Generation of Radicals in Pairs and the Kinetics of Radical Chain Reactions in Solution

Abstract

FREE radicals responsible for the propagation of chain reactions are commonly formed by the photochemical or thermal decomposition of an appropriate labile substance, and are, in such cases, inevitably produced in pairs. The restricted mobilities of radicals in solution have some special influence on the fate of these radicals, in that many pairs of radicals recom-bine before bringing about any reaction¹. This effect, sometimes known as the cage effect but more recently called² geminate recombination has been widely discussed. A further consequence of the production of radicals in pairs has not, however, received consideration Even if a free radical avoids recombination with its twin radical in the primary cage, the chain carrier will in general, at least in the earlier part of its life, lie closer to its twin than to another chain carrier. There is therefore a finite and sometimes appreciable probability of the mutual termination of geminate chains. The proportion of mutual termination of geminate chains must depend, on one hand, on factors controlling the rate of diffusion of the radicals in the medium and, on the other, the average relative distances of a radical to its twin and other radicals, and this will depend on the rate of decomposition of the initiating molecule.