Kinetics of the Reactions of Alkyl Iodides with HI

Abstract

A reexamination of Ogg's data on the kinetics of the reactions of RI+HI→RH+I₂ where R=CH₃, C₂H₅ or n—C₃H₇, shows that the mechanism is a simple I‐atom catalyzed, radical reaction with the steps $$\begin{array}{c}\mathrm{I}+\mathrm{RI}\rightleftarrows {\displaystyle {lim}_2^1}\mathrm{R}+{\mathrm{I}}_2\\ \mathrm{R}+\mathrm{HI}\to {\displaystyle {lim}_3}\mathrm{RH}+\mathrm{I}.\end{array}$$ The rate expression is $$\mathrm{d\; (}{\mathrm{I}}_2{\mathrm{)/dt=k}}_1{k}_3{K}^{\frac{1}{2}}(\mathrm{RI}\mathrm{)\; (}\mathrm{HI}{\mathrm{)/[k}}_2({\mathrm{I}}_2{\mathrm{)+k}}_3(\mathrm{HI}\mathrm{)],}$$ where K is the equilibrium constant for dissociation of I₂. Values are obtained for k₁, and the inhibition ratio k₃/k₂ at one temperature and for k₁k₃/k₂ over a temperature range. Using some entropy estimates, values are deduced for A₁, A₂, A₃, and A₄ (reverse of step 3) all of which seem quite reasonable and self‐consistent. The difference (E₃—E₂) appears to lie between 0 and 2 kcal/mole while E₁ is in excess of the estimated endothermicity of reaction (1) by not more than 1.5 kcal mole. There seems to be good agreement with independent data on the kinetics of pyrolysis of alkyl and aryl iodides which also turn out to be I‐atom catalyzed, radical reactions.