Kinetics of the Gas-Phase Reaction of CH3I with HI

By modification of a Beckmann DB spectrophotometer for use with high‐temperature gas systems it has been possible to follow the kinetics of reaction of CH₃I+HI which goes quantitatively to CH₄+I₂ in the temperature range, 260° to 316°C. The kinetics fit the mechanism proposed earlier by Benson and O'Neal very well. $$\begin{array}{cc}{\mathrm{I}}_2\text{⇋2}\mathrm{I},& (\mathrm{K}\mathrm{),}\\ \mathrm{I}+{\mathrm{CH}}_3\mathrm{I}\leftrightharpoons {\displaystyle {lim}_2^1}{\mathrm{CH}}_3+{\mathrm{I}}_2,& \\ {\mathrm{CH}}_3+\mathrm{HI}\leftrightharpoons {\displaystyle {lim}_4^3}{\mathrm{CH}}_4+\mathrm{I}.& \end{array}$$ The rate law for this system is (neglecting 4) $$\mathrm{d(}{\mathrm{I}}_2{\mathrm{)/dt=k}}_1{k}_3{K}^{\frac{1}{2}}({\mathrm{CH}}_3\mathrm{I}\mathrm{)(}\mathrm{HI}\mathrm{)(}{\mathrm{I}}_2{)}^{\frac{1}{2}}{\mathrm{/[k}}_2({\mathrm{I}}_2{\mathrm{)+k}}_3(\mathrm{HI}\mathrm{)].}$$ Although this equation can be integrated exactly, the integrated forms are not sensitive to the data, and various expansions must be used. Values for k₁ and k₃/k₂ are found to be in excellent agreement with earlier estimates. From the activation energy E₁ and the assumption that E₂≤0.5 kcal, the bond energy of CH₃I is found to be about 55±1 kcal/mole, in excellent agreement with other estimates. Values are given, in addition, for the individual rate constants, k₂, k₃ and k₄, as well as their Arrhenius parameters. The A factors for reactions 1 and 4 are shown to be in excess of collision frequencies, indicating steric factors slightly in excess of unity and thus the formation of very loose transition‐state complexes. This appears to be typical of iodine‐atom or I₂‐molecule reactions and thus far quite unique.