Oxidation of the benzyl radical: Mechanism, thermochemistry, and kinetics for the reactions of benzyl hydroperoxide

Document Type

Article

Publication Date

12-1-2009

Abstract

Oxidation of the benzyl radical plays a key role in the autoignition, combustion, and atmospheric degradation of toluene and other alkylated aromatic hydrocarbons. Under relevant autoignition conditions of moderate temperature and high pressure, and in the atmosphere, benzyl reacts with O2 to form the benzylperoxy radical, and the further oxidation reactions of this radical are not yet fully characterized. In this contribution, we further develop the reaction chemistry, thermodynamics, and kinetics of benzyl radical oxidation, highlighting the important role of benzyl hydroperoxide and the benzoxyl (benzyloxyl) radical. The benzylperoxy + H reaction mechanism is studied using computational chemistry and statistical reaction rate theory. High-pressure limit rate constants in the barrierless benzylperoxy + H association are obtained from variational transition state theory calculations, with internal rotor contributions. The benzylperoxy + H reaction is seen to produce an activated benzyl hydroperoxide adduct that has 87 kcal mol-1 excess energy over the ground state. We show that this activated adduct proceeds almost exclusively to the benzoxyl radical + OH across a wide range of temperature and pressure conditions. Minor reaction paths include benzyl + HO2, α-hydroxylbenzyl + OH, and benzaldehyde + H2O, each constituting around 1% of the total reaction rate at higher temperatures. Thermal decomposition of benzyl hydroperoxide, formed by hydrogen abstraction reactions in the benzylperoxy radical and at low temperatures in the benzylperoxy + H and benzyl + HO2 reactions, is also investigated. Decomposition to benzoxyl + OH is fast at temperatures of 900 K and above. The contribution of benzyl hydroperoxide chemistry to the ignition and oxidation of alkylated aromatics is discussed. Benzyl radical oxidation chemistry achieves the conversion of toluene to benzaldehyde, aiding autoignition via processes that either release large amounts of energy or form reactive free radicals through chain-branching. © 2009 American Chemical Society.

Identifier

73949117775 (Scopus)

Publication Title

Journal of Chemical Theory and Computation

External Full Text Location

https://doi.org/10.1021/ct900352f

ISSN

15499618

First Page

3185

Last Page

3194

Issue

12

Volume

5

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