Hydrogen Abstraction by Halogens

Hydrogen abstraction by halogen atoms such as fluorine and chlorine... 

The initial OH chemistry of HCFC-123 (CHC12CF3) produces a fully halogenated alkoxy radical where further hydrogen abstraction by 02 is not an option ... 

Tables 9.6-9.8 illustrate some of the trends observed, and Table 9.9 shows activation parameters for abstraction by halogen atoms of different types of hydrogen. Note that in alkanes, hydrogens become more easily abstracted on...

Such normal free-radical substitution processes might be geometrically directed to otherwise unremarkable positions in substrates, as the benzophenone reactions had been. Consequently, in our first example, we found that intramolecular chlorination could be directed by attachment of a PhICl2 group to the steroid [156], i.e., compound 82 directed chlorination exclusively to C-14 (83), while 84 directed halogenation exclusively to C-9 (85). As shown in Scheme 1.1, an intramolecular hydrogen abstraction by... 

The crosslinking reaction mechanisms for substituted polystyrenes were reported and it was shown experimentally that the abstraction by halogen and hydrogen radicals exists in a crosslinking reaction 12 ). However, the individual monomer concentration dependence of the ratio for the abstraction reaction has not been clear yet. In calculation, Iim(xm) and hisj(x(sj) were approximated to be independent of the individual monomer concentration. 

Volume 1 Chapter 6.2, Volume 2). Because of space limitation, hydrogen atom abstractions by halogen atoms (radical halogenation) will not be presented in this review with the exception of hydrogen abstraction by complexed chlorine atoms. 

A very important tactic for a-amino radical generation involves translocation via hydrogen abstraction by an aromatic free radical generated from a halogen five bonds away. Thus chain extension at the a-position of an amide or amine can be accomplished with great success. 

In alkaline solution, the anion of acetone will be produced by proton transfer from the radical of 2-propanol to the hydroxide ion (R3). Not only electron attachment of the solvated electron (R7), but also electron transfer from the anion of acetone (R4) will be a significant process to form the anion of the halogenated carbons. The radical of 2-propanol will be reproduced due to hydrogen abstraction by radicals formed by the dissociation of an anion of the halogenated carbons (R6). This is a chain reaction and causes effective degradation. On the contrary, dissociative electron attachment is the only process to decompose halogenated hydrocarbons in the pure alcohol solution. 

Radical mechanisms are characterized by six different kinds of steps (1) homolytic cleavage, (2) addition to a it bond, (3) hydrogen abstraction, (4) halogen abstraction, (5) elimination, and (6) coupling. 

Hydrogen Abstraction by Heteroatom-centred (Non-halogen-centred) Radicals. 140... 

In a chain reaction, the step that determines what the product will be is most often an abstraction step. What is abstracted by a free radical is almost never a tetra- or tervalent atom (except in strained systems, see p. 989) and seldom a divalent one. Nearly always it is univalent, and so, for organic compounds, it is hydrogen or halogen. For example, a reaction between a chlorine atom and ethane gives an ethyl radical, not a hydrogen atom ... 

Some radicals (e.g., tert-butyl, benzyl, and cyclopropyl), are nucleophilic (they tend to abstract electron-poor hydrogen atoms). The phenyl radical appears to have a very small degree of nucleophilic character. " For longer chains, the field effect continues, and the P position is also deactivated to attack by halogen, though much less so than the a position. We have already mentioned (p. 896) that abstraction of an a hydrogen atom from ring-substituted toluenes can be correlated by the Hammett equation. 

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. 

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. 

A. Cyclizations of halides terminated by hydrogen atom abstraction or halogen atom transfer 1a... 

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