Kinetic and mechanistic study of the reaction of OH radicals with methylated benzenes: 1,4-dimethyl-, 1,3,5-trimethyl-, 1,2,4,5-, 1,2,3,5- and 1,2,3,4-tetramethyl-, pentamethyl-, and hexamethylbenzene

Abstract

The reaction of OH radicals with a series of methylated benzenes was studied in a temperature range 300-350 K using a flash-photolysis resonance fluorescence technique. Reversible OH additions led to complex OH decays dependent on the number of distinguishable adducts. Except for hexamethylbenzene, triexponential OH decay curves were obtained, consistent with formation of at least two adduct species. For three compounds that can strictly form two adduct isomers for symmetry reasons (1,4-dimethyl-, 1,3,5-trimethyl-, and 1,2,4,5-tetramethylbenzene) with OH bound ortho or ipso with respect to the methyl groups, OH decay curves were analysed in terms of a reaction mechanism in which the two adducts can be formed directly by OH addition or indirectly by isomerization. In all cases one adduct (add1) is dominating the decomposition back to OH. The other (add2) is more elusive and only detectable at elevated temperatures, similar to the single OH adduct of hexamethylbenzene. Two limiting cases of the general reaction mechanism could be examined quantitatively: reversible formation of add2 exclusively in the OH reaction or by isomerization of add1. Total OH rate constants, adduct loss rate constants and products of forward and reverse rate constants of reversible reactions were determined. From these quantities, adduct yields, equilibrium constants, as well as reaction enthalpies and entropies were derived for the three aromatics. Adduct yields strongly depend on the selected reaction model but generally formation of add1 predominates. For both models equilibrium constants of OH reactions lie between those of OH + benzene from the literature and those obtained for OH + hexamethylbenzene. The corresponding reaction enthalpies of add1 and add2 formations are in a range -87 ± 20 kJ mol(-1), less exothermic than for hexamethylbenzene (-101 kJ mol(-1)). Reaction enthalpies of possible add1 → add2 isomerizations are comparatively small. Because results for 1,3,5-trimethylbenzene are partly inconsistent with a direct formation of add2, we promote the existence of isomerization reactions. Moreover, based on available theoretical work in the literature, add1 and add2 are tentatively identified as ortho and ipso adducts, respectively. Total OH rate constants were obtained for all title compounds. They can be described by Arrhenius equations: kOH = A × exp(-B/T). The parameters ln(A/(cm(3) s(-1))) = -25.6 ± 0.3, -25.3 ± 0.6, -27.3 ± 0.3, -24.6 ± 0.3, -26.2 ± 0.4, -26.2 ± 0.4 and -24.5 ± 0.2, and B/K = -160 ± 90, -550 ± 180, -1120 ± 90, -330 ± 100, -820 ± 100, -980 ± 130, and -570 ± 40 were determined for 1,4-dimethyl-, 1,3,5-trimethyl-, 1,2,4,5-, 1,2,3,5- and 1,2,3,4-tetramethyl-, pentamethyl-, and hexamethylbenzene.

Rate coefficients for reactions of OH with aromatic and aliphatic volatile organic compounds determined by the Multivariate Relative Rate Technique

“…In particular, the results for the tetramethylbenzene + OH reactions were in good agreement with their literature counterparts, despite their only being measured once or twice previously (Aschmann et al, 2013;Alarcón et al, 2015). Figure S2 shows sections of typical ion chromatograms (TIC) obtained for Mixture 2: one chromatogram with the reactor lamp turned off (blue) and one for a sample with the reactor lamp turned on (black).…”

Rate coefficients for reactions of OH with aromatic and aliphatic volatile organic compounds determined by the multivariate relative rate technique

“…In particular, the results for the tetramethylbenzene + OH reactions were in good agreement with their literature counterparts, despite their only being measured once or twice previously (Aschmann et al, 2013;Alarcón et al, 2015). Figure S2 in the Supplement shows sections of typical ion chromatograms (TICs) obtained for Mixture 2: one chromatogram with the reactor lamp turned off (blue) and one for a sample with the reactor lamp turned on (black).…”

Gas-Phase Reaction Kinetics of the Ortho and Ipso Adducts 1,2,4,5-Tetramethylbenzene–OH with O2

“…Monocyclic aromatic hydrocarbons with varying degrees of methylation react with OH radicals with rate constants ranging from 1.2 × 10 –12 cm 3 s –1 for unsubstituted benzene to 1.5 × 10 –10 cm 3 s –1 for fully substituted hexamethylbenzene at 298 K. − For 1,2,4,5-tetramethylbenzene (1245-TeMB), rate constants of around 5.8 × 10 –11 cm 3 s –1 were determined previously , with a slightly negative temperature coefficient B in k OH = A exp­(− B / T ). The major OH reaction pathway at atmospheric temperatures is addition to the aromatic ring, while H atom abstraction from CH 3 substituents is minor, accounting for about 11% of the total rate constant for the monosubstituted derivative toluene at room temperature .…”

“…Several isomers can be formed, including ipso isomers, where OH adds at already-substituted positions on the aromatic ring, − apart from stereoisomers. Deviations from biexponential OH decay behavior were observed for several aromatic compounds in a narrow temperature range between about 300 and 340 K, and an evaluation scheme for the analysis of triexponential OH decay curves was developed and applied in previous work. , This scheme is strictly applicable when two isomers are formed and can be extended to investigate the kinetic behavior of the adduct isomers in the presence of scavengers like O 2 .…”

“…In this work, the rate constants for the reactions of O 2 with the ortho and ipso isomers of 1245-TeMB–OH adducts were determined. 1245-TeMB was selected because the triexponential behavior of the OH decay was distinctive in previous work . Even though the exact mechanism of adduct isomer formation through OH addition and possible isomerization reactions remained unclear, the ortho isomer was tentatively identified to be formed preferentially and to decompose more readily back to OH.…”