Radiation-induced debromination of sym-dibromotetrafluoroethane: Effect of fluorine atoms on the stability of monobromo substituted ethyl radicals

Abstract

The kinetics of the γ radiation induced free-radical chain reactions in solutions of sym-C2F4Br2 in cyclohexane (RH) have been studied in the temperature range 150–250°C. Bromotetrafluoroethyl radicals were produced by reaction of radiolytically generated cyclohexyl radicals with sym-C2F4Br2. The kinetics of the following reactions have been investigated: C2F4Br + RH→C2F4BrH + R (3), C2F4Br→C2F4+ Br. (4) The following rate expression is obtained: log10[(k4/k3)/mol dm–3]= 7.86 ± 0.13–[(54 600 ± 1260) J mol–1/2.3 RT]. From the activation parameter of the rate expression and the assumptions that E3=E(C2F5+ RH) and D(CF2BrCF2—Br)=D(C2F5—Br) then E4= 81.50 ± 2.85 and E–4= 8.7 kJ mol–1.

Arguments based on bond energy terms are presented to show that fluorine substitution affects the C—C bond energy in normal alkanes and that the difference in activation energy in bromine atom elimination reactions from C2F4Br and C2H4Br radicals cannot be correlated with the π bond destabilization effect of fluorine atoms in C2F4 as measured by the difference in enthalpies of polymerization of C2F4 and C2H4.

It is also suggested that in the case of C2F4Br, fluorine atoms affect the stability at the radical centre.