The influence of orbital asymmetry on the kinetics of the gas-phase reactions of ozone with unsaturated compounds

Abstract

The relative-rate method was used to obtain room temperature rate constants for the gas-phase reactions of ozone with selected chlorinated alkenes, methylene-substituted cycloalkanes and monoterpenes. Measurements were carried out at 298±2 K and 760±10 Torr. The following rate constants, in units of 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹, were obtained: 2.79±0.32 (methylenecyclopropane), 19.3±2.6 (methylenecyclobutane), 89.5±8.6 (methylenecyclopentane), 28.2±3.6 (methylenecyclohexane), 43.8±5.4 (1-chloro-3-methyl-2-butene), 3.71±0.49 (1-chloro-2-methyl-2-propene), 2.42±0.57 (3-chloro-1-butene), 22.9±1.7 (1-chloro-2-butene), 0.45±0.05 (camphene) and 23.5±2.7 (β-pinene). These rate parameters are interpreted in terms of frontier orbital theory and a correlation with calculated orbital energies is investigated. Scatter in the relationship is examined in terms of the asymmetry of the highest occupied molecular orbital of the alkenes. Differences in the magnitude of the orbital coefficients at either end of the reacting double bond are shown to be consistent with a direct retardation of reaction rate, consistent with the production of a cyclic intermediate via a concerted process. Geometric and spatial requirements of the ozone–alkene reaction mechanism are discussed.