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The hypothesis put forward in 1926, namely, that unimolecular reactions are mainly confined to molecules with large numbers of internal degrees of freedom, has proved a great stimulus to the investigation of more complex molecules, such as those encountered in the encountered in the field of organic chemistry. The hypothesis was suggested by an investigation on the thermal decomposition of propionic aldehyde. Although this work was detailed enough for its original purpose, a re-investigation has become desirable in view of recent advances in chemical kinetics. According to the collision theory of activation and deactivation, the unimolecular velocity constant should decrease at low pressures. Reactions which behave in this way (so-called quasi-unimolecular reactions) are kinetically bimolecular at low pressures, but become unimolecular at high pressures. More complex behaviour is observed with gaseous acetaldehyde. This compound, while decomposing bimolecularly at high pressures, undergoes additional quasi-unimolecular decomposition at low pressures. The occurrence of independent quasi-unimolecular reactions in the acetaldehyde decomposition can be attributes to the existence of different types of activated state in the molecule. The mode of decomposition would be, on this assumption, a function of the manner in which the energy imparted to the molecule is distributed among the various molecular motions, and also the transformation probability characteristic of the different distributions.

The thermal decomposition of propionic aldehyde