Evidence for alkoxy radical isomerization in photooxidations of C 4C 6 alkanes under simulated atmospheric conditions

Abstract

We have calculated approximate room temperature rate constants for intramolecular hydrogen shift isomerizations of alkyl, alkoxy and alkylperoxy radical intermediates in photochemical smog and found that alkoxy radicals with δ hydrogens appear to undergo 1,5 hydrogen shifts at ambient conditions. Product distributions observed during irradiations of alkane ( n-butane, n-pentane and n-hexane)NO x air systems in two quite-different ≈6000-liter environmental chambers were found to be consistent with this prediction. For example, in the n-pentaneNO x air system the observed large yields of 3-pentanone relative to 2-pentanone were consistent with room temperature rate constants which we have estimated for the corresponding hydrogen shift isomerizations. While such isomerizations have been well recognized in studies of free radicals and high temperature hydrocarbon oxidations, they are not taken into account in current kinetic mechanisms for alkane oxidation under ambient atmospheric conditions. This has implications for the construction of computer models of photo-chemical smog formation.