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.