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.