Addition of toluene and ethylbenzene to mixtures of H2 and O2 at 773 K: Part I: Kinetic measurements for H and HO2 reactions with the additives and a data base for H abstraction by HO2 from alkanes, aromatics and related compounds

Abstract Relative rate constants have been determined for the reactions of H atoms and HO 2 radicals with toluene and ethylbenzene by adding traces of these compounds to mixtures of H 2 + O 2 at 773 K. The values k 24t = (5.5 ± 1.5) × 10 4 and k 24e = (1.65 ± 0.63) × 10 5 dm 3 mol −1 s −1 are the first reliable kinetic data obtained for the abstraction of an H atom from any aromatic compound by HO 2 radicals. It is shown that the values are significantly lower than expected on the grounds of enthalpy of reaction, and it is concluded the explanation lies in a combination of a lower A factor than observed with alkanes (because of the loss of entropy of activation in the emerging electron-delocalized radicals), and a slightly higher activation energy. A comprehensive database for HO 2 abstraction reactions from alkanes, aromatics, alkenes, and related compounds has been assembled and recommended for use over the temperature range 600 to 1200 K. (24t) HO 2 + C 6 H 5 CH 3 → H 2 O 2 + C 6 H 5 CH 2 (24e) HO 2 + C 6 H 5 C 2 H 5 → H 2 O 2 + C 6 H 5 CHCH 3 / C 6 H 5 CH 2 CH 2 The overall rate constants k 22t =(5.0 ± 1.0) × 10 8 and k 22e = (1.07 ± 0.25) × 10 9 dm 3 mol −1 s −1 are obtained for H attack on toluene and ethylbenzene at 773 K. As found for the HO 2 reactions with toluene and ethylbenzene, the values are lower than expected, despite the fact that at least one other pathway besides abstraction is known to occur. (22t) H + C 6 H 5 CH 3 → products (22e) H + C 6 H 5 C 2 H 5 → products