Alkoxy radicals isomerization, hydrogen abstraction

The alkyl radical initially formed reacts readily with oxygen to give the corresponding alkylperoxy radical, which may abstract hydrogen from a fuel molecule to form the alkylhydroperoxide or alternatively decompose to yield an aldehyde and an alkoxy radical. Some workers thought that this decomposition was preceded by an isomerization of the alkylperoxy radical, the activation energy of which had been estimated by Semenov [3] to be ca. 20 kcal. mole. Shtern was of the opinion that the major, if not the only, fate of the alkylperoxy radical was decomposition, but in contrast to other workers he believed that it must involve scission of a C—C bond and could not lead to the formation of a carbonyl compound and hydroxyl radical. 

The right-hand side of Table 6-13 shows relative rates for alkoxy radical reactions in the atmosphere for boundary layer conditions. Comparison of the rates makes it immediately clear that reactions with N02 (or NO) are of little importance. For the smaller alkoxy radicals the reaction with oxygen is preponderant, whereas for alkoxy radicals largerthan butoxy, decomposition and isomerization reactions become competitive. Tertiary butoxy radicals have no abstractable hydrogen atom and thus cannot react with oxygen. In this case, decomposition is dominant. 

Alkoxy (RO) radicals are formed in the reaction of alkyl peroxy (R02) radicals with NO. Subsequent reactions of alkoxy radicals determine to a large extent the products resulting from the atmospheric oxidation of VOCs (Orlando et al. 2003). Alkoxy radicals react under tropospheric conditions via a variety of processes unimolecular decomposition, unimolecular isomerization, or reaction with 02. Alkoxy radicals with fewer than five carbon atoms are too short to undergo isomerization for these the competitive processes are unimolecular decomposition versus reaction with 02. The general alkoxy radical-02 reaction involves abstraction of a hydrogen atom by 02 to produce an H02 radical and a carbonyl species ... 

In the above Reaction Scheme 7.1, pathway (m) corresponds this process, and the formed n-butanol radicals produce hydroxyl butanal and 1 -hydroxy butyl nitrate following the similar process as butyl radicals mentioned above (pathways (r) and (p), respectively). The rate constant for the isomerization reaction of alkoxy to alcohol radicals is in general 10 s while the rate constants for the production of aldehydes or ketones by hydrogen abstraction by O2,... 

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