Haloaliphatic compounds

Fluoroaliphatic compounds have been found to be non-reactive towards e q, as can be seen from the behaviour of fluoro- and trifluoro-acetate ions (Anbar and Hart, 1965a). The same conclusion may be inferred from the behaviour of trifluoroacetone and methyl trifluoro-acetate (Hart et al., 1967), the reactivity of which has been shown to be fully accounted for by that of the carbonylic functional group. 

The specific rates of chloro-aliphatic derivatives range from 4 x 108 m-1 see-1 for compounds carrying a deactivated chlorine atom, e.g. chloro-ethanol, to the diffusion-controlled CF3C1, an activated chloroderivative (Anbar and Neta, 1967a). Polychloro-derivatives, e.g. CHC1S or CC14, react at diffusion-controlled rates (k = 3x 1010 m-1 sec-1) (Hart et al., 1964a). This is expected in view of the inductive effect of adjacent chlorine atoms in addition to the statistical factor in polychloro-derivatives. 

Bromo-aliphatic compounds behave similarly to their chloro-analogues, and their specific rates range from 1-6 x 109 m-1 sec-1 for deactivated bromoethanol to diffusion-controlled rates. Iodo-derivatives react with e q at diffusion-controlled rates (Anbar and Neta, 1967a). 

Chloroaliphatic compounds have been shown to undergo quantitative dechlorination on reaction with e-q (Hayon and Allen, 1961 Stockdale and Sangster, 1966). Bromo- and iodo-derivatives follow a similar pattern (Anbar, 1965 Anbar and Neta, 1967c). This dehalogenation is not a result of a dissociative electron capture 

Evidence that the cleavage of the RX bond does not take place in the rate-determiningstep could be derived from the absence of a solvent effect on the rate of e +EX reactions (Anbar and Hart, 1965a) and from the fact that the activation energy of the (e q + RX) reaction is the same for compounds with quite different R—Cl bond strengths (Anbar and Hart, 1967). The decomposition of RX-, which is probably a very fast reaction, is induced by the solvation of X- and may be regarded as a typical SN1 process. 

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The functionality of either the haloaliphatic compound or the alkene can some times be maintained in this type of radical addition [15 16] (equations 14 and 15)... 

The reactivity of haloaliphatic compounds decreases in ascending order. This seems to be related to polarizability. RX ion has been postulated as an intermediate, but it has not been experimentally observed. Again, the reactivity is increased by adjacent electron-accepting groups. 

Damborsky, J., K. Manova, and M. Kuty, A mechanistic approach to deriving quantitative structure biodegradability relationships. A case study Dehalogenation of haloaliphatic compounds . In Biodegradability Prediction, W. J. G. M. Peijnenburg and J. Damborsky, Eds., Kluwer Academic, Dordrecht, 1996. 

Selectivity versus Reactivity. The "reactivity-selectivity principle" (RSP) states that "in a set of similar reactions, the less reactive the reagent, the more selective it is in its attack" (451. This principle is widely held, but its generality has recently been questioned (28.46-481. The reactivity data assembled in Tables IV through VIII appear to provide an opportunity to examine the validity of the RSP, using log(kA/ka)sN as a measure of the selectivity of a haloaliphatic compound between nucleophiles A and B. 

A haloaliphatic compound whose rate of reaction with a sulfur nucleophile (Nu) exceeds its rate of reaction with H2O is one for which the following inequality is satisfied ... 

Several alkyl radicals, including (CH3)260H, CH3.CHOH, CH2OH, CH3.6H.CO.OH and 6H2. CO. OH, were found to transfer an electron to haloaliphatic compounds (Anbar and Neta, 1967b). 

The photocatalytic destruction of haloaliphatic compounds has been studied in detail by Hoffmann and co-workers [107,108]. A detailed study of the mechanism of the photocatalytic destruction of chloroform using Ti(>2 was investigated by Kormann et al. [107]. The process was believed to be initiated by hydrogen abstraction by the hydroxyl radicals generated on the surface of the TiC>2 (Eqs. 27-32). The chloroform radical subsequently reacts with oxygen forming trichloromethanol oxidation to trichloroformaldehyde and ultimately complete mineralisation. 

The reaction of the hydrated electron with monochloracetic acid to give chloride ion has already been discussed (see p. 430). This is only one of a general type of reaction involving haloaliphatic compounds [55, 56]. The reactions occurring are... 

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