The mechanism of the acetaldehyde pyrolysis

Abstract

Previous work on the acetaldehyde pyrolysis is shown to be vitiated by the presence, in the acetaldehyde, of impurities, mainly ethanol and crotonaldehyde. The reaction has been remvestigated with the use of acetaldehyde, prepared from paraldehyde, which is free from these and other impurities. On the basis of a study of the kinetics of formation of the major products (methane and carbon monoxide) and of a number of minor products (hydrogen, acetone, propionaldehyde, ethane and ethylene) a reaction mechanism is proposed. This includes all of the reactions in the original Rice-Herzfeld scheme, together with a number of other elementary processes, in particular $CH_3 + CH_3CHO \rightarrow CH_4 + CH_2CHO.$ The decomposition of the radical CH$_2$CHO into CH$_2$CO and H provides an additional source of hydrogen, the rate of production of which is therefore not a measure of the rate of the initiation process. Acetone is believed to arise mainly by the reaction $CH_3 + CH_3CHO \rightarrow CH_3COCH_3 + H,$ and only to a negligible extent by the combination of CH$_3$ and CH$_3$CO. The main chainending step is concluded to be $CH_3 + CH_3 \rightarrow C_2H_6,$ with a small contribution from $CH_3 + CH_2CHO \rightarrow CH_3CH_2CHO.$ The work provides further evidence for the falling off, at low pressures, of the second order coefficient for the combination of methyl radicals. Rate constants for various elementary processes are deduced from the rates of formation of the various products, and are shown to be consistent with values obtained directly.