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 6 37-733 K and at pressures from 6 to 86 kPa. The yields of the major hydroca rbon products, C2H2, C2H4, and 1,3-C4H6, are second order in vinyl bromide over the entire range of temperatures investigated. At the higher temperatu res, initiation by molecular elimination of HBr dominates, while at lower t emperatures a free radical initiation channel becomes increasingly importan t. 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 t o previous results at temperatures from 800 to over 2000 K. The combination of our data and the earlier measurements of the HBr elimination is reasona bly represented by In(k) = 37 - (3 x 10(4))/T. Our data suggest that the fr ee radical pathway is disproportionation rather than unimolecular cleavage of the C-Br bond, a situation analogous to that in the low-temperature ther mal decomposition of ethylene. Kinetic analysis indicates that the activati on energy of this new free radical initiation channel is approximately 150 kJ mol-', much less than the C-Br bond energy.