Dechlorination mechanism

Van Dyke RA. 1977. Dechlorination mechanisms of chlorinated olefins. Environ Health Perspect 21 121-124. 

As noted earlier there are various photoreductive dechlorination mechanisms. The triplet excited state is believed to undergo homolytic cleavage in the case of prthp-chlorinated PCB, a hypothesis supported by... 

SO far, to contain the previously unknown 176-nor-cobamide norpseudovitamin Bi2 (17) (54). The cofactor role of the corrinoid and its presumed participation in the reduction suggest a radical dechlorination mechanism (see Fig. 11) (55). 

Investigations conducted to elucidate the dechlorination mechanism of 4-chlorophenol have revealed that elimination of HCl via heterolytic C-Cl bond scission from aqueous 4-chlorophenol in the presence of oxygen produces an initial transient carbene, 4-oxocyclohexa-2,5-dienyhdene that has a triplet character and is stable in aqueous solutions. The addition of Oj to this intermediate affords a benzo-quinone-O-oxide, from which loss of an oxygen atom yields the major reaction product p-benzoquinone. The involvement of a carbene intermediate has also been indicated for 2-chlorophenoL ... 

Apparatus Ketene generator (500 ml distillation flask) (see Ref. 20, p. 529) for. the preparation of tetramethylallene 1-1 round-bottomed, three-necked flask, provided with a mechanical stirrer, a thermometer and a vent for the addition of dichlorocarbene 500-ml flask (see Fig. 1) for the 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). 

Reactions with strongly basic nucleophiles such as potassium amide in liquid ammonia may prove much more complex than direct substitution. 2-Chloro-4,6,7-triphenylpteridine reacts under these conditions via an S ANRORC mechanism to form 2-amino-4,6,7-triphenylpteridine and the dechlorinated analogue (78TL2021). The attack of the nucleophile exclusively at C-4 is thereby in good accord with the general observation that the presence of a chloro substituent on a carbon position adjacent to a ring nitrogen activates the position meta to the chlorine atom for amide attack. 

It would be expected that if linkage was by the latter mechanism complete dechlorination would occur, the adjacent chlorine atoms being removed together. In the case of head-tail polymerisation it would be expected that because the reaction does not occur in steps along the chain, but at random, a number of unreacted chlorine atoms would become isolated and thus complete dechlorination could not occur (Figure 12.11). 

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

Considerable attention has been directed to dehalogenation mediated by corrinoids and porphyrins in the presence of a chemical reductant (references in Gantzer and Wackett 1991 Glod et al. 1997 Workman et al. 1997). Illustrations are provided by the dechlorination and elimination reactions carried out by titanium(III) citrate and hydroxocobala-min (Bosma et al. 1988 Glod et al. 1997). The involvement of corrinoids and porphyrins is consistent with the occurrence of analogous mechanisms for biological reactions that... 

Cultures of a number of anaerobic bacteria are able to dechlorinate tetrachloromethane and Acetobacterium woodii formed dichloromethane as the final chlorinated metabolite by successive dechlorination, although CO2 was also produced by an unknown mechanism (Egli et al. 1988). 

Subsequent studies of the physical properties of this series of E-V copolymers obtained via the (n-Bu)3SnH reduction of PVC have revealed that their properties, both in the solid state and in solution, are sensitive to their detailed microstructure (7-10). These observations prompted the present study concerning the mechanisms of the reductive dechlorination of PVC with (n-Bu)3SnH. 

With the (n-Bu)jSnH reduction of PVC successfully simulated via the kinetic studies of DCP and TCH reduction, it remains to explain the mechanisms (14) of this reductive dechlorination. We need only consider the mechanisms of the reduction of DCP and TCH, because we have demonstrated that the kinetics of their reduction are the same as those observed for PVC when also reduced by (n-Bu)3SnH. 

Atom Transfer Atom Transfer (AT) takes place typically in the case of d7 complexes, which abstract the halogen atom from RX. The radical formed combines then with a second metal [193, 194]. A classical example of this mechanism is the hydrodehalogenation with cyanocobaltates(II) (see Section 18.2.1) [8, 9], but an analogous pathway was suggested recently for the Co(II) corrin-catalyzed dechlorination of CC14 in the presence of S2 /cysteine as reductant (Eqs. (11)—(12))... 

What is the mechanism for formation of dechlorinated product 25 Presumably, oxidative... 

Oxidative dehalogenation of aromatic halogens should not occur because there is no hydrogen atom on the carbon involved however, it often does occur. One mechanism likely involves ipso addition as will be discussed later and as proposed for the dechlorination of pentachlorophenol (Fig. 4.65) (131). 

Quensen et al. [69] showed that micro-organisms isolated from Hudson river sediment dechlorinated most polychlorinated biphenyls in Aroclor 1242 under anaerobic conditions in the laboratory. The higher the polychlorobiphenyl concentration, the more rapid the rate of dechlorination. The possible mechanisms involved are discussed. The products of dechlorination were less toxic than the original compounds and were more readily degraded by aerobic bacteria wastewater containing... 

Mirex is a very persistent compound in the environment and is highly resistant to both chemical and biological degradation. The primary process for the degradation of mirex is photolysis in water or on soil surfaces photomirex is the major transformation product of photolysis. In soil or sediments, anaerobic biodegradation is also a major removal mechanism whereby mirex is slowly dechlorinated to the 10-monohydro derivative. Aerobic biodegradation on soil is a very slow and minor degradation process. Twelve years after the application of mirex to soil, 50% of the mirex and mirex-related compounds remained on the soil. Between 65--73% of the residues recovered were mirex and 3-6% were chlordecone, a transformation product (Carlson et al. 1976). 

Under anaerobic conditions, mirex was slowly dechlorinated to the 10-monohydro derivative by incubation with sewage sludge bacteria for two months (Andrade and Wheeler 1974 Andrade et al. 1975 Williams 1977). The primary removal mechanism for mirex was anaerobic degradation as demonstrated by the 6-month stability of the compound in nine aerobic soils and lake sediments (Jones and Hodges 1974). 

Fig. 11.5. Mechanism of hydrolytic dechlorination of mechlorethamine (11.31, R = Me) and other antitumor nitrogen mustards [65] [66]. Also shown is the mechanism of the nonenzymat-ic conjugation with reduced glutathione (GSH, L-glutamyl-L-cysteinylglycine).

The relative ratio of regioisomers of PCDD/F and other chlorinated compounds formed in incinerators is called the incineration pattern. The pattern can be derived from statistical analysis of a large number of measurements of the same plants, and can be used for elucidation of thermal formation mechanisms in plants. In principle regioisomers can be formed either by stereospecific chlorination or dechlorination processes. The pattern has also been used as a part for explaining of the formation mechanism of PCDD/F and other chlorinated compounds formed in incinerations (see Figure 8.4). 

Photochemical decomposition can also be carried out in the presence of a suspension of photoactive material such as Ti02 where substrate absorption onto the uv activated surface can initiate chemical reactions e. g. the oxidation of sulphides to sul-phones and sulphoxides [37]. This technology has been adapted to the destruction of polychlorobiphenyls (PCB s) in wastewater and is of considerable interest in environmental protection. Using pentachlorophenol as a model substrate in the presence of 0.2 % TiOj uv irradiation is relatively efficient in dechlorination (Tab. 4.5) [38]. When ultrasound is used in conjunction with photolysis, dechlorination is dramatically improved. This improvement is the result of three mechanical effects of sonochemistry namely surface cleaning, particle size reduction and increased mass transport to the powder surface. 

Hantzsch 1,4-dihydropyridines were oxidized quantitatively to give the corresponding pyridine derivatives by irradiation in CCI4. A photo-induced electron-transfer mechanism is involved. The critical step in this mechanism is the fast dechlorination of CCI4 (Scheme A)P... 

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