Haloalkanes, Halogenation, and Radical Reactions

A generic scheme for the atmospheric oxidation of a C2 haloalkane is given in Fig. 6. Values in parentheses are order of magnitude lifetime estimates. Reaction with OH radicals gives a halogenated alkyl radical which reacts with O2 to give the corresponding peroxy radical (RO2). As discussed in previous sections, peroxy radicals can react with three important trace species in the atmosphere NO, NO2, and HO2 radicals. 

As assumed, the small and positive valne of H/D kinetic isotope effect may be used as a criterion for an electron-transfer pathway. For example, anion-radicals of a-benzoyl-co-haloalkanes can react in two routes (Kimura and Takamnkn 1994). The first ronte is the common one—an electron is transferred from the oxygen anion of the carbonyl gronp to a terminal halogen. The transfer provokes fission of the carbon-halogen bond. The second ronte is the S 2 reaction, leading to a cyclic product as shown in Scheme 2.37. 

Recent studies on simple haloalkanes, especially iodoalkanes. suggest that the initial process is always homolytic carbon-halogen bond cleavage, but that iodo-systerns are especially susceptible to subsequent electron transfer from the alky) radical to the fairly unreactive iodine atom. This gives the alkyl carbonium ion that reacts with, for example, a nucleophilic solvent. The operation of this mechanism provides for the generation and reaction of vinyl carbonium ions from vinyl iodides fS.66), and this offers one of the lew ways of generating such intermediates. 

In this chapter, we begin with the structure and physical properties of haloalkanes. We then study radical halogenation of alkanes as a vehicle to introduce an important type of reaction mechanism, namely the mechanism of radical chain reactions. Reactions of oxygen with alkenes and a radical mechanism for HBr addition to alkenes complete the chapter. 

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