A method is proposed for the derivation of a mathematical equation that relates the kinetic parameters of the propagation reactions of homogeneous free-radical chain reactions to the overall order of the reaction. The equation may be used in evaluating the predominant chain-termination reaction, provided that the mechanism of propagation of the chain is well understood. Application of the technique is demonstrated for the pyrolysis of n-butane. It predicts the predominance of the methyl-methyl radical recombination reaction.Canadian Journal of Chemistry, 47, 2707 (1969) Introduction Goldfinger, Letort, and Niclause (1) have shown that, for pyrolytic reactions, the overall order of the reaction depends on: (a) the order of initiation (first or second order), (b) the order of termination (second or third order), and (c) the type of radical participating in the chaintermination process. They referred to radicals involved in bimolecular propagation reactions as

The thermal decomposition of n-butane was investigated at temperatures ranging from 330 to 420 "C. This extension of the temperature range for such studies was made possible by the use of a highly sensitive analytical system, consisting of a gas-liquid chromatograph equipped with a hydrogen-flame-ionization detector which permitted the determination of the major products of the reaction at reactant conversions as low as 0.0002 %. At temperatures below 400 "C the reaction order was both temperature and pressure dependent, a lower order, approaching 0.5, being favored at lower temperatures and higher pressures. The contribution to the reaction mechanism of a chain-termination step consisting of the combination of 2 secondary butyl radicals explains the observed phenomena.