Thermodynamics of Iodine Addition to Ethylene, Propylene, and Cyclopropane

Abstract

The gas‐phase equilibrium in the systems $$\begin{array}{c}{\mathrm{C}}_2{\mathrm{H}}_4+{\mathrm{I}}_2\rightleftharpoons {\mathrm{CH}}_2\mathrm{I}–{\mathrm{CH}}_2\mathrm{I},\\ \mathrm{propylene}+{\mathrm{I}}_2\rightleftharpoons {\mathrm{CH}}_3\mathrm{CHI}–{\mathrm{CH}}_2\mathrm{I},\end{array}$$ have been studied in the temperature range 365° to 476°K using a new technique for the determination of di‐iodides. In addition, existing data are re‐examined for the gas system: $$\mathrm{cyclopropane}+{\mathrm{I}}_2\rightleftharpoons {\mathrm{CH}}_2{\mathrm{ICH}}_2{\mathrm{CH}}_2\mathrm{I}.$$ Using other available data on the ethylene system which with ours cover a range of 200°K it is possible to obtain for 1,2‐C₂H₄I₂: C_p⁰ = 19.2±2 eu, S_p⁰ = 83.3±0.5 eu, ΔH_f⁰ = 15.9±0.2 kcal/mole. For 1,2‐C₃H₆I₂: C_p⁰ = 24.1±3 eu, S_p⁰ = 94.6±1.0 eu, ΔH_f⁰ = 8.6±0.4 kcal/mole. For 1,3‐C₃H₆I₂, the values of C_p⁰ and S_p⁰ are estimated by additivity rules to be 24.7 eu±1.0 eu and 93.0±1.0 eu, respectively. Based on these estimates and the measured ΔF we find ΔH_f⁰ = 10.6±0.5 kcal/mole. A possibility of slightly lower values for 1,3‐diiodopropane is discussed with reference to corrections due to the kinetics of the reaction system as well as the conformations of the molecule. For use with additivity rules, the following partial contributions are assigned for the C–I bond, C–(I) (H)₂(C) group and C–(I) (H) (C)₂ group respectively: S⁰ = 24.9, 42.4, 22.6 eu; and ΔH_f⁰ = 14.1 7.95, 10.7 kcal/mole. The group additivity rule is found to be valid for di‐iodides to ±0.5 eu for S⁰ and to ±0.5 kcal/mole for ΔH_f⁰. On the basis of this rule, thermodynamic values are assigned to a number of alkyl iodides and their C–I bond dissociation energies derived relative to the parent hydrocarbons.