Studies of the decomposition of oxirane and of its addition to slowly reacting mixtures of hydrogen and oxygen at 480 °C

Abstract

Elementary reactions involved in the oxidation and decomposition of oxirane have been studied in two ways at 480 °C. First, by adding small amounts of oxirane to slowly reacting mixtures of H₂+O₂, rate constants have been determined for H and OH attack on the additive: H + C₂H₄O → C₂H₃O + H₂(22H) OH + C₂H₄O → C₂H₃O + H₂O. (21H) Combination with independent low-temperature data gives values of log₁₀(A_22H/dm³ mol^–1 s^–1)= 10.9±0.2, E_22H= 41±2 kJ mol^–1 and log₁₀(A_21H/dm³ mol^–1 s^–1)= 10.25±0.15, E_21H= 15.1 ± 1.9 kJ mol^–1. The major products are CH₄ and CO, and CO, and a value of (7.5 ± 2)× 10⁵ dm³ mol^–1 s^–1 is obtained for the rate constant of the CH₃+H₂ reaction. Secondly, decomposition studies have been carried out at total pressures (with N₂) of 60 and 500 Torr with oxirane pressures varied between 1 and 20 Torr. The major products are CH₄, CO, CH₃CHO, C₂H₆, C₂H₄ and H₂, and a comprehensive mechanism is presented to account for their occurrence. In addition to consumption in free-radical processes through H and CH₃ attack, oxirane isomerises to give excited CH₃CHO^* molecules which undergo three competing reactions: [graphic omitted] Little effect of total pressure is observed on the relative rate of each process. From the relative yields of CH₄ and C₂H₆, a value of k₄=(4.0 ± 0.2)× 10⁵ dm³ mol^–1 s^–1 is obtained, which in combination with low-temperature data gives log₁₀(A₄/dm³ mol^–1 s^–1)= 9.03±0.15 and E₄= 49.5±1.3 kJ mol^–1. It is suggested that C₂H₄ is formed in radical–radical reactions of the CH₂CHO radical. Evidence is presented to support the view that the C—H bond dissociation energy in oxirane is very similar to that in C₂H₆, and values of D([graphic omitted])= 420.2±4.0 kJ mol^–1 and Δ_fH^o₂₉₈(C₂H₃O)= 149.6 kJ mol^–1 are suggested: CH₃+ C₂H₄O → CH₄+ C₂H₃O. (4)