Chlorobenzene degradation compounds

The structural range of industrially important representatives of these groups is enormous, and includes chlorobenzenes (solvents), polychlorinated biphenyls (PCBs) (hydraulic and insulating fluids), and polybrominated biphenyls and diphenyl ethers (flame retardants). There is widespread concern over both the persistence and the potential toxicity of all these compounds, and sites that have become contaminated during their production represent a threat both to the environment and to human health. Pathways for the aerobic bacterial degradation of chlorobenzenes and chlorobiphe-nyls, and their brominated analogs have been discussed in Chapter 9, Part 1. 

Soil Under aerobic conditions, indigenous microbes in contaminated soil produced pentachlorocyclohexane. However, under methanogenic conditions, a-BHC was converted to chlorobenzene, 3,5-dichlorophenol, and the tentatively identified compound 2,4,5-trichlorophenol (Bachmann et al., 1988). Manonmani et al. (2000) isolated a microbial consortium from sewage and soil that could completely mineralize a-BHC in 14 d at 30 °C. The acclimated consortium could degrade up to 100 mg/L of a-BHC within 72 h at a degradation rate of 58 mg/L-day. 

Degradation products of chlorobenzene in environmental media are difficult to determine. This difficulty is not so much an analytical problem as it is a problem of knowing the fundamental environmental chemistry of these compounds in water, soil, air, and biological systems. 

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]. 

Dioxygenases present in the blocked mutants of Pseudomonas putida, a soil bacterium, degrades benzene and its derivatives into cyclohexa-3,5-diene-l,2-diols. With chlorobenzene, diol 387 is obtained with > 99% ee. This compound is converted in a few chemical steps into... 

Pyrolysis products of chlorinated polyethylene contain molecules similar to those found in polyethylene pyrolysates and, in addition, compounds similar to that obtained from vinyl chloride (significant amount of HCI). Chlorosulfonated polyethylene typically contains only about 1.5% sulfur, but sulfur-containing compounds such as SO2 can be detected among its pyrolysis products. The distribution of chlorine atoms in chlorinated polyethylene has been investigated using Py-GC [55, 56]. The polymer was considered equivalent with a terpoiymer poly[ethylene-co-(vinyl chloride)-co-(1,2-dichloroethylene)]. The level of specific degradation products such as aromatic molecules (benzene + toluene + styrene + naphthalene), chlorobenzene, and dichlorobenzenes correlates well with the carbon/chlorine ratio in the polymer. 

Inherent biodegradabilty also appears to affect the adaptation process. Substrates which are easily metabolized may not exhibit an adaptive response because initial rates of degradation are sufficient. For instance, 2-chlorophenol was readily metabolized in both unexposed and preexposed groundwater microcosms (46). More recalcitrant compounds require an adaptive period. Thus, 2,4,5-trichlorophenol added to microcosms similar to those used for the 2-chlorophenol experiments was degraded rapidly after a 1-day lag in the preexposed microcosm. A 6-day lag was required in the unacclimated microcosm. Other substrates show no adaptive response, such as chlorobenzene or 1,2,4-trichlorobenzene (43). 

Figure 6.6 Reaction sequences for the photochemical degradation of p-chlorophenol. [Reproduced with permission from P. Boule, C. Guyon, A. Tissot, and J. Lemaire, Specific Phototransformation of Xenobiotic Compounds Chlorobenzenes and Halophenols , in R. G. Zika and W. J. Cooper, Eds, Photochemistry of Environmental Aquatic Systems, ACS Symposium Series No. 327, pp. 10-26. Copyright 1985, American Chemical Society.]...

Chlorinated aromatic compounds are mi or environmental pollutants, because they are often released in substantial quantities. They are toxic and resistant to degradation, and accumulate in sediment and biota. Although some compovmds are degraded only slowly b7 soil and aquatic microorganisms, others are metabolized relatively quickly. There have been many studies on the ability of soil and aquatic microorganisms to dissimilate chlorinated aromatic hydrocarbons, such as chlorotoluenes [35,36], chlorobenzenes [37-42], chlorobenzoates [43-50], chlorophenols [51-66], chloroacetamide [67], 4-chlorophenylacetate [33,68], and chlorophenoxyacetates [69-83]. 

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