Reactions of ethyl radicals with oxygen over the temperature range 400–540° C

Abstract

The reactions of C₂H₅ radicals with O₂ have been studied by adding ethane to mixtures of 2,2,3,3-tetramethylbutane + O₂ and of H₂+ O₂. The only initial products from C₂H₅ radicals are ethylene and ethylene oxide in relative yields of ≈ 100:1. For all mixtures studied, the ratio [C₂H₄]/[C₂H₄O] is constant at low extents of reaction but then decreases due to formation of ethylene oxide by reaction (14). The initial value of the product ratio is independent of variations in mixture composition, total pressure and vessel surface. Over the temperature range 400–540°C, values of A₁/A₂= 13.6 ± 2.1 and E₂–E₁= 12.5 ± 1.0 kJ mol^–1 are obtained, where reaction (2) is the overall reaction for the formation of ethylene oxide. C₂H₅+ O₂→ C₂H₄+ HO₂(1) C₂H₅+ O₂→ C₂H₄O + OH (2) HO₂+ C₂H₄→ C₂H₄O + OH (14) By the use of the trace amounts of butane formed at 540°C to calculate the concentration of C₂H₅ radicals, k₁= 7.7 × 10⁷ dm³ mol^–1 s^–1 is obtained; combination of this with independent results at 440°C gives E₁= 16.2 kJ mole^–1 and A₁= 10^8.93 dm³ mol^–1 s^–1, which are considered the most reliable parameters available for this reaction.

Based on the formation of ethylene oxide in a peroxy radical isomerisation and decomposition sequence, values of log₁₀(A₁₆/s^–1)= 13.29 ± 0.50 and E₁₆= 143.7 ± 10 kJ mol^–1 are calculated for the transfer of a primary H atom (1,4p) in reaction (16). This A factor is higher, by a factor of about ten, than the figure suggested by Benson for a (1,5) H atom transfer. C₂H₅OO → C₂H₄OOH (16)