Thermal unimolecular decomposition of 1,1-difluorocyclobutane

Abstract

The thermal decomposition of 1,1-difluorocyclobutane has been studied in the gas phase in the temperature range 444–516°C at pressures of 6–8 Torr. There is a homogeneous first order decomposition pathway to yield ethylene and 1,1-difluoroethylene which is probably a true unimolecular reaction. The Arrhenius equation obtained for this process for the pressure range covered was log k/s^–1= 15.60 ± 0.13 –(289 740 ± 1820) J mol^–1/RT ln 10 and the Arrhenius parameters are likely to be very close to the infinite pressure values. Rate constants have been determined at 480.4°C at various pressures down to 8 × 10^–3 Torr, i.e., well into the fall-off region and the results obtained compared with a theoretical curve calculated on the basis of RRKM theory. The experimental points for k/k_∞ are shifted to lower pressures when compared with data for cyclobutane itself, as theory predicts. Some preliminary remarks about the energetics of the decomposition are made on the basis of a comparison of this work with studies on cyclobutane and octafluorocyclobutane.

Another decomposition pathway to yield hydrogen fluoride and 2-fluorobuta-1,3-diene takes place largely by a surface reaction which exhibits auto-catalysis. Provided reaction vessels were “aged” this heterogeneous pathway could be largely suppressed in the initial stages of decomposition. There is evidence that the remaining reaction to give these products was a homogeneous elimination process, which constituted ≈ 20 % of the overall decomposition.