Thermal unimolecular decomposition of β-propiolactone (oxetan-2-one)

Abstract

The thermal decomposition of oxetan-2-one has been investigated in the gas phase in the temperature range 262–322 °C. The reaction, which yields ethylene and carbon dioxide, is homogeneous and obeys first-order kinetics. There is a minor heterogeneous isomerization to yield acrylic acid. The decomposition is almost certainly a unimolecular process, which in the pressure range studied, ⩽ 6 Torr, is in the fall-off region. High-pressure rate constants were determined by extrapolation using three procedures that are discussed. The preferred method using plots of k^–1 against p^–0.61 yielded the Arrhenius equation logk/s^–1= 14.86±0.30 – 180.46±3.20 kJ mol^–1/RT ln10. Theoretical (RRKM) calculations are close to the experimental fall-off curves on the basis of the strong-collision assumption or with 〈ΔE〉_d(stepladder model) 24 kJ mol^–1, from which it is hardly distinguishable. Certainly it is necessary to assume very efficient intermolecular energy transfer. Comparison with the result from other studies suggests a concerted decomposition via an activated complex with zwitterionic character rather similar to those involved in cyclobutanone decompositions.