The effect of fluorine as a substituent on bond dissociation energies

Abstract

The kinetics of the thermal and chemically activated elimination of HF from 1,1,1-trifluoroethane was analyzed resulting in DH₂₉₈₋ (CH₃-CF₃) = 101.2±1.1 kcal mol�¹ and ΔH°[subscript f](CH₃-CF₃,g,298) = -178.6±1.5 kcal mol�¹. The C-C bond dissociation energy turned out to be stronger than that predicted by group additivity for a redistribution reaction such as: | CF₃CF₃+ CH₃CH₃ -> 2CH₃CF₃ (1) | A value of ΔH°[subscript r](1,g,298) = -16±1.0 kcal mol�¹ was obtained although one expects reactions such as (1) to be thermosneutral. The deviation from additivity may be explained by taking into account interactions of the dipoles associated with the CF₃ and CH₃ moieties. Such dipole-dipole interactions would predict that | CF₃CF₃+ CH₂ICH₂I -> 2CF₃CH₂I (2) | CF₃CF₃+ CH₂BrCH₂Br -> 2CF₃CH₂Br (3) | CF₃CF₃+ CH₂ClCH₂Cl -> 2CF₃CH₂Cl (4) | ΔH°[subscript r] for the series of reactions (1)-(4) should approach zero. Previous work had shown that ΔH°[subscript r](2,g,298) = -3.8 kcal mol�¹ and this study was undertaken to determine ΔH°[subscript r] for (3) and (4). ...