The thermal decomposition of gaseous benzaldehyde

Abstract

The thermal decompositions of the lower aliphatic aldehydes, including chloral, are in general of no definite integral or half integral order except with formaldehyde. The order of the reaction changes from the second with formaldehyde to nearer the first with the more complex aldehydes (Hinshelwood, Fletcher, Verhoek and Winkler 1934). To explain this, one theory postulates the existence of several simultaneously operating activation mechanisms. The more complex mechanisms predominating with larger molecules would have a longer time lag between activation and decomposition since they involve the transfer of energy from the more distant parts of the molecule. Hence they would be more nearly of the first order at moderate pressures. The superposition of different mechanisms accounts for some of the complex behaviour observed. The acetaldehyde decomposition is not inhibited by small amounts of nitric oxide and responds negatively to other tests for chains (Staveley and Hinshelwood 1936; Smith 1939). With propaldehyde and butaldehyde, nitric oxide shows both a chain reaction and one insensitive to nitric oxide (Staveley and Hinshelwood 1937). Over a limited range of pressures the acetaldehyde decomposition is nearly of the 3/2 order, and attempts have been made to explain this by a chain mechanism, the chains being supposed immune to the action of nitric oxide (Letort 1937; Rice 1934). Experiments have definitely shown the presence of chains in the photochemical decomposition (which is inhibited by nitric oxide), and free radicals can be detected in a stream of decomposing aldehyde at temperatures of 1000° C. But direct evidence for the existence of chains in the range of measurably slow thermal decomposition is lacking.