Hydroxyalkoxy radical isomerization

The /3-hydroxyalkoxy radicals such as that formed in (36a) can, in principle, react as discussed earlier, i.e., with 02, decompose, or, for the larger radicals, undergo intramolecular isomerization. For the small alkenes (< C4), decomposition appears to be the most important process (e.g., Tuazon et al., 1998a). For example, for the jS-hydroxyalkoxy radical formed in (36a), the primary fate is... 

For the /3-hydroxyalkoxy radicals formed from alkenes C5 and larger, there is experimental evidence that isomerization starts to dominate (e.g., see Atkinson et al., 1995d Kwok et al., 1996b). Thus, isomerization followed by reaction with 02, NO, etc., ultimately leads to the formation of dihydroxycarbonyl compounds. For the reaction of OH with 1-butene, for example, isomerization of one of the alkoxy radicals ultimately leads to 3,4-dihydroxybutanal in competition with its decomposition and reaction with 02 ... 

The P-hydroxyalkoxy radicals can then decompose, react with 02, or isomerize. Available data show that, apart from ethene, for which reaction of the H0CH2CH20- radical with 02 and decomposition are competitive, the P-hydroxyalkoxy radicals formed subsequent to OH addition to > C3 alkenes undergo decomposition and the reaction with 02 is negligible. 

The hydroxyalkoxy radicals formed in pathway (k) can take place isomerization by intramolecular shift of H-atom via a six-membered ring as in the case of the alkoxy radical in the previous paragraph. Dihydroxyl aldehyde is then formed in pathways (q), (r), (s) though dihydroxyl radicals which has two OH groups in a molecule. The formation of dihydroxyl aldehyde has been confirmed in the laboratory experiment, and the yield of dihydroxyl aldehyde (3,4-dihydroxyl butanal) is 0.04 for 1-butene but it is as high as 0.6 for 1-octene (Kwok et al. 1996b). Under the low NOx concentrations, a part of hydroxyperoxy radicals react with HO2, and is known to produce hydroxyhydroperoxy butane (pathways (f), (m)) (Hatakeyama et al. 1995 Tuazon et al. 1998). 

Kwok, E.S.C., Atkinson, R., Arey, J. Isomerization of p-hydroxyalkoxy radicals formed from the OH radical-initiated reactions of C4-C8 1-alkenes. Environ. Sci. Technol. 30, 1048-1052 (1996b)... 

The products of the OH-radical-initiated oxidation of c/5 -3-hexen-l-ol in air have been determined by Aschmann et al. (1997) and Reisen et al. (2003). Using different analytical techniques, both groups found that 3-hydroxypropanal is a major reaction product. 3-Hydroxypropanal is the co-product of propanal and therefore formed in identical yield ( 75%). This indicates that the decomposition pathway of both jS-hydroxyalkoxy radicals dominates. The production of dihydroxycarbonyl (CH3CH2CH(0H)CH(0H)CH2CH0) arising from the isomerization channel and dihy-droxynitrates was experimentally confirmed these products may account for 25 7% of the reaction (Reisen et al., 2003). 

CH3)2C(0)CH(0H)CH2CH2C(CH3)(0H)CH=CH2. Shu et al. (1997) have discussed the fate of these radicals. The first j8-hydroxyalkoxy radical can react with O2 or decompose, but cannot isomerize by a 1,5-H shift via a six-member transition state. The decomposition pathway leading to acetone and 4-hydroxy-4-methyl-5-hexen-l-al dominates reaction with O2 by a factor of 7 x 10. The second /3-hydroxyalkoxy radical cannot react with O2 because it lacks a hydrogen atom in the a-position, but it can isomerize and/or decompose. The observation of a substantial yield of acetone and 4-hydroxy-4-methyl-5-hexen-l-al, and the lack of any detectable methyl vinyl ketone, shows that decomposition dominates (see figure II-D-22). Shu et al. reported that 4-hydroxy-4-methyl-5-hexen-l-al undergoes cyclization to form the thermodynamically favored isomeric compound 2-ethenyl-2-methyl-5-hydroxytetrahydrofiiran ... 

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