Halogenated Alkanes and Alkenes

The stability of perchlorofluoroalkanes is due to the absence of hydrogen atoms that may be abstracted by reaction with hydroxyl radicals. Attention has therefore been directed to chlorofluo-roalkanes containing at least one hydrogen atom (Hayman and Derwent 1997). Considerable effort has also been directed to the reactions of chloroalkanes and chloroalkenes, and this deserves a rather more detailed examination in the light of interest in the products that are formed. 

CI3C-CCI2O CI3C-COCI + Cl COCI2 + CCI3 

An overview of the reactions involving trihalomethanes (haloforms) CHXYZ, where X, Y, and Z are halogen atoms, has been given in the context of ozone depletion (Hayman and Derwent 1997). Interest in the formation of trichloroacetaldehyde formed from trichloroethane and tetrachloroethene is heightened by the phytotoxicity of trichloroacetic acid (Frank et al. 1994), and by its occurrence in rainwater that seems to be a major source of this contaminant (Muller et al. 1996). The situation in Japan seems, however, to underscore the possible significance of other sources including chlorinated wastewater (Hashimoto et al. 1998). Whereas there is no doubt about the occurrence of trichloroacetic acid in rainwater (Stidson et al. 2004), its major source is unresolved since questions remain on the rate of hydrolysis of trichloroacetaldehyde (Jordan et al. 1999). 

Low concentrations of trifluoroacetate have been found in lakes in California and Nevada (Wujcik et al. 1998). It is formed by atmospheric reactions from 1,1,1,2-tetrafluoroethane and from the chlorofluorocarbon replacement compound CF3-CH2F (HFC-134a) in an estimated yield of 7-20% (Wallington et al. 1996). CF3OH that is formed from CF3 in the stratosphere is apparently a sink for its oxidation products (Wallington and Schneider 1994). 

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A variety of halogenated alkanes and alkenes such as hexachlorobutadiene, chlorotrifluoro-ethylene, tetrafluoroethylene, and trichloroethylene (Fig. 7.28) are nephrotoxic. Studies have shown that metabolic activation is necessary for toxicity, but this does not involve cytochromes P-450. Thus, hexachlorobutadiene (HCBD) is a potent nephrotoxin in a variety of mammalian species, and the kidney is the major target. 

Nirmalakhandan and Speece [36] introduced the polarizability factor,, as an additional molecular descriptor. They derived the following model for halogenated alkanes and alkenes, alkylbenzenes, halobenzenes, and alkanols ... 

The first term, representing acid-"catalyzed" hydrolysis, is important in reactions of carboxylic acid esters but is relatively unimportant in loss of phosphate triesters and is totally absent for the halogenated alkanes and alkenes. Alkaline hydrolysis, the mechanism indicated by the third term in Equation (2), dominates degradation of pentachloroethane and 1,1,2,2-tetrachloroethane, even at pH 7. Carbon tetrachloride, TCA, 2,2-dichloropropane, and other "gem" haloalkanes hydrolyze only by the neutral mechanism (Fells and Molewyn-Hughes, 1958 Molewyn-Hughes, 1953). Monohaloalkanes show alkaline hydrolysis only in basic solutions as concentrated as 0.01-1.0 molar OH- (Mabey and Mill, 1978). In fact, the terms in Equation(2) can be even more complex both elimination and substitution pathways can operate, leading to different products, and a true unimolecular process can result from initial bond breaking in the reactant molecule. 

Chlorpyrifos can be fit to a QSAR Wolfe (1980) developed. The calculated as well as observed half-life of about 80 days falls in the middle of the set of diethyl-aryl-phospho-rothioates. The reactivity span of these compounds is much lower than that observed for the halogenated alkanes and alkenes, with half-lives generally within the range of several days to several years. 

Fishbein L. 1985. A survey of the analysis of halogenated alkanes and alkenes in biological samples. Chap. 7. In Fishbein L, O Neill IK, eds. Environmental carcinogens selected methods of analysis. Vol. 7-Some volatile halogentated hydrocarbons. Publication No. 68, International Agency for Research on Cancer (IARC), Lyon France, 141-168. 

Degradation of Halogenated Alkanes and Alkenes 6.4.1 Elimination Reactions... 

A variety of halogenated alkanes and alkenes such as hexachlorobutadiene, chlorotrifluoroethylene,... 

Alkynylsilanes—reactive precursors and versatile synthons for various reactions—can be obtained mainly by silylation techniques of metallated alkynes (including Grignard derivatives), or by elimination reactions of halogenated alkanes and alkenes in the presence of a silylating agent. The main classes of alkynylsilanes will be discussed as well as some derivatives which may be of further interest to organic synthesis. 

The disappearance rate constants for a large number of halogenated hydrocarbons, which include halogenated alkanes and alkenes, in anoxic sediment-water systems were correlated with several molecular descriptors (Peijnenburg et al.,... 

Equation 3.51) and experimentally determined rate constants (k p) for the series of halogenated alkanes and alkenes. 

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