Kinetic study of the mechanism of the low-temperature pyrolysis of vinyl bromide

The pyrolysis of vinyl bromide has been examined in the temperature range 637-733 K and at pressures from 6 to 86 kPa. The yields of the major hydrocarbon products, C2H2, C2H4, and 1,3-C4H6, are second order in vinyl bromide over the entire range of temperatures investigated. At the higher temperatures, initiation by molecular elimination of HBr dominates, while at lower temperatures a free radical initiation channel becomes increasingly important. Our data for the overall process leading to HBr fit the relation In(k) = (30.7 +/- 4.8) - ((26.6 +/- 3.3) x 10(3))/T, with the rate constant in the units L mol(-1) s(-1), indicating an activation energy of 220 kJ mol(-1) +/- 12% for the HBr elimination. A simple Arrhenius extrapolation is close to previous results at temperatures from 800 to over 2000 K. The combination of our data and the earlier measurements of the HBr elimination is reasonably represented by In(k) = 37 - (3 x 10(4))/T. Our data suggest that the free radical pathway is disproportionation rather than unimolecular cleavage of the C-Br bond, a situation analogous to that in the low-temperature thermal decomposition of ethylene. Kinetic analysis indicates that the activation energy of this new free radical initiation channel is approximately 150 kJ mol-', much less than the C-Br bond energy.