The gas-phase thermal isomerization of some tri- and tetra-fluoro-dewar benzenes

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Abstract
The kinetics of the gas-phase thermal isomerization of 1,2,3,5- and 1,2,4,5-tetrafluorobicyclo[2.2.0]hexa-2,5-diene into the corresponding tetrafluorobenzenes have been studied over a temperature range of 320–390 K and a pressure range of 100–2 000 N m–2. The reactions are homogeneous, obey a first-order law, and are unimolecular. The temperature dependence of the rate constants is given by the Arrhenius equations (1) for 1,2,3,5-tetrafluorobicyclo[2.2.0]hexa-2,5-diene, and (2) for 1,2,4,5-tetrafluorobicyclo[2.2.0]hexa-2,5-diene. log (k/s–1)= 13.73 ± 0.30 –116.9 ± 2(kj mol –1)//2.303 RT (1), log (k/s–1)= 10.42 ± 0.99 –96.4 ± 7(kj mol –1)//2.303 RT (2) The rate constants of the thermal isomerization of 1,3,5-, 1,2,5-, and 2,3,5-trifluorobicyclo[2.2.0]hexa-2,5-diene into corresponding trifluorobenzenes have been measured at 353 K, and the following values of – log (k/s–1) were obtained : 2.3 ± 0.2, 2.4 ± 0.2 and 2.6 ± 0.2, respectively. The influence of fluorine substitution on the thermal stability of the Dewar structure is discussed.

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