Reaction of hydrogen bromide with fluoroethylenes in the gas phase. Part 2.—Photoaddition of hydrogen bromide to trifluoroethylene, cis- and trans-difluoroethylene, and vinyl fluoride

Abstract

The rate laws of the photoaddition of hydrogen bromide to C₂F₃H, cis- and trans-C₂F₂H₂, and C₂FH₃ have been determined in the gas phase. The kinetic order n in HBr for the photoaddition is 1.5. The orders in the olefins vary with fluorine substitution : n= 0.5 for C₂F₃H, n=∼0.7 for cis- and trans-C₂F₂H₂ and C₂FH₃ at low pressures of olefin, and n=∼0.6 for C₂F₂H₂ and C₂FH₃ at higher olefin pressures. The light intensity exponent for the HBr+C₂F₃H reaction is 0.52, and 0.6–0.7 for the reaction of HBr with C₂F₂H₂ and C₂FH₃. The kinetic orders vary with changes in the magnitude of the activation energy (E^≠₄) of the decomposition of intermediate radicals. For C₂F₃-HBr, E^≠₄=∼12 kcal/mol, and for both C₂F₂H₂Br and C₂FH₃Br, E^≠₄ 7 kcal/mol. The variation of E^≠₄ is due to the π-destabilization effect of the fluorine atoms and to their combined electron-withdrawing and electron-repelling effects. The various activation energies in the chain sequence account for different kinetically observed termination reactions. The kinetic results and the lack of an inert-gas effect suggest that the intermediate radicals are thermally equilibrated.