Arrhenius parameters for the reaction C2H5+ O2? C2H4+ HO2

Abstract

Studies have been made of the relative yields of C₂H₄ and C₂H₆ produced in the oxidation of propionaldehyde over the temperature range 593–753 K and the results have been used to obtain the values E₂–E₁= 36.7 ± 2.5 kJ mol^–1 and A₂/A₁= 14.8 ± 4.5: C₂H₅+ O₂→ C₂H₄+ HO₂(1), C₂H₅+ C₂H₅CHO → C₂H₆+ C₂H₅CO. (2), Use of the literature values of A₂ and E₂, with suitable corrections, gives E₁=–6.3 ± 3.5 kJ mol^–1 and A₂= 10^{7.05 ± 0.18} dm³ mol^–1 s^–1, which confirm a recent experimental observation that reaction (1) has a small negative activation energy. Use of these parameters to extrapolate down to 295 K gives (subject to considerable uncertainty) a value for k₁ in almost exact agreement with a value obtained independently from direct measurements of C₂H₄ formation. The mechanism of the formation of C₂H₄ from C₂H₅+ O₂ is discussed in the light of the present results and those from independent studies. A direct bimolecular reaction appears to be inconsistent with the negative temperature coefficient, and the possibility that the reaction path involves formation and decomposition of C₂H₄OOH radicals is ruled out by independent work. C₂H₄ formation from the decomposition of C₂H₅O^* ₂ radicals is consistent with the present results, but if the same mechanism operates over the range 295–750 K then a different route is required. The possibility of a modified abstraction process which involves a cyclic intermediate of some stability is considered in an attempt to explain the negative temperature coefficient.