Addition of toluene and ethylbenzene to mixtures of H_2 and O_2 at 772 K: Part 2: Formation of products and determination of kinetic data for H+ additive and for other elementary reactions involved

摘要

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 reaction (26) and in the overall reaction (31). C_6H_5CH_2 + HO_2 → OH + C_6H_5CH_2O (+ O_2 → C_6H_5CHO) (26) C_6H_5CH_2 + O_2 → C_6H_5CHO + OH (31) 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) X 10~9 dm~3 mol~(-1) s~(-1) and k_(29) = 2.8 X 10~3 s~(-1) (for the 1,3 Hatom transfer involved in (31)) are obtained. C_6H_5CH_2OO → C_6H_5CHOOH (29) Styrene is the major initial product from EB. The electron-localised radical C_6H_5CH_2CH_2 reacts almost completely in a fast reaction (k = 2 X 10~8 dm~3 mol~(-1) s~(-1)) with O_2 to give styrene, but the more stable electron-delocalised C_6H_5CHCH_3 radicals also undergo radical-radical reactions to give benzaldehyde with k_(38) = (7.3 +- 3.0) X 10~9 dm~3 mol~(-1) s~(-1). C_6H_5CHCH_3 + HO_2 → C_6H_5CH(OOH)CH_3 (38) Measurements of the yields of benzene from TOL and EB gave rate constants for the reaction H + TOL/EB → C_6H_6 + CH_3/C_2H_5, and combination with independent data gives k = 950T~2exp(―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 a-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.