Thermal unimolecular isomerization of bicyclo [2.1.0]pent-2-ene

Abstract

The gas-phase unimolecular isomerization of bicyclo [2.1.0]pent-2-ene to cyclopentadiene has been followed spectrophotometrically in the temperature range 26.0–108.4° and at total pressures from ∼0.2 to ∼760 torr. The rate constants fit the Arrhenius equation: log [k/sec^–1]= 14.21 ± .20–(26.88 ± .32)/θ; θ= 2.303 RT kcal/mole. These parameters, as well as those for similar systems, are consistent with a transition state whose energy can be calculated from a biradical model. The parameters for this disrotatory process, when analyzed in contrast with those for the conrotatory isomerization of substituted cyclobutenes, yield a quantitative measure of the energy difference between orbital symmetry allowed and non-allowed processes.