Arrhenius parameters for the reaction HO2 + cyclohexane between 673 and 773 K, and for H atom transfer in cyclohexylperoxy radicals

Abstract

Studies have been made of the addition of cyclohexane (CHX) to O₂ + tetramethylbutane (TMB) mixtures between 673 and 773 K in aged boric-acid-coated Pyrex vessels to obtain kinetic data for the reaction of HO₂ radicals with CHX. The contribution by OH radicals to the removal of CHX is minimised by the use of a total pressure of 15 Torr, and by product analysis well within 5% consumption of CHX and TMB. From measurements of the relative rate of consumption of CHX and TMB, values of k₁₂/k₇^1/2 were obtained at 673, 713, 753 and 773 K which were relatively insensitive to any other parameter associated with the mechanism.

Values of E₁₂ − ½E₇ = 71.1 ± 4.5 kJ mol⁻¹ and log[(A₁₂/A₇^1/2)/(dm³ mol⁻¹ s⁻¹)^1/2] = 5.67 ± 0.41 were obtained, which with k₇ = 1.87 × 10⁹ exp(−775/T) dm³ mol⁻¹ s⁻¹ give E₁₂ = 74.3 ± 4.9 kJ mol⁻¹ and log(A₁₂/dm³ mol⁻¹ s⁻¹) = 10.31 ± 0.47. These parameters are the first determined experimentally for the reaction of HO₂ radicals with secondary C–H bonds. For temperatures outside the range 673–773 K, k₁₂ = 1.12 × 10²T^2.5exp(−7120/T) dm³ mol⁻¹ s⁻¹ is recommended. From measurements of the initial yields of 1,4-epoxycyclohexane, hex-5-en-1-al and 1,2-epoxycyclohexane relative to that of cyclohexene, rate constants and Arrhenius parameters have been determined for 1,6s, 1,5s and 1,4s H atom transfers, respectively, in cyclohexylperoxy radicals. The role of the peculiar stereochemistry of cyclohexane and its derivatives in determining the value of these parameters has been discussed. The initial products show that pyrolysis is unimportant at the temperatures of the present study. However, through a careful analysis of the small yields of butadiene, the large amounts obtained by other workers at about 1000 K are explained by the high temperature coefficients associated with both the formation of linear-C₆H₁₁ radicals, which is in equilibrium with the regeneration of cyclo-C₆H₁₁, and the homolysis of linear-C₆H₁₁ radicals. Values of E₁₆ = 130 ± 8 kJ mol⁻¹ and log(A₁₆/s⁻¹) = 13.7 ± 0.7 are recommended