Thermal decomposition of azomethane

Abstract

The thermal decomposition of azomethane alone and in the presence of added gases has been reinvestigated in a static reaction system to see if the reaction could still be regarded as a true unimolecular reaction and, as such, used to test the detailed theories of unimolecular reactions and to obtain information about the efficiencies of added gases in energy transfers with the reactant molecules.

The reaction was followed in the temperature and pressure ranges (502 - 593°K) and (0.2 - 975 mm.) by measurement of the nitrogen which was produced by the primary reaction

CH₃N=NCH₃ --> CH₃ + N₂ + CH₃

As predicted by theory, the experimental first -order rate constant and the activation energy both fall away from their high -pressure values with decreasing pressure. However, work with toluene and propylene as added gases showed that the reaction is complicated by a short chain process. A possible mechanism for the chain decomposition of azomethane is discussed and suggestions are made as to how this may be verified by experiment. These chains account for about 50 per cent of the reaction at higher pressures, but they can be entirely inhibited by the presence of excess propylene. The rate constants for the fully inhibited reaction are given by

k∞ = 10¹⁵.⁷ exp (-51,200/RT) sec.⁻¹

Surface reaction was eliminated by seasoning the reaction vessel with allyl bromide.

Because of the complexity of the reaction the interpretation of the rate /pressure and activation -energy /pressure curves as a detailed test of unimolecular theory is doubtful but the indications are that these can only be accounted for by the quasi- unimolecular nature of the reaction. There are four current theories of unimolecular reactions which are chiefly associated with the names of Hinshelwood, Kassel, Eyring and Slater.

Little attention has been paid in the past to treating these theories as a whole. In this thesis the particular theories are developed from general arguments and then compared. In particular, the dependence of the first -order rate constant and the activation energy on pressure is noted. The significance of the experimental activation energies and frequency factors in terms of the theories is discussed in detail. In general, the high- pressure activation energy should be equal to the bond dissociation energy of the rupturing bond and the high- pressure frequency factor should lie within the range of molecular vibration frequencies. However, there is an important exception: if reaction involves the simultaneous rupture of more than one bond then the high- pressure activation energy is not necessarily related to the dissociation energies of the bonds in a simple manner and the high- pressure frequency factor is greater than normal.

The previous work on the thermal and photochemical decomposition of azomethane and some of the more reliable work on energy transfer in other systems are also summarised.