Halogenated hydrocarbons elimination

Halon - As employed in the fire protection industry, a gaseous fire suppression agent. Halon is an acronym for halogenated hydrocarbons, commonly bromotrifluoromethane (Halon 1301) and bromochlorodifluoro-methane (Halon 1211). Considered obsolete for fire protection purposes due to a possible environmental impact to the Earth s atmospheric ozone layer and beginning to be phased out or eliminated. 

Molecular reactions are generally more difficult to treat because of the complexity of the possible transition states. The most widely studied complex molecular reaction class is HX elimination from halogenated hydrocarbons. These reactions proceed primarily via the formation of polar, four-centered tight transition states, and examples include... 

Most of the toxic and environmentally unwanted chemicals a decade ago have been eliminated from the common workplace. This includes halogenated hydrocarbons, such as... 

G. Atomic versus Molecular Elimination in Halogenated Hydrocarbons... 

The apparent rate of hydrolysis and the relative abundance of reaction products is often a function of pH because alternative reaction pathways are preferred at different pH. In the case of halogenated hydrocarbons, base-catalyzed hydrolysis will result in elimination reactions while neutral hydrolysis will take place via nucleophilic displacement reactions. An example of the pH dependence of hydrolysis is illustrated by the base-catalyzed hydrolysis of the structurally similar insecticides DDT and methoxy-chlor. Under a common range of natural pH (5 to 8) the hydrolysis rate of methoxychlor is invariant while the hydrolysis of DDT is about 15-fold faster at pH 8 compared to pH 5. Only at higher pH (>8) does the hydrolysis rate of methoxychlor increase. In addition the major product of DDT hydrolysis throughout this pH range is the same (DDE), while the methoxychlor hydrolysis product shifts from the alcohol at pH 5-8 (nucleophilic substitution) to the dehydrochlorinated DMDE at pH > 8 (elimination). This illustrates the necessity to conduct detailed mechanistic experiments as a function of pH for hydrolytic reactions. 

For lindane, structural considerations suggest that stepwise 1,2-HCl elimination should occur with relative ease, and that the final product should be 1,3,5-trichlorobenzene. Roberts et al. (1993) included lindane in a QSAR and found close agreement between the predicted and experimentally reported rate constant, with an environmental half-life of about 6 years. The QSAR is useful for compounds that can be analyzed by that approach, especially because the range of reactivity is so vast for the halogenated hydrocarbons. However, the limitations of QSARs are also evident, in that various mechanisms can operate, and a QSAR is developed only for one mechanistic pathway. 

There has been considerable interest in the elimination of hydrogen halides from halogenated hydrocarbon molecules and radicals which have been vibration-ally excited by chemical activation, by photochemical methods or by shock-tube techniques. Studies on fluorinated species have been reported by Trotman-Dickenson et a/.7 3.839-84i,846.908,909 by Pritchard et fl/.747,748.905.919.920 ... 

I, 1,1 -trichloroethane and other halogenated hydrocarbons are generally supportive and rely on the body s ability to eliminate rapidly 1,1,1-trichloroethane and its metabolites. Animal studies indicate that intravenous injection or infusion of calcium gluconate or phenylephrine are protective against acute blood pressure reduction caused by exposure to 1,1,1-trichloroethane (Herd et al. 1974). Further animal testing is needed to assess whether these compounds might be used to resuscitate individuals exposed to high concentrations of 1,1,1-trichloroethane. 

When both C02and a substituent such as halogen are eliminated, no dehydrogenation is required to produce the catechol. This is also the case in the degradation of 3,4-dichlorobenzoate where spontaneous loss of HCI from the initially produced 4,5-rfs-diol formed 3-chloro-4,5-dihydroxybenzoate without intervention of a dehydrogenase that is necessary for 3-chlorobenzoate (Nakatsu et al. 1997). These examples stand in contrast to the situation for aromatic hydrocarbons in which a dehydrogenase is necessary. 

Oxidative dehydrohalogenation is a common metabolic pathway for many halogenated hydrocarbons (25,29,30,31). The CYP450-catalyzed oxidation generates the transient gem-halohydrin (analogous to alkane hydroxylation) that can eliminate the hydrohalic acid to form carbonyl derivatives (aldehydes, ketones, acyl halides, and carbonyl halides) (Fig. 10.7). This reaction... 

Yet another advantage of paraffin and silicone oils is their (f) low toxicity, which is now - in the era of ecologically oriented sciences - almost a principal requirement that has practically eliminated the third group of classical pasting liquids - (iii) halogenated hydrocarbons-, by the way, the very first CP-binders in the pioneering years of the field [1,7). 

In reactions with organic molecules e q reacts as nucleophilic reagent it attacks molecules with low-lying molecular orbital, like aromatic hydrocarbons, conjugated olefinic molecules, carboxyl compounds, and halogenated hydrocarbons (Swallow 1982 Buxton 1982, 1987 Buxton et al. 1988). In the latter case, addition is usually followed by halide ion elimination, so the reaction can be considered as a dissociative electron capture. For instance, the reaction with chlorobenzene yields phenyl radical and chloride ion... 

Elimination of halogen by sodium (Wurtz s reaction) gives a higher hydrocarbon. 

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