Hydrogen atom abstraction susceptibility

In reactions with organic substrates, the 0 anion-radical is usually bad at electron transfer, but good at hydrogen atom abstraction. Because of the O spherical symmetry, its reactions are vulnerable by steric factors. Because of the O charged nature, its reactions with organic ions are of course susceptible to the effects of coulombic interaction also. Armstrong et al. (2004) showed that the predominant reactions with the anions of alanine and methylalanine proceed as follows ... 

The polar properties of the alpha-C-H bonds and their reactivity towards hydrogen atom abstraction in acetals are enhanced by the presence of two ether groups. These hydrogens are susceptible to abstraction by excited carbonyl compounds (77). Cleavage of C—0 bonds in acetals occurs readily when an acetal radical is formed (38) even at room temperature. It has been shown that irradiation of cyclic acetals of aldehydes at room temperature in the presence of acetone leads to the appropriate carboxylic esters (22),... 

The efficiency of hydrogen atom abstraction is often influenced by several factors involving thermodynamic, polar, stereoelectronic and steric effects. For instance, C-H bonds a to oxygen are not always susceptible to abstraction when nucleophilic radical initiators are used. P. Kaushal, P. L. H. Mok,... 

Polymers vary greatly in their susceptibility to autooxidation. Polystyrenes, because they have benzylic carbon-hydrogen bonds that afford highly stabilized free radicals on hydrogen atom abstraction, are among the most readily oxidized polymers under environmental conditions. As would be expected on these grounds, polymers that contain alkene groups, such as rubber (1), are readily autooxidized. The... 

Data from kinetic studies on hydrogen-atom abstraction by trifluoromethyl radicals ( nerated by photolysis of hexafluoroacetone) from the esters RpCOaMe (Rf = CFs, C2F5, or n-CaF ) have been presented and discussed. The susceptibilities of the methoxy-groups towards hydrogen abstraction are less by a factor of 2—3 than those found with methyl formate and acetate, which contrasts with the claim (based on a study of the esters FCOaMe and CFa COaMe) that fluorine substitution enhances the ease of abstraction. ... 

The hydrogen atoms of the -CH2- group located between the two double bonds of linoleic ester (Lin-H) are especially susceptible to abstraction by radicals. 

Atom abstraction from the cr-carbon atom in spin adducts, which leads to nitrones, and thence in some instances to spin adducts of these nitrones (e.g. Scheme 9), is undoubtedly a general phenomenon. For this sequence to be observed it is necessary that some, or all, of the following conditions are fulfilled. (0 The nitrone is more reactive towards radical addition than is the original spin trap and/or the latter is substantially consumed in the reaction. (h) The initial spin adduct is particularly susceptible to hydrogen-atom loss (and probably reacts this way by disproportionation). ( 7) The secondary spin adduct is particularly persistent. 

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. 

Figure 2.9 Abstraction of a hydrogen atom by an alkoxy radical from a susceptible molecule.

---