A Reinvestigation of the Deceptively Simple Reaction of Toluene with ●OH, and the Fate of the Benzyl Radical. I. a Combined Thermodynamic and Kinetic Study on the Competition Between ●OH Addition and Hydrogen Abstraction Reactions.

Abstract

This work reports density functional and composite model chemistry calculations performed on the reactions of toluene with the hydroxyl radical. Both experimentally observed H-abstraction from the methyl group and possible additions to the phenyl ring were investigated. Reaction enthalpies and heights of the barriers suggest that H-abstraction is more favorable than ●OH addition to the ring. The calculated reaction rates at room temperature and the radical-intermediate product fractions support this view. This is somehow contradictory with the fact that, under most experimental conditions, cresols are observed in a larger concentration than benzaldehyde. Since the accepted mechanism for benzaldehyde formation involves H-abstraction, a contradiction arises that begs for an explanation. In this first part of our work we give the evidences that support the preference of hydrogen abstraction over ●OH addition and suggest an alternative mechanism which shows that cresols can actually arise also from the former reaction and not only from the latter.

Enthalpies of formation of the benzyloxyl, benzylperoxyl, hydroxyphenyl radicals and related species on the potential energy surface for the reaction of toluene with the hydroxyl radical

“…Quantum mechanical calculations have been performed on radicals and closed shell species lying on the potential energy surface of the reaction of benzyl radical with O2, OH • , and H2O. These species arise in different channels in the reaction mechanism of toluene reaction with the OH • radical (see, for instance, Metcalfe et al [5], Murakami et al [10], Bounaceur et al [11] and ourselves [12,13]) and the enthalpies of formation were known for about half of the 17 compounds studied. Non-isodesmic reactions connecting products and reactants on these reaction paths, starting either from toluene or the benzyl radical, were used to calculate the enthalpies of formation.…”

“…The mechanisms studied in our work on the reaction between toluene and OH • are shown in Fig. 1 [12,13]. We framed there those species that are studied in this paper, in black for those with known experimental enthalpy of formation, and in red for the other ones.…”

“…The reactions employed to obtain the enthalpy of formation of the benzyl radical are the following T + OH • → TR + H2O (12) T + CH3 • → TR + CH4…”

“…A non-exhaustive list of recent studies can be found in references [5][6][7][8][9][10][11]. We have recently performed a detailed study of the possible routes for the reaction of toluene (T) with OH • , and further reactions of the intermediate radicals with both OH • and O2 [12,13]. Several intermediates were identified theoretically, for which scarce or no experimental information is available.…”

“…Enthalpies of reaction and formation, as well as error of the theoretical enthalpy of formation of TR calculated using reactions(12) and(13). All values are in kcal/mol.…”