Thermal isomerization of cyclobutenes. Part 19.—3,3,4,4-Tetrafluorocyclobutene

Abstract

The thermal isomerization of 3,3,4,4-tetrafluorocyclobutene has been shown to yield 1,1,4,4-tetrafluorobuta-1,3-diene quantitatively. In the temperature range 306 to 360°C the reaction is a homogeneous first order essentially irreversible reaction which is almost certainly unimolecular. At pressures above 5.3 kN m^–2(∼40 Torr) the rate constant is independent of pressure. High pressure rate constants determined at 10 temperatures in the above range yielded the Arrhenius equation, log k/s^–1= 14.09 ± 0.05 – 200 600 ± 500 J mol^–1/2.303 RT. At 333.7°C the rate constant was determined over a range of pressures from 0.7 to 9000 N m^–2(0.005 to 70 Torr) and a fall-off curve obtained.

The isomerization of this compound is discussed in relation to that of perfluorocyclobutene and perfluoro-1,2-dimethylcyclobutene. Rice–Ramsperger–Kassel–Marcus calculations have been carried out, and found to fit the fall-off curve quite well. In addition, the effect on the calculated fall-off curve of making extreme changes to the vibrational assignment of the activated complex has been investigated. Finally, various computational procedures have been discussed in relation to the calculation of fall-off curves using deconvolution of the experimental results.