Relative rate constants for the reaction of hydrogen atoms with some fluorinated mono-olefins

Abstract

Relative rate constants for the addition of hydrogen atoms to fluorinated ethylenes, propenes and butenes have been determined by a competitive technique. Values relative to C₂H₄= 1.00 are C₂H₃F = 0.44, CH₂CF₂= 0.36, trans-CHFCHF = 0.09, cis-CHFCHF = 0.06, C₂HF₃= 0.05, C₂F₄= 0.08, CH₂FCHCH₂= 0.90, CH₃CFCH₂= 1.3, cis-CH₃CHCHF = 0.16, trans-CH₃ CHCHF = 0.50, CF₃CHCH₂= 0.40, CF₃CFCF₂ < 0.03, CH₂FCH₂CHCH₂= 1.5, C₂F₅CHCH₂= 0.48, CH₃(CF₃)CCH₂= 1.1. In the case of the fluoroethylenes, the rate constants correlate qualitatively with minimum atom localisation energies if a term is included to allow for changes in charge density arising from fluorine substitution. In the case of the fluoro-propenes and -butenes, the introduction of fluorine into the molecule lowers its reactivity relative to the hydrocarbon olefin and this is discussed in terms of the (–I) and (+ M) effects. The results are consistent with an attack at a specific site via a σ-complex transition state.