Rate constants for the reaction of HO2 radicals with cyclopentane and propane between 673 and 783 K

Abstract

Studies have been made of the separate addition of cyclopentane (c-C₅H₁₀) and propane (C₃H₈) to mixtures of O₂ and tetramethylbutane (TMB) between 673 and 783 K in aged boric-acid-coated vessels to obtain kinetic data for the reaction of HO₂ radicals with each of the additives. The contribution by OH radicals to the removal of c-C₅H₁₀ and C₃H₈ has been minimised by use of a total pressure of 15 Torr and by measurements well within 5% consumption of TMB and the additives. A full product analysis was carried out for each kinetic data point which shows that at least 85% of the radicals produced from c-C₅H₁₀ and C₃H₈ give conjugate alkene + HO₂ and which permits a precise correction for the small percentage of OH radicals formed. From measurements of the relative rates of consumption of TMB and the additives, values of k_12p/k₇^1/2 and k_12c/k₇^1/2 were obtained at each temperature used and were shown by sensitivity analysis to be relatively insensitive to any other parameter associated with the mechanism.

 HO₂ + C₃H₈ → H₂O₂ + C₃H₇(12p)HO₂ + c-C₅H₁₀ → H₂O₂ + c-C₅H₉(12c)HO₂ + HO₂ → H₂O₂ + O₂(7) Values of E_12p − 1/2E₇ = 75.6 ± 5.8 kJ mol⁻¹, log[(A_12p/A₇^1/2)/(dm³ mol⁻¹ s⁻¹)^1/2] = 5.53 ± 0.52 and E_12c − 1/2E₇ = 71.2 ± 3.9 kJ mol⁻¹, log[(A_12c/A₇^1/2)/(dm³ mol⁻¹ s⁻¹)^1/2] = 5.58 ± 0.40 were obtained which with k₇ = 1.87 × 10⁹ exp(− 775/T) dm³ mol⁻¹ s⁻¹ give E_12p = 78.8 ± 6.6 kJ mol⁻¹, log(A_12p/dm³ mol⁻¹s⁻¹) = 10.17 ± 0.57 and E_12c = 74.4 ± 4.7 kJ mol⁻¹, log(A_12c/dm³ mol⁻¹ s⁻¹) = 10.22 ± 0.45. The parameters for reaction (12c) are in excellent agreement with E = 73.2 ± 5.0 kJ mol⁻¹, log(A/dm³ mol⁻¹ s⁻¹) = 10.23 ± 0.49 obtained in a previous study for the reaction of HO₂ radicals with cyclohexane. For use outside the temperature range of this study, the three-parameter function k = AT^2.5exp(−B/T) is recommended. From a combination of the data obtained for c-C₅H₁₀ and cyclohexane, values of A = 7.9 dm³ mol⁻¹ s⁻¹ K^−2.5 and B = 6970 K are recommended for the abstraction by HO₂ radicals of a single secondary H atom from alkanes with a carbon number larger than 4. Use has been made of all the available data for HO₂ + alkane reactions to derive a data base for HO₂ attack at primary, secondary, and tertiary C–H sites in alkanes with an accuracy expressed as Δlog = ± 0.15 between 600 and 800 K rising to ± 0.3 at 1200 K.