Kinetic isotope effect in the thermolysis of methylenecyclobutane

The intramolecular kinetic isotope effect for the thermolysis of equilibrated methylenecyclobutane-d2 was investigated at 5150C as a function of pressure. A high-pressure value of k/sub H/k/sub D/ (ethylene/ethylene-d2) = 0.9 was obtained at 13 cm of N2 pressure. This value decreased to 0.86 at 70 μm total pressure. No intermolecular kinetic isotope effect was measured for the formation of ethylene from labeled and unlabeled methylenecyclobutane. The pressure and temperature dependence of the intramolecular kinetic isotope effect was used as evidence in establishing the inverse nature of the effect. The isotope effect observed was explained in terms of competing equilibrium and kinetic isotope effects in which the equilibrium isotope effects dominate. It was concluded on the bases of these results that an acyclic intermediate is involved in the fragmentation of methylenecyclobutane to ethylene and allene. The results also support the notion that deuterium prefers to accumulate at the methylene group with the greatest p character in the carbon--hydrogen bond. 1 figure, 4 tables