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

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 k12p/k71/2 and k12c/k71/2 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 E12p − 1/2E7 = 75.6 ± 5.8 kJ mol−1, log[(A12p/A71/2)/(dm3 mol−1 s−1)1/2] = 5.53 ± 0.52 and E12c − 1/2E7 = 71.2 ± 3.9 kJ mol−1, log[(A12c/A71/2)/(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 E12p = 78.8 ± 6.6 kJ mol−1, log(A12p/dm3 mol−1s−1) = 10.17 ± 0.57 and E12c = 74.4 ± 4.7 kJ mol−1, log(A12c/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 = AT2.5exp(−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.