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

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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.

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