Intramolecular Hydrogen-shift Reactions of Peroxy Radicals

Citation

Praske, Eric (2019) Intramolecular Hydrogen-shift Reactions of Peroxy Radicals. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/KBX8-H010. https://resolver.caltech.edu/CaltechTHESIS:12122018-171618053

Abstract

Straight chain alkanes with more than five carbons, emitted in cities due to incomplete combustion and fuel evaporation, undergo atmospheric gas-phase oxidation with the hydroxyl radical to produce alkyl radicals. These alkyl radicals subsequently add O₂, leading to the formation of peroxy radicals. Following further reaction of these radicals in urban areas, hydroxy-substituted peroxy radicals are formed. Previously, the fate of these peroxy radicals was assumed to be dominated by reaction with nitric oxide, a common air pollutant. Computational and experimental investigations of the oxidation mechanism of 2-hexanol, however, demonstrate that hydrogens α to the hydroxy group exhibit a significantly reduced energetic barrier toward intramolecular hydrogen shifts to the peroxy radical. The barrier reduction for these hydrogen shift reactions results in rate constants that are orders of magnitude larger than for alkyl hydrogens that lack α substitution. Due to significant reductions of nitric oxide emissions in North America, these rate constants are sufficiently large to suggest that this chemistry is competitive even in large cities, particularly during warm summer days. Gas-phase alkyl hydroperoxides, a class of compounds previously expected to exist in negligible quantities in cities, are major products of this chemistry.

Further oxidation of alkyl hydroperoxides leads to the formation of hydroperoxy-substituted peroxy radicals. The chemistry of such peroxy radicals is evaluated through the oxidation of 2-hydroperoxy-2-methylpentane. Experimental observations confirm the previously reported computational result that these peroxy radicals rapidly isomerize by intramolecular hydrogen shift of the hydroperoxide hydrogen. This isomerization occurs on timescales that are much faster than those of bimolecular reaction in essentially all regions of the troposphere. As a consequence of the isomerization, one peroxy radical isomer produced in the oxidation of 2-hydroperoxy-2-methylpentane exhibits an α hydroperoxide hydrogen shift. This reaction rate constant is similar to that reported for the α hydroxy hydrogen shift in the 2-hexanol system.

Alkoxy radicals produced in the oxidation of 2-hydroperoxy-2-methylpentane are similarly shown to undergo a very rapid hydrogen shift of the hydroperoxide hydrogen. One of these shifts results in a peroxy radical that exhibits an α hydroxy hydrogen shift. Thus, the rapid scrambling of hydroperoxy-subsituted alkoxy and peroxy radicals is a key process that can enable additional unimolecular pathways that are otherwise inaccessible. This chemistry has the potential to introduce significant mechanistic complexity and, due to the rapid nature of the reactions, cannot be neglected even under typical "high NO" conditions employed in chamber studies.

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