Rate constants for the H abstraction from alkanes (R–H) by [formula omitted] radicals: A systematic study on the impact of R and R′

Abstract

A possible source of chain-branching in low temperature combustion is thermal decomposition of alkyl hydroperoxides (R′OOH). One way these species can be produced is via H atom abstraction reactions from alkanes (RH) by alkylperoxy radicals R ′ O 2 .An earlier study focussing on the abstraction from ethane by HO 2 , CH 3 O 2 and C 2 H 5 O 2 revealed that these reactions have a noticeable impact on calculated ignition times of ethane/O 2 mixtures.Another outcome was that the abstraction rate constants for CH 3 O 2 and C 2 H 5 O 2 are virtually identical but smaller than that for HO 2 .The associated activation energies followed an Evans–Polanyi relationship while a common A-factor could be used to describe the kinetics of all three reactions within a factor of about 2–3.In this current study, we extend the investigation by (1) considering a set of alkanes(RH = CH 4, C 2H 6, C 3H 8, C 4H 10) and (2) by including additional peroxy species ( R ′ O 2 with R′ = H, CH 3, C 2H 5, C 3H 7, C 4H 9, HC O, and CH 3C O). We present rate constants for a total of 32 reactions and analyze the data with respect to systematic trends in the reactivity. The results reveal that the rate constants decrease in the order acylperoxy > HO 2 > alkylperoxy. The reactivity of different C–H bonds follows the bond strengths. Overall the heat of reaction is found to be the dominant but not the only rate constant controlling parameter. The accuracy of the calculations and implications of the results are discussed.