The Reactions of Methyl Radicals with Deuterium, Ethane, Neopentane, Butane and Isobutane

Abstract

Methyl radicals produced by photodecomposition of mercury dimethyl have been allowed to react with deuterium, ethane, neopentane, butane and isobutane in the temperature range 100–300°C. The kinetics of the processes have been ascertained. Ethane production was found to be bimolecular, methane production between first and second order with respect to methyl radical concentrations. From the rate of methane formation as a function of temperature the following activation energies for the reaction of methyl radicals were obtained: with deuterium 8.1, ethane and neopentane 8.3, butane 5.5 and isobutane 4.2 kcal. These values are based on an assumed activation energy E=0 for ethane formation. The data of Kistiakowsky on the heats of hydrogenation of the substituted ethylenes suggest that the bond energies of primary, secondary and tertiary C–H bonds are measurably different. The differences being approximately 2.5 kcal. between primary and secondary and 4.0 kcal. between primary and tertiary C–H bonds in saturated aliphatic hydrocarbons. These differences correspond approximately to the differences in the activation energies noted above. It is proposed that such differences be accepted as a second approximation to the prevailing assumption of equal energies for such C–H bonds.