The self-inhibited pyrolysis of neopentane at small extents of reaction

Abstract

A detailed study of the formation of the products in the initial stages (less than 3∙5% conversion) of the pyrolysis of neopentane at pressures between 20 and 550 Torr and in the temperature range 713 to 823°K has been carried out. The reaction is found to be both homogeneous and strongly self-inhibited. The pyrolysis is assumed to be entirely free radical in character. The proposed mechanism is based on a simple Rice-Herzfeld scheme. At the start of reaction, chain termination is due to methyl-methyl recombination. As reaction proceeds hydrogen atom abstraction from iso­-butene by methyl becomes increasingly important, in consequence the chain length decreases and the rate of formation of the major products falls off due to the occurrence of termination reactions involving the methylallyl radical. This mechanism gives a satisfactory quantitative description of the observations. Rate constants for the unimolecular C-C scission of neopentane have been obtained. Their values are some two orders of magnitude less than those which would be predicted, from available thermochemical data, if the rate of recombination of methyl and t -butyl radicals is normal, i. e. about 10 13.3 mole -1 ml. s -1 . These rate constants are in remarkable agreement with those of Tsang (1966) for a much higher temperature range. It is suggested that there may be some error in the literature value for the heat of formation of the t -butyl radical.