Hydrogen abstraction reactions, halogenation

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

The hydrogen abstraction reactions from trifluoromethanol by halogen atoms... 

The N-haloimide halogenations are also controlled partially by the fact that Cl or the iV-succinimidyl radical are much more reactive than Br in hydrogen abstracting reactions and, towards a hydrocarbon of low reactivity such as neopentane, a bromine atom chain would be quite ineffective. With mixtures of NBS and CI2, halogenation occurs to form the alkyl bromides but with the selectivity expected for chlorine atom attack Apparently ClBr is formed and reacts with the alkyl radical to form RBr and a chlorine atom. A similar situation exists for the bromination of alkanes using a mixture... 

Calculate the exothermicity or endothermicity of the following hydrogen abstraction reactions involved in free radical halogenation of alkanes (the BDEsof HF, HQ, HBr, and HI are 136,103,87, and 71 kcal/mol, respectively). Which reaction would you predict is the least selective ... 

One of the older preparative free-radical reactions is the addition of polyhalomethanes to alkenes. Examples of addition of carbon tetrabromide, carbon tetrachloride, and bromoform have been recorded. The reactions are chain processes that depend on facile abstraction of halogen or hydrogen from the halomethane ... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

It is also worth emphasizing that the initiation and termination steps are not included in the central chain process. For instance, in metal hydride-promoted domino reactions the initial halogen abstraction (or SePh displacement, etc.) and the final hydrogen abstraction from R MH are not classified as part of the domino sequence. More precisely, only the propagation steps within the mechanism of this process will be considered as a strict integral part of the domino reaction. 

Because many of the alternates and replacements for CFCs have an abstractable hydrogen atom, reaction with OH in the troposphere dominates their loss. Table 13.4 gives some rate constants for the reaction of OH with these compounds the kinetics summary of De-More et al. (1997) should be consulted for other compounds. It is seen that the rate constants at 298 K are typically in the range of 10-l3-10-ls cm3 molecule-1 s-1, depending on the degree of halogen substitution and the nature of the halogen, e.g., F, Cl, or Br. Typical A factors are of the order of 1 X 10 12 cm3 molecule-1 s-1 per H atom (DeMore, 1996). 

Wohl in 1919 reported that A -bromoacetamide (CH CONHBr) induced allylic bromination. " Then iV-bromosuccinimide (30) was described in 1942 by Ziegler and co-workers to be useful in such free radical bromination reactions (equation 41), " and this widely utilized procedure is known as the Wohl-Ziegler reaction. In 1963 the mechanism of the reaction was proposed to involve halogen atoms in the hydrogen abstraction step " " " instead of succinimidyl radicals as had been commonly supposed. The halogen atom mechanism had previously been proposed by Gosselain et al. for reactions of yV-chlorosuccinimide. " ... 

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