Rate constants for H and OH attack on propanone, butanone and pentan-3-one at 753 K, and the oxidation chemistry of the radicals formed

Abstract

Elementary reactions involved in the oxidation of propanone, butanone and pentan-3-one were elucidated by adding very small amounts of the compound individually to slowly reacting mixtures of H₂ + O₂ at 753 K. Kinetic studies of the relative rates of consumption of the additive and H₂ show that (21) and (22) are the key reactions consuming the ketones. Values of k₂₁ = 1.76 × 10⁹, 3.16 × 10⁹ and 3.83 × 10⁹ dm³ mol⁻¹ s⁻¹, for propanone, butanone and pentan-3-one, respectively, are obtained, together with k₂₂ = 2.34 × 10⁸, 7.2 × 10⁸ and 1.20 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively.

The values of k₂₁ show a 30–40% decrease by comparison with those of the analogous rate constants obtained in this laboratory for the structurally-related alkanes, whereas the values of k₂₂ show a 10–15% increase. Although independent data are extremely limited, Arrhenius parameters were obtained by combination with the results of other workers. Arguments are presented for the occurrence of a reaction not previously reported in the literature, namely the addition of H atoms to propanone, which may occur prominently at relatively high temperatures.

Analytical studies of the oxidation chemistry of the ketones at 753 K were carried out by adding individually 5 Torr of each to a mixture containing 70, 140 and 285 Torr of O₂, H₂ and N₂, respectively. With this approach, each oxidation occurs under effectively the same highly controllable conditions. The mechanism for the formation of products is discussed in the context of modern understanding of hydrocarbon oxidation, and the use of a self-consistent set of kinetic data gives a good prediction of the product yields in the initial stages of reaction. The low yields of methyl vinyl ketone from butanone and ethyl vinyl ketone from pentan-3-one were rationalised through considerations of the thermochemistry and kinetics involved. Values of log(A/s⁻¹) = 11.72 and E = 136 kJ mol⁻¹ are recommended for the 1,5p H atom transfer reaction in CH₃COCH₂O₂ radicals