Arrhenius parameters for the unimolecular decompositions of azomethane and n-propyl and isopropyl radicals and for methyl radical attack on propane

Abstract

A flow system study of the azomethane sensitized pyrolysis of propane over the ranges 676–813 K and 10–26 Torr total pressure has allowed evaluation of the Arrhenius parameters for the reactions: (CH₃)₂N₂→ 2CH₃+N₂(1), i-C₃H₇⇌ H + C₃H₆(3), n-C₃H₇⇌ CH₃+C₂H₄.(4). These are k₁/s^–1= 10^{13.3 ± 0.3} exp(– 189 ± 4 kJ mol^–1/RT), k₃/S^–1= 10^{13.4 ± 0.7} exp(– 171 ± 10 kJ mol^–1/RT), k₄/s^–1= 10^{12.1 ± 0.5} exp(– 136 ± 8 kJ mol^–1/RT). The parameters quoted for reaction (1) are based on the present and previously published work. The parameters for reactions (3) and (4) are calculated on the basis that mutual recombinations of n-propyl and of isopropyl radicals have the rate constant 10^11.6 cm³ mol^–1 s^–1 as indicated by recent studies. Published thermochemical data permit the further evaluations k_–3/cm³ mol^–1 s^–1= 10^12.8 exp(– 7 kJ mol^–1/RT) which is in remarkably good agreement with the best recent measurement, and k_–4/cm³ mol^–1 s^–1= 10^10.5 exp(– 33 kJ mol^–1/RT) the activation energy of which agrees with previous measurements. However, only the lowest published values of k_–4 are in agreement with the above; other values are up to 15 times as great. The above value of A_–4 is compatible with published values for addition of other alkyl radicals to ethene.

The Arrhenius plot of the rate constant for the reaction CH₃+C₃H₈→ CH₄+n- or i-C₃H₇(2) is strongly curved. In the low and in the high temperature regions of the present study k₂ may be adequately represented by the Arrhenius expressions T < 743 K, k₂/cm³mol^–1 s^–1= 10^{12.30 ± 0.52} exp(– 47.3 ± 6.8 kJ mol^–1/RT); and, T > 743 K, k₂/cm³mol^–1 s^–1= 10^{15.70 ± 0.51} exp(– 96.4 ± 7.5 kJ mol^–1/RT). These expressions are in good agreement with previous work in both temperature ranges.