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.^[1]

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.^[2] These reactions depend on a molecule which tends to be very aggressively sought after by some microbes, vitamin B12.^[3]

In many instances, microbiological reductive dechlorination of chlorinated organic molecules is important for bioremediation of polluted groundwater. One particularly important example for public health^[4] 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.^[5]

Electrochemical

The electrochemical reduction of chlorinated chemicals such as chlorinated hydrocarbons and chlorofluorocarbons (CFCs) can be carried out by electrolysis in appropriate solvents, such as mixtures of water and alcohol. The cathode transfers electrons to the molecule, which decomposes to produce the corresponding hydrocarbon (hydrogen atoms substitute the original chlorine atoms) and free chloride ions. For instance, the reductive dechlorination of CFCs is complete and produces several HFCs plus chloride.^[6]

Radiation

It is known that by gamma irradiation of PCBs that they can be converted into biphenyl and inorganic chloride, this is formally a reduction of the organic compound as hydrogen is added. See the Polychlorinated biphenyl page for more details of this destruction method. This reductive destruction method works for many organochlorine compounds, for instance carbon tetrachloride when irradiated tends to form chloroform and chloride anions.

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