Reinvestigation of the Deceptively Simple Reaction of Toluene with OH and the Fate of the Benzyl Radical: The “Hidden” Routes to Cresols and Benzaldehyde

Abstract

In a previous work, we have investigated the initial steps of the reaction of toluene with the hydroxyl radical using several quantum chemical approaches including density functional and composite post-Hartree–Fock models. Comparison of H-abstraction from the methyl group and additions at different positions of the phenyl ring showed that the former reaction channel is favored at room temperature. This conclusion appears at first sight incompatible with the experimental observation of a lower abundance of the product obtained from abstraction (benzaldehyde) with respect to those originating from addition (cresols). Further reactions of the intermediate radicals with oxygen, water, and additional OH radicals are explored in this paper through theoretical calculations on more than 120 species on the corresponding potential energy surface. The study of the addition reactions, to obtain the cresols through hydroxy methylcyclodienyl intermediate radicals, showed that only in the case of o -cresol the reaction proceeds by addition of O 2 to the ring, internal H-transfer, and hydroperoxyl abstraction and not through direct H-abstraction. For both p - and m -cresol, instead, the reaction occurs through a higher-energy direct H-abstraction, thus explaining in part the observed larger concentration of the ortho isomer in the final products. It was also found that the benzyl radical, formed by H-abstraction from the methyl group, is able to react further if additional OH is present. Two reaction paths leading to o -cresol, two leading to p -cresol, and one leading to m -cresol were determined. Moreover, in this situation, the benzyl radical is predicted to produce benzyl alcohol, as was found in some experiments. The commonly accepted route to benzaldehyde was found to be not the energetically favored one. Instead, a route leading to the benzoyl radical (and ultimately to benzoic acid) with the participation of one water molecule was clearly more favorable, both thermodynamically and kinetically.

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

“…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 (13) Both are isodesmic reactions and we showed above that the theoretically calculated values obtained for H2O and CH4 employing these radicals are reasonably accurate. For both these reasons, one would expect to get good error cancellation and to obtain reasonable values of ∆ 298 .…”

“…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.…”

“…Upper and lower limits of these average errors are approximated by twice the standard deviation (±2 Table 2. 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.…”

“…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.…”

“…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.…”