The Activation Energy of the Reaction CH3+HBr = CH4+Br and the Carbon-Hydrogen Bond Strength in Methane

Abstract

Mixtures of hydrogen bromide, methyl iodide, and iodine have been illuminated with ultraviolet light, and the rate of formation of methane as a function of the relative concentrations has been determined. With iodine absent from the system, a maximum amount of methane is formed, due to the absence of recombining reactions of methyl radicals and iodine. A comparison of the amount of methane produced with iodine present in various amounts with that produced when iodine is absent enables one to calculate the relative rates of the reactions $$\begin{array}{c}{\mathrm{CH}}_3+\mathrm{HBr}={\mathrm{CH}}_4+\mathrm{Br},\\ {\mathrm{CH}}_3+{\mathrm{I}}_2={\mathrm{CH}}_3\mathrm{I}+\mathrm{I},\end{array}$$ when the relative steric factors of the two reactions are assumed to be approximately the same as for the reactions $$\begin{array}{c}\mathrm{H}+\mathrm{HBr}={\mathrm{H}}_2+\mathrm{Br},\\ \mathrm{H}+{\mathrm{Br}}_2=\mathrm{HBr}+\mathrm{Br}.\end{array}$$ This leads to an activation energy of 1.5 kcal. for the methyl‐hydrogen bromide reaction, which, combined with the activation energy of the reverse reaction as determined by Van Artsdalen, yields a binding energy of 102 kcal. for CH₃–H.