of aromatic compounds to afford tetrahalobenzobarrelene derivatives in good yields, frequently in the range of 55 to 75%. The dehalogenation of a variety of alkenyl chlorides with alkali metals in tetrahydrofu-ran containing tert-butyl alcohol suggested this approach to the dechlorination of tetrachlorobenzobarrelenes. 

DeWeerd KA, DL Bedard (1999) Use of balogenated benzoates and other halogenated aromatic compounds to stimnlate the microbial dechlorination of PCBs. Environ Sci Technol 33 2057-2063. 

The radical anion can participate in electron transfer reactions, in which the electron is transferred to a compound with suitable electron affinity. The parent aromatic compound is regenerated. Such a reaction is shown in Eq. (83), where chlorobenzene is dechlorinated by electron transfer from the benzene radical anion ... 

Aromatic compounds are dechlorinated by the general mechanism shown in Sch. 1. Electron transfer to a ir-antibonding orbital forms an aromatic radical anion, which then ejects Cl" to give an aromatic radical. This radical picks up a second electron to give a very basic cx-anion, which abstracts a proton either from NH3 or from a more acidic source like water, when water is present. If water is not present, then an NH2 anion can be formed. The presence of ME can lead to the formation of aminated products via the benzyne mechanism. Aminated products were formed in dry NH3 but not when water was present [24], A further reduction via radical anion formation and proton abstraction can give dihydroaromatics or tetrahydroaromatics, or dimerization may occur. In soils, both water and... 

Hagenmaier H, Brunner H, Haag R, Kraft M (1987b), Environ. Sci. Technol. 21 1085-1088. Copper-catalyzed dechlorination/hydrogenation of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, and chlorinated aromatic compounds"... 

Lahaniatis et al. found that a thermal degradation of toxaphene at 400°C to 800°C leads to the formation of aromatic compounds, among them tri- to hexa-chlorobenzenes, low-chlorinated naphthalenes, biphenyls, and even dibenzofu-rans [176]. The major processes of degradation seem to be dechlorination and dehydrochlorination [54]. However, Chandurkar and Matsumura reported that at least some hydroxylated toxaphene metabolites are formed [81]. So far, only very few oxidation products of toxaphene have been identified [177,178,179] which seem to be rather unstable [177]. 

Although considerable effort has been directed to the dechlorination of aromatic compounds, fewer studies have examined debromination. Some examples will be given to illustrate the probably widespread distribution of this reaction ... 

Because compounds like PCB are mineralized by P. chrysosporium, it is reasonable to suggest that these otherwise unsubstituted chlorinated aromatic compounds might undergo reductive dechlorination (substitution of Cl by H). There is, however, no currently... 

Thus, various chlorinated aliphatic and aromatic compounds were dechlorinated in a flow-through electrochemical cell with a graphite fibre cathode, a Nafion (cation-permeable) membrane and a Pt gauze anode. The concentration of pentachlorophenol decreased from 50 to about 1 mg per litre after 20 min of electrolysis at a current efficiency of about 1 %, and the product was phenol. Similar results were obtained with other chlorode-rivatives. The expected total costs of the process are of the order of 10 DM per 1 m of waste water, which is comparable with the cost of adsorption on active carbon [42]. 

Reductions Microbial and mammalian nitroreductase reduces nitro compounds to amines. Chlorinated alkanes and alkenes are common contaminants in ground water and chlorinated aromatics, PCBs, organochlorine pesticides, are often detected in soils and sediments and it has been of interest to evaluate the potential for these compounds to be metabolized. A number of microorganisms are able to dechlorinate both halogenated aliphatic and aromatic compounds in a reduction reaction." It has been observed that the more highly chlorinated congeners are more reactive in these systems in contrast to the response in oxidative dechlorinations. 

Chlorinated benzoic acids have been shown to be intermediates in the biodegradation of several chlorinated aromatic compounds, for example, 4-chlorobenzoic acid is formed in the biodegradation of both polychlorinated biphenyls [103]. Mixed microbial cultures, which have been studied by a number of groups under aerobic conditions, can degrade a wide range of chlorinated benzoic acids, including 2-, 3- and 4-chlorobenzoic acid as well as 3,4-dichloro- and 2,4-dichlorobenzoic acid [104]. By contrast, under anaerobic conditions, only reductive dechlorination of meta-substituted benzoic acids has been observed [49,105,106]. Cometabolism of chlorobenzoic acids in the presence of unsubstituted benzoic adds leads to the formation of the corresponding... 

As with the ethenes, chlorinated ethanes will also undergo halorespiration. Dechlorination of 1,1,1 -TCA has been described by Vogel and McCarty and Cox et al. but understanding this pathway is complicated by the rapid hydrolysis reactions (e.g., half-life is 0.5-2.5 yrs) that can affect TCA. " Finally, halorespiration has been observed with highly chlorinated benzenes such as hexachlorobenzene, pentachlorobenzene, tetrachlorobenzene, and trichlorobenzene. " As discussed by Suflita and Townsend, halorespiration of aromatic compounds has been observed in a variety of anaerobic habitats, including aquifer materials, marine and freshwater sediments, sewage sludges, and soil samples. However, isolation of specific microbes capable of these reactions has been difficult. 

Many substituted aromatic compounds are also degraded under anaerobic conditions, but the mechanisms are less well understood. There is some evidence that dechlorination can take place also that chlorinated nitro compounds are converted to amino derivatives which will then follow the above degradation pathways [16]. 

Reductive DechIorina.tion. Such reduction of chlorinated aUphatic hydrocarbons, eg, lindane, has been known since the 1960s. More recentiy, the dechlorination of aromatic pesticides, eg, 2,4,5-T, or pesticide products, eg, chlorophenols, has also been documented (eq. 10) (20). These reactions are of particular interest because chlorinated compounds are generally persistent under aerobic conditions. 

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). 

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