Valence-bond isomer chemistry. Part 10. Kinetics and thermodynamics of the thermal gas-phase interconversion of hexakis(pentafluoroethyl)benzene and its para-bonded (‘Dewar’) isomer

Abstract

The gas-phase interconversion of hexakis(pentafluoroethyl)benzene (1) and its para-bonded isomer (2) is a clean reversible isomerisation at 7–60 mmHg and 457–602 K. Measurements of the equilibrium constant give ΔH^⊖= 37.6 ± 0.3 kJ mol^–1 and ΔS^⊖= 68.2 ± 0.5 J K^–1 mol^–1 for reaction (1), error limits being standard errors. The reaction at 457–525 K shows reversible first-order kinetics with the Arrhenius equations (i) and (ii) for the forward and backward reactions, respectively. The reaction is completely unaffected by packing the vessel with log₁₀k₁/s^–1=(16.25 ± 0.12)–(186.6 ± 1.1) kJ mol^–1/RT In 10 (i), log₁₀k_–1/s^–1=(12.99 ± 0.16)–(151.7 ± 1.5) kJ mol^–1/RT In 10 (ii) glass tubes or by adding but-2-ene to the system, and it is concluded that the interconversions are unimolecular. The reaction probably involves a biradical intermediate, and the rate-determining activated complex is similar in its structure to the para-bonded isomer rather than the benzene.