Addition of toluene and ethylbenzene to mixtures of H2 and O2 at 772 K: Part 2: formation of products and determination of kinetic data for H+ additive and for other elementary reactions involved

Abstract A detailed product analysis was carried out at 773 K when toluene (TOL) and ethylbenzene (EB) were added separately in small amounts to H 2 + O 2 + N 2 mixtures at 500 Torr total pressure. Benzaldehyde is the major initial product from TOL, formed in , , and in the overall , . (26) C 6 H 5 CH 2 +HO 2 → OH+C 6 H 5 CH 2 O (+O 2 → C 6 H 5 CHO) (31) C 6 H 5 CH 2 +O 2 → C 6 H 5 CHO+OH The electron-delocalised benzyl radical reacts very slowly with O 2 , and the importance of radical-radical reactions is confirmed by the observation that bibenzyl is formed in relatively high yields. Values of k 26 = (5.1 ± 1.5) × 10 9 dm 3 mol −1 s −1 and k 29 = 2.8 × 10 3 s −1 (for the 1,3 Hatom transfer involved in , ) are obtained. (29) C 6 H 5 CH 2 OO→C 6 H 5 CHOOH Styrene is the major initial product from EB. The electron-localised radical C 6 H 5 CH 2 CH 2 reacts almost completely in a fast reaction (k = 2 × 10 8 dm 3 mol −1 s −1 ) with O 2 to give styrene, but the more stable electron-delocalised C 6 H 5 CHCH 3 radicals also undergo radical-radical reactions to give benzaldehyde with k 38 = (7.3 ± 3.0) × 10 9 dm 3 mol −1 s −1 . (38) C 6 H 5 CHCH 3 +HO 2 → C 6 H 5 CH(OOH)CH 3 Measurements of the yields of benzene from TOL and EB gave rate constants for the reaction H + TOL/EB → C 6 H 6 + CH 3 /C 2 H 5 , and combination with independent data gives k = 950T 2 exp(−475) dm 3 mol −1 s −1 for the TOL reaction. Rate expressions are given in non-Arrhenius form for all H abstractions by H atoms from TOL and EB. Further evidence is provided that H abstraction from the α-carbon atom in alkyl benzenes is considerably slower than expected on thermochemical grounds. It is, however, concluded that abstraction from the benzene ring by H atoms is significantly more important than hitherto suggested.Keywords: aromatic oxidation, kinetics, mechanism