The rate of the gas phase iodination of cyclobutane. The heat of formation of the cyclobutyl radical

The gas phase iodination of cyclobutane was studied spectrophotometrically in a static system over the temperature range 589° to 662°K. The early stage of the reaction was found to correspond to the general mechanism where the Arrenius parameters describing k1 are given by log k1/M−1 sec−1 = 11.66 ± 0.11 – 26.83 ± .31/θ, θ = 2.303RT in kcal/mole. The measured value of E1, together with the fact that E−1 = 1 ± 1 kcal/mole, provides ΔH(c-C4H7.) = 51.14 ± 1.0 kcal/mole, and the corresponding bond dissociation energy, D(c-C4H7H) = 96.8 ± 1.0 kcal/mole. A bond dissociation energy of 1.8 kcal/mole higher than that for a normal secondary CH bond corresponds to one half of the extra strain energy in cyclobutene compared to cyclobutane and is in excellent agreement with the recent value of Whittle, determined in a completely different system. Estimates of ΔH and entropy of cyclobutyl iodide are in very good agreement with the equilibrium constant K12 deduced from the kinetic data. Also in good agreement with estimates of Arrhenius parameters is the rate of HI elimination from cyclobutyl iodide.

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