Activation energies of hydrogen atom abstractions from alkanes by free radicals

Abstract

Activation energies for the gas phase reaction X + RH = HX + R where X is an atom or free radical and RH is an alkane conform to equation (i) where σ*_XCH2 is the Taft parameter for the group XCH₂ in aliphatic systems undergoing reaction in aqueous solution at 25 °C E= 12·2 – 7·2σ*_XCH2+ 0·55ΔH+ΔH²/(40 + 2·2|ΔH|)(i). Calculated activation energies for X = Cl and particularly for X = CF₃ differ significantly from the experimental values, and possible explanations have been given. With substrates (HY) other than alkanes, thermochemical data and Arrhenius parameters are less accurately known, but within experimental error the activation energies are generally consistent with the above equation, indicating that the electronegativity of Y has little influence on the activation energy. For the substrates H₂S and HCl, the experimental activation energies are significantly lower than the calculated values.