Kinetics of the Vapor Phase Reaction of Cyclopropane with Iodine

The thermal and photoreaction (light absorbed by iodine) between gaseous iodine and cyclopropane has been studied, and found to lead to the reversible formation of 1,3 diiodopropane. A slow iodine catalyzed isomerization of cyclopropane to propylene is the only important side reaction. The reaction is essentially homogeneous. For the thermal reaction, in the range 245–280°C, the rate expression is $$\frac{\mathrm{d(}{\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{I}}_2)}{\mathrm{dt}}{\mathrm{=k}}_1\left[({\mathrm{C}}_3{\mathrm{H}}_6\mathrm{)(}{\mathrm{I}}_2{)}^{\frac{1}{2}}{\mathrm{-K}}_6\frac{({\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{I}}_2)}{{\mathrm{(I}}_2{)}^{\frac{1}{2}}}\right].$$ For the photoreaction, in the range 180–230°C, the rate expression is $$\mathrm{d(}{\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{I}}_2{\mathrm{)/dt=k}}_3{I}_{\mathrm{abs}}^{\frac{1}{2}}({\mathrm{C}}_3{\mathrm{H}}_6\mathrm{).}$$ The respective values of k₁ and k₃ were found to be $${\text{1.17×10}}^{13}{e}^{\text{−35,230/RT}}(\mathrm{mole}/\mathrm{cc}{)}^{-\frac{1}{2}}\sec {.}^{\text{−1}}$$ and $${\text{8.5×10}}^6{e}^{\text{−18,800/RT}}(\mathrm{Einstein}/\mathrm{cc}{)}^{-\frac{1}{2}}\sec {.}^{-\frac{1}{2}}.$$ The mechanism has been established, the rate determining step being the reaction I+C₃H₆→ICH₂CH₂CH₂, with rate constant $${\text{7.36×10}}^{12}{e}^{\text{−17,280/RT}}(\mathrm{mole}/\mathrm{cc}{)}^{\text{−1}}\sec {.}^{\text{−1}}.$$ The equilibrium constant, K_e, in the temperature range 255°‐280°C, is some $${\text{11.7e}}^{\text{−17,200/RT}}(\mathrm{mole}/\mathrm{cc}\mathrm{).}$$ Hence for the reaction C₃H₆+I₂→C₃H₆I₂, ΔE=‐17,200 calories per mole. If the energy of the carbon‐iodine linkages is taken as 43,000 calories, that of the carbon‐carbon linkage in the cyclopropane ring is 32,900 calories per mole.