The kinetics of the oxidation of gaseous benzene

Abstract

The comparatively few exothermic gaseous oxidation reactions which have been investigated kinetically nearly all exhibit interesting peculiarities of behaviour connected with the existence of the “chain” mechanism. The oxidation of hydrocarbons is an example to which considerable attention is at present being given. The rate of oxidation is sometimes retarded by an increase in the surface of the containing vessel—a fact which points directly to the existence of reaction chains—and the relation between the rate and the concentrations of the gases is often a remarkable one. Thus ethylene is oxidised at a speed which is relatively little affected by the oxygen concentration but depends upon a high power of the ethylene concentration. Similar relations hold good for acetylene, It seemed, therefore, of interest to study the oxidation of gaseous benzene, partly to ascertain what would be the behaviour of a hydrocarbon of a quite different kind of structure, and partly because benzene is a substance of inherent chemical importance. The results indicate that the oxidation of benzene is a homogeneous reaction in which chains play a part, though not so important a part as, for example, in the combination of hydrogen and oxygen. Kinetically the reaction resembles the oxidation of ethylene in many respects. In particular, rapid oxidation both of benzene and ethylene is markedly favoured by a high concentration of the hydrocarbon, and, other things being equal, by a high ratio of hydrocarbon to oxygen. From this it appears that the primary product of oxidation gives rise to chains by reacting with more hydrocarbon, but not so readily by reacting with oxygen.