The Abstraction of Hydrogen and Halogen Atoms

The Abstraction of Hydrogen and Halogen Atoms 5.2.1 The Effect of the Structure of the Radical... 

Bottoni also reported the results of computational studies into the abstraction of hydrogen atom by silyl and trichlorosilyl radicals from chloromethane, dichloromethane and chloroform (equation 15)42, and concluded that hydrogen abstraction does not effectively compete with halogen abstraction in the systems investigated. B3LYP/6-31G calculated energy barriers for these reactions fall in the approximate range of 46-72 kJmol-1. 

By the effect of ionization irradiation side substituents are abstracted from excited macromolecules in parallel to the scission of main chains. Elimination of side substitutions such as hydrogen and halogen atoms may be the main primary process. For instance, in polyethylene, although the C —H bond is stronger than the C—C bond, excited macromolecules eliminate hydrogen atoms preferentially through a C —C bond cleavage of a macromolecular backbone. Part of the... 

When an N-halogenated amine contains two 6 carbons, the abstraction of hydrogen atom preferably takes place from secondary and tertiary 8-carbons. For example, N-chloro N-butyl N-pentyl amine 25 gives only l-n-butyl-2-methylpyrrolidine 26. 1-Amylpyrrolidine 27 is not detected. 

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. 

Many of HCFC or HFC alternatives to CFCs are two-carbon compounds, and are therefore haloethanes of general formula CX3CX2H, where each X may be any of H, F or Cl. These compounds should be susceptible to tropospheric degradation by hydroxyl-mediated abstraction of the hydrogen atom which each one contains, and should not reach the stratosphere. This strategy is effective if the tropospheric lifetime of the HFC or HCFC is short relative to its rate of transport into the stratosphere. Obviously it is important to consider whether tropospheric oxidation of these compounds may lead to the production of any other halogenated species which might themselves be transported into the stratosphere. 

To explain the difference between chlorination and bromination, we return to the Hammond postulate (Section 7.15) to estimate the relative energy of the transition states of the rate-determining steps of these reactions. The rate-determining step is the abstraction of a hydrogen atom by the halogen radical, so we must compare these steps for bromination and chlorination. Keep in mind ... 

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