Kinetic study of the gas‐phase reaction c‐C5H8 + I2 ⇄ c‐C5H6 + 2HI the heat of formation and the stabilization energy of the cyclopentenyl radical

The gas-phase dehydrogenation of cyclopentene to cyclopentadiene catalyzed by iodine in the range 178–283°C has been found to obey a rate law consistent with the slow rate-determining step, , log [k4/(1 mole−1 sec−1)] = 10.25 ± 0.08 - (12.26 ± 0.18)/θ, where θ = 2.303RT in kcal/mole. Surface effects are not important. This value of E4 leads to a value of DH = 82.3 ± 1 kcal/mole and ΔHf298 = 38.4 ± 1 kcal/mole. From difference in bond strengths in the alkane and the alkene, the allylic resonance stabilization in the cyclopentenyl radical is 12.6 ± 1.0 kcal/mole, in excellent agreement with the value for the butenyl radical. Arrhenius parameters for the other steps in the mechanism are evaluated. The low value of A4 (compared with A4 for cyclopentane) suggests a “tighter” transition state for H-atom abstraction from alkenes than from alkanes.