KINETICS AND THERMOCHEMISTRY OF THE REACTION CL + CYCLOPROPANE REVERSIBLE HCL + CYCLOPROPYL - HEAT OF FORMATION OF THE CYCLOPROPYL RADICAL

The bimolecular rate constant for the reaction of chlorine atoms with cyclopropane, Cl + c-C3H6 ⇄ HCI + c-C3H5 (1, -1), was measured at 298 K using the very low pressure reactor (VLPR). The rate constant was found to be k1298 = (1.21 ± 0.05) × 10-13 cm3 molecule-1 s-1. The equilibrium constant K1 = k1/k-1 was observed at 298 K to be 0.91 ± 0.1. This gives a heat of formation of the cyclopropyl radical ΔHr°298 (cyclopropyl) = 66.9 ± 0.25 kcal mol-1 and a C-H bond dissociation energy of BDE (c-C3H5-H) = 106.3 ± 0.25 kcal mol-1. This is the highest C-H bond energy for any saturated hydrocarbon. It is reconciled with a model for the cyclopropane ring which associates an excess strain energy of about 13 kcal with each π-bond center in the ring, external and internal. Such a strain model is also shown to explain the lack of olefinic conjugation between cyclopropane and external unsaturation. Copyright © 1979, American Chemical Society. All rights reserved.