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

Elementary reactions involved in the oxidation of propanone, butanone and p entan-3-one were elucidated by adding very small amounts of the compound in dividually to slowly reacting mixtures of H-2+O-2 at 753 K. Kinetic studies of the relative rates of consumption of the additive and H-2 show that (21 ) and (22) are the key reactions consuming the ketones. Values of k(21)=1.7 6x10(9), 3.16x10(9) and 3.83x10(9) d mol(-1) s(-1), for propanone, butanone and pentan-3-one, respectively, are obtained, together with k(22)=2.34x10( 8), 7.2x10(8) and 1.20x10(9) d mol(-1) s(-1), respectively. OH + ketone-->H2O + ketonyl (21) H + ketone-->H-2 + ketonyl (22) The values of k(21) show a 30-40% decrease by comparison with those of the analogous rate constants obtained in this laboratory for the structurally-r elated alkanes, whereas the values of k(22) show a 10-15% increase. Althoug h independent data are extremely limited, Arrhenius parameters were obtaine d 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. H + CH3COCH3 --> CH3C(OH)CH3 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 7 0, 140 and 285 Torr of O-2, H-2 and N-2, respectively. With this approach, each oxidation occurs under effectively the same highly controllable condit ions. The mechanism for the formation of products is discussed in the conte xt of modern understanding of hydrocarbon oxidation, and the use of a self- consistent set of kinetic data gives a good prediction of the product yield s 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 th rough considerations of the thermochemistry and kinetics involved. Values o f log(A/s(-1))=11.72 and E=136 kJ mol(-1) are recommended for the 1,5p H at om transfer reaction in CH3COCH2O2 radicals CH3COCH2O2-->CH2COCH2O2H.