Reaction of Cyclopropane with Iodine and Some Observations on the Isomerization of Cyclopropane

Abstract

The kinetic and thermodynamic data of Ogg and Priest on the system I₂+cyclopropane⇌ICH₂CH₂CH₂I are reexamined. Using an entropy for ICH₂CH₂CH₂I, estimated from additivity rules of 93.0±1.5 eu, and the experimental value of K_eq, ΔH° is found to be −19.4±0.9 kcal. This gives H_f° (ICH₂CH₂CH₂I) = 8.2 kcal/mole and the use of group additivity rules allow the values of ΔH_f° for EtI and n—C₃H₇I to be fixed at −3.3±0.5 and −8.3±0.5 kcal/mole, respectively. For CH₃I, ΔH_f°=1.6±1 kcal/mole. The kinetic mechanism is also reexamined and from entropy and energy estimates for the free radical species, values of all of the rate constants are evaluated. These are in good agreement with values for similar reactions. It is proposed that the I₂‐catalyzed isomerization of cyclopropane proceeds through the step I+ICH₂CH₂CH₂I→HI+ICH₂–CH=CH₂+I, followed by a rapid, atom‐catalyzed reaction of allyl iodide with HI to give propylene. The rate constants for such a step are also in good agreement with those for similar reactions. Extension of the data to cyclopropane‐propylene isomerization is made and it is shown that the most reasonable path involves the trimethylene radical as an intermediate. Its lifetime is, however, very short∼10⁻¹¹ sec. Slater's model for this reaction is shown to be implausible. These data indicate an activation energy for the cyclization of the trimethylene free radical of about 8 kcal. This is reasonable only if the end CH₂ groups in trimethylene are interacting via a ``bent'' bond. In such case the activation energy arises from the necessity for eclipsed configurations. A similar analysis is made for cyclobutane and the tetramethylene radical with compatible results. For cyclopropane the strain energy is 30 kcal and for cyclobutane 28 kcal.