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Reductive dechlorination
Reductive dechlorination is degradation of chlorinated organic compounds by chemical reduction with release of inorganic chloride ions.
== Biological ==
In a biological context chlorine behaves similarly to other atoms in the halogen chemical series, and thus reductive dechlorination can be considered to fall within a somewhat broader class of biological reactions known as reductive dehalogenation reactions, in which the removal of a halogen substituent from an organic molecule occurs with a simultaneous addition of electrons to the molecule. This can be further subdivided into two types of reaction processes, the first of which, hydrogenolysis, is the replacement of the halogen atom with a hydrogen atom. The second, vicinal reduction (sometimes called, dihaloelimination), involves the removal of two halogen atoms that are adjacent on the same alkane or alkene molecule, leading to the formation of an additional carbon-carbon bond.〔Mohn and Tiedje. Microbial reductive dehalogenation. Microbiol Rev (1992) vol. 56 (3) pp. 482-507 PMID 1406492〕
Biological reductive dechlorination is often catalyzed by certain species of bacteria. Sometimes the bacterial species are highly specialized for organochlorine respiration and even a particular electron donor, as in the case of ''Dehalococcoides'' and ''Dehalobacter''. In other examples, such as ''Anaeromyxobacter'', bacteria have been isolated that are capable of using a variety of electron donors and acceptors, with a subset of possible electron acceptors being organochlorines.〔Smidt and de Vos. Anaerobic microbial dehalogenation. Annu Rev Microbiol (2004) vol. 58 pp. 43-73 PMID 15487929〕
In many instances, microbiological reductive dechlorination of chlorinated organic molecules is important for bioremediation of polluted groundwater. One particularly important example for public health〔Kielhorn et al. Vinyl chloride: still a cause for concern. Environ Health Perspect (2000) vol. 108 (7) pp. 579-88 PMID 10905993〕 is the organochloride respiration of the dry-cleaning solvent, tetrachloroethylene (PCE), and the engine degreasing solvent trichloroethylene (TCE) by naturally occurring anaerobic bacteria, often members of the candidate genera ''Dehalococcoides''. Bioremediation of these chloroethenes can occur when other microorganisms at the contaminated site provide H2 as a natural byproduct of various fermentation reactions. The dechlorinating bacteria use this H2 as their electron donor, ultimately replacing chlorine atoms in the chloroethenes with hydrogen atoms via hydrogenolytic reductive dechlorination. If the soil and groundwater contain enough organic electron donor and the appropriate strains of ''Dehalococcoides'', this process can proceed until all of the chlorine atoms are removed, and TCE is dechlorinated completely via dichloroethene (DCE) and vinyl chloride (VC) to ethene, a harmless end-product.〔McCarty. Breathing with chlorinated solvents. Science (1997) vol. 276 (5318) pp. 1521-2 PMID 9190688〕
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