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