Reactions of CF3 radicals with aromatics. Part 4.—Hydrogen abstraction from benzene at high temperatures and the bond dissociation energy D(C6H5—H)

Abstract

The reactions of CF₃ radicals with benzene in the range 100–400°C were studied using the photolysis of hexafluoroacetone as a source of radicals. The addition of CF₃ radicals to the benzene ring becomes reversible above ∼140°C. Below 140°C, pesudo H-abstraction occurs via the reactions CF₃+ C₆H₆→CF₃C₆H₆, CF₃+ CF₃C₆H₆→CF₃H + C₆H₅CF₃, but at higher temperatures, genuine H-abstraction by reaction (3) occurs CF₃+ C₆H₆→CF₃H + C₆H₅(3), and we obtain log k₃(cm³ mol^–1 s^–1)=(11.31±0.28)–(9110±760)/θ, where θ= 2.303 RT cal mol^–1. Combination of E₃ and published data on E_–3, (for the reverse of reaction (3)), gives ΔH^°₃ from which ΔH^°_f(C₆H₅)= 77.9±2.0, D(C₆H₅–H)= 110.2±2.0 kcal mol^–1, both at 298 K. These values are compared with published data and the following are recommended: ΔH^°_f(C₆H₅)= 77.7±2.0 and D(C₆H₅—H)= 110.0±2.0, both in kcal mol^–1.