Kinetic study of the gas‐phase reaction c‐C5H10 + I2 ⇄ c‐C5H8 + 2HI the heat of formation of cyclopentyl radical

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Abstract
The kinetics of the gas‐phase dehydrogenation of cyclopentane to cyclopentene is found to be consistent with a slow attack by an I atom (step 4, text) on cyclopentane in the range 282–382°C. The measured rate constants fit the Arrhenius equation, log k4 = 11.95 ± 0.08 – (24.9 ± 0.23)/θ 1 mole−1 sec−1, where θ = 2.303RT in kcal/mole. This leads to a value of ΔH magnified image = 24.3 ± 1 kcal/mole and a bond dissociation energy DH magnified image = 94.9 ± 1 kcal/mole. The latter value is identical with DH0(i‐Pr‐H) = 95 ± 1 kcal/mole and signifies that cyclopentane and the cyclopentyl radical have the same strain energy. Arrhenius parameters are deduced for all six steps in the reaction mechanism. Surface reactions are shown to be unimportant.Cyclopentyl iodide is an unstable intermediate in the reaction and the rate constant for its bimolecular formation from HI + cyclopentene is found to be log k6 = 8.40 ± 0.29 ‐ (26.9 ± 0.8)/θ 1 mole−1 sec−1. Together with the equilibrium constant, this yields for the unimolecular elimination of HI from cyclopentyl iodide, the rate constant, log k5 = 13.3 ± 0.3 – (42.8 ± 1.2)/θ sec−1.
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