On the thermal decomposition of acetaldehyde and ethylene oxide

The object of this investigation was as follows. It had been observed in this laboratory that the thermal decomposition of acetaldehyde, and of a number of other organic compounds, was, over a wide range of experimental conditions, associated with phenomena which are generally assumed to be criteria of the operation of chain mechanism. The rate of chemical change at any moment seemed to be determined rather by antecedent conditions than by condition obtaining at the moment. It did not diminish with time during the earlier stages, as the classical theory of chemical reaction demands, and the rate graphs showed discontinuities. Further, the reactions proceeded more slowly in packed than in erupts vessels. the processes referred to differed only from those which are considered to he definitely influenced by the operation of chain mechanism, such as the oxides of chlorine, in that in the latter case the total energy change in the processes taking place between the formation of a primary centre, in accordance with the Maxwell-Boltzmann principle, and the completion of reaction, represented by the quantity (E + Q), is relatively large, while for such compound as acetaldehyde it is relatively small. However, the dispersal of this energy must ultimately be by collision. In any process represented by 2A ⇌ A' A' → B, dispersal of the energy, or part of it, in the second stage, by collision with a molecule of A will increase the probability of the formation of a new primary centre. Where (E + Q) is large and the molecule is simple, the probability is large, and where (E + Q) is small and the molecule large, the probability is small. However, it is never zero. Something more seemed to he wanted to bring cases such as that of acetaldehyde into line with com pound such as the oxides of chlorine, but the idea furnished a basis for an attack on the problem. It seemed to be possible to test this view of the case by study in the thermal decomposition of acetaldehyde, and ethylene oxide, hold of which decompose mainly in accordance with the equation C2 H4 O = CH4 + CO, the rates of decomposition being closely comparable at 400°, so that the neighbourhood of this temperature was selected for the investigation. The thermal decomposition of ethylene oxide has been studied by Heikert and Mack, using the pressure difference method. According to Parks and Huffman, the free energy of chemical change (∆F) accompanying the thermal decomposition of gaseous acetaldehyde to CH4 and CO at 400° is —19.6 k. cals. No definite data are available for ethylene oxide, but the total heat change (∆H) in the process, (CH2 )2 O → CH3 CHO, derived from the heats of combustion of the two compounds is —30.7 K. cals. Since the thermal properties of the two compound are very similar, the free energy of decomposition of ethylene oxide at 400° cannot be very far from —50 k. cals. It was therefore expected that the decomposition of acetaldehyde would proceed in a manner more closely in accord with the classical theory than would ethylene oxide; or if departure from Ike classical theory was observed, and could be accounted for by means of the chain theory, it would he Ike more marked for ethylene oxide.