Oxidation of the Benzyl Radical: Mechanism, Thermochemistry, and Kinetics for the Reactions of Benzyl Hydroperoxide

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.

The pyrolysis of 2-methylfuran: a quantum chemical, statistical rate theory and kinetic modelling study

“…hDE d i estimates have ranged from 2000 cm À1 for the benzyl radical, 52,53 1000 cm À1 for a C 7 heterocyclic ring, 55 718 cm À1 for a C 10 H 10 biheterocycle, 54 500 cm À1 for pyrazole, 56 fulveneallene 57 and benzyl hydroperoxide, 58 and 260 cm À1 for 2,5-dimethylfuran 29-31 and furan. Many recent literature studies have assumed constant hDE d i values for similar systems (large aromatic molecules).…”

Theoretical considerations of secondary organic aerosol formation from H-abstraction of p-xylene

“…In daytime, the reaction of aromatic hydrocarbons with hydroxyl radicals is the major atmospheric loss process [14][15][16]. Several previous experimental [17][18][19] and theoretical [20][21][22][23][24][25][26] studies have unraveled the elementary reactions involved in aromatic oxidation.…”

“…Previous studies showed that in the presence of OH radical the main reaction path is OH addition to the aromatic ring to form a xylene-OH adduct (consuming $90% of OH radicals) with Habstraction from one methyl group to form a methylbenzyl radical (consuming $10% of OH radicals) being the minor route [18,[31][32][33][34]. In addition, theoretical investigations have mainly focused on OH-addition to xylene and the fate of xylene-OH adducts and intermediates [15,26,31,[34][35][36], whereas the H-abstraction mechanism for p-xylene oxidation following the initial OH attack and subsequent products have only been studied experimentally [22,25,37,38].…”

Theoretical Determination of Enthalpies of Formation of Benzyloxy, Benzylperoxy, Hydroxyphenyl Radicals and Related Species in the Reaction of Toluene with the Hydroxyl Radical

“…For salicylic alcohol, an approximate determination using the group additivity Joback-Reid method [40] is present in the literature [41], giving a value of -67.12 kcal/mol in good agreement with our recommended value. Da Silva and Bozzelli [42,43], using a simpler theoretical method (G3B3) suggested a value of 31.1 kcal/mol for the benzyloxy radical and 29.6 kcal/mol for the benzylperoxy radical. Both values are larger than the best ones we found and, moreover, they suggest that the latter is more stable than the former, contrary to what we found.…”