Oxidation Reaction Mechanism of Alkanes by OH

The aUcyl radicals formed by the abstraction reaction react exclusively with O2 in the atmosphere to form alkylperoxy radicals (pathways (c) (h)), and most of them are then transformed to alkoxy radicals by oxidizing NO to NO2 in the presence of NOx (pathways (d) and (i)). However, some alkylperoxy radicals react with NO by the recombination isomerization reactions (e) and (j), to form alkyl nitrate (see Sect. 5.3.3). Although the yields of 2-butyl and 1-butyl nitrate are not large in the case of 2-butyl and 1-butyl radicals, 0.083 and 0.04, respectively, the production yields of alkyl nitrates increase with the increase of carbon number of alkyl radicals as described in Sect. 5.3.3, and it is as large as 0.2 for the Cg, and C7 secondary n-aUcyl radicals (2-hexyl, 3-hexyl, 2-heptyl, and 3-heptyl radicals) (Lightfoot et al. 1992 Arey et al. 2001). Since these reactions act as termination reactions for the OH chain reaction, they are important as parameters in determining the ozone formation efficiency in model calculations. The production yields of RONO2 

The 2-butoxy, and 1-butoxy radicals formed in the reactions of 2-butyl and 1-butyl peroxy radicals with NO (path (d) and (j)), react with O2 to produce HO2 together with carbonyl compounds such as methyl ethyl ketone and butanal (path (g) and (1)), and complete the HO cycle. Under the typical NO concentrations in the polluted atmosphere, the formation rate of alkyl nitrates by the reactions of alkoxy radicals with NO2 is negligible, the recombination isomerization reaction by the alkylperoxy radicals with NO is the major pathway as the formation process of alkyl nitrate. 

It is known that for straight-chain alkoxy radicals with a carbon number 4, such as 1-butoxy radicals, isomerization forms alkoxy radicals to alcohol radicals though six-members rings can occur (Atkinson 1997a, b) as follows. 

Furthermore, as for alkoxy radicals with a carbon number C4, it is known that the following type of unimolecular decomposition to rupture C-C bond between the carbon attached to carboxyl group and the adjacent carbon atom occurs. 

Since the unimolecular decomposition rate constants for the secondary alkoxy radicals with 4 carbons, are 10 s (Atkinson and Arey 2003), the reactions of the types (7.23), (7.24) and (7.25) can occur in parallel to give hydroxyl aldehyde, aldehyde with the same carbon number as the reactant alkanes, and aldehydes with one carbon less than the original alkane, respectively. The rate constants of for the isomerization reaction, reaction with O2, and the unimolecular 

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