KINETICS OF THE REACTIONS OF UNSATURATED HYDROCARBON FREE-RADICALS (VINYL, PROPARGYL, AND ALLYL) WITH MOLECULAR BROMINE

The kinetics of the reactions of three unsaturated free radicals (vinyl, propargyl, and allyl) with molecular bromine have been studied by using a tubular reactor coupled to a photoionization mass spectrometer. The radicals were homogeneously generated by the pulsed photolysis of precursor molecules at 193 nm. The subsequent decays of the radical concentrations were monitored in time-resolved experiments as a function of Br2 concentration to obtain the rate constants of these Br-atom metathesis reactions. Rate constants were measured as a function of temperature to obtain Arrhenius parameters. The following rate constant expressions were obtained (units of the preexponential factor are cm3 molecule-1 s-1 and activation energies are kJ mol-1; the temperature range covered in each study is also indicated): C2H3 + Br2 {(4.0 +/- 0.7) x 10(-11) exp((2.4 +/- 1.5)/RT), 297-532 K}, C3H3 + Br2 {(2.8 +/- 0.5) x 10(-12) exp((-2.3 +/- 1.2)/RT), 296-532 K}, and C3H5 + Br2 {(4.8 +/- 0.8) x 10(-12) exp((1.6 +/- 0.8)/RT), 298-532 K}. The kinetics of R + Br2 reactions is reviewed, and the factors governing the reactivity of polyatomic free radicals in R + Br2 reactions are discussed.