Kinetics of the Reaction of HI with Ethyl Iodide and the Heat of Formation of the Ethyl Radical

Abstract

The kinetics of the reaction between EtI and HI have been studied in the temperature range 263° to 303°C using a spectrophotometric technique for the determination of I₂. The mechanism obeyed is the following: $$\begin{array}{c}{\mathrm{I}}_2\text{⇌2}\mathrm{I}{\mathrm{,\hspace{0.17em}\hspace{0.17em}K}}_{{\mathrm{I}}_2}\\ \mathrm{I}+\mathrm{EtI}\rightleftharpoons {\displaystyle {lim}_2^1}\mathrm{Et}+{\mathrm{I}}_2,\\ \mathrm{Et}+\mathrm{HI}\rightleftharpoons {\displaystyle {lim}_4^3}\mathrm{EtH}+\mathrm{I},\end{array}$$ , which has the rate law (neglecting Step 4): $$\frac{\mathrm{d(}{\mathrm{I}}_2)}{\mathrm{dt}}=\frac{k_1{k}_3{K}_{{\mathrm{I}}_2^{\frac{1}{2}}}(\mathrm{EtI}\mathrm{)(}\mathrm{HI}\mathrm{)(}{\mathrm{I}}_2{)}^{\frac{1}{2}}}{k_2({\mathrm{I}}_2{\mathrm{)+k}}_3(\mathrm{HI})}.$$ . Decomposition of EtI by unimolecular elimination of HI is negligible. A differential method was employed to determine the rate constants k₁ and the ratio k₂/k₃. The Arrhenius parameters of these constants were evaluated experimentally and combined with known thermodynamic data to give the parameters of the individual constants k₂, k₃, k₄. As with other I₂ molecule systems, the A factors of Reactions (1) and (4) are high, being approximately equal to collision frequencies. With the assumption E₂=0.2 kcal and the experimental activation energy E₁, the C–I bond‐dissociation energy in EtI is found to be 52.9±1.0 kcal at 25°C. The heat of formation of the ethyl radical as determined here [ΔH_f°(Et)=25.3±1.2 kcal] indicates the C–H bond energy in ethane to be 97.7±1.2 kcal. These values agree well with independent estimates.