METHYL RADICAL PRODUCTION IN THE RADIOLYSIS OF HYDROCARBONS1

Abstract

Radioiodine scavenging methods were employed in the radiolysis of liquid hydrocarbons to examine details of the production and reaction of methyl radicals. Carrierfree chromatographic separation of the radio-methyl iodide enabled studies to be carried out at iodine concentrations down to 5 x 10/sup -7/ M. In the radiolysis of 2,2,4trimethylpentaneiodine solutions at room temperature the methyl iodide yield is shown to be independent of iodine concentration above 10/ sup -5/ M. Competition between the scavenging reaction and abstraction of hydrogen from the solvent by the methyl radicals was observed in the region of 10/ sup -6/ M. In the absence of iodine the methane yield is shown to be dependent on absorbed dose rate in the region of 10/sup 7/ rads/hr. At this dose rate the abstraction process competes effectively with the reaction between methyl radicals and other alkyl radicals. Measurement of the competition rates of the above reactions, together with rate information from paramagnetic resonance experiments, allows an estimate of 3 x 10/sup 8/ liters mole/sup -1/ sec/sup -1/ to be made for the absolute second-order rate constant for the reaction of methyl radicals with molecular iodine in liquid 2,2,4-trimethylpentane. The efficiency of reaction per encounter is shown tomore » be close to unity with little or no activation energy involved. A survey of methyl radical production from various hydrocarbons showed that carbon-methyl bond rupture deviates substantially from that predicted by a simple model involving only a statistical consideration of the number of methyl groups in the molecule. The yields were found to be very markedly dependent on specific details of the structure of the species being irradiated. For individual homologous series the methyl radical yields were observed to be decreasing monotonic functions of the chain length of the hydrocarbon. For the paraffins and isoparaffins the yield was proportional to the inverse square of the number of carboncarbon bonds in the molecule. By a generalization of this relationship, an empirical rule was developed which allows reasonable quantitative estimates to be made for the methyl radical yields from even highly branched hydrocarbons. (auth)« less