Thermal reactions of 1,2-dibromopropane and isomerization of its reaction product

Abstract

The kinetics of competitive four-channel thermal decomposition reactions of 1,2-dibromopropane(1,2-DBP) at 600.0 to 636.2 K and thermal cis-trans isomerization reaction of 1-bromopropene(1-BP) at 620.8 to 753.2 K were studied over the pressure range of 0.17-50.3 Torr. Four monobromopropenes were obtained as main products of 1,2-DBP thermal reaction by molecular hydrogen bromide elimination process or by radical chain process or else by heterogeneous reaction. The product ratios showed a pressure dependence. And the surface condition of reaction vessel also influenced on the product ratio. Inhibition effects of cyclohexene and propene represented that the radical chain process participated in thermal decomposition of 1,2-DBP. Propene appeared to be less effective than cyclohexene in its inhibition effect. Inhibition efficiency of cyclohexene and propene was studied in succession through the cistrans isomerization of 1-BP being one of the products of 1,2-DBP thermal reaction. Both the inhibition effect by cyclohexene or propene and the autocatalytic effect of $O_2$ or HBr showed a radical process as the main mechanism of the cistrans isomerization of 1-BP. In the suppression of the radical process by the inhibitors, the molecular process also contributed to the overall reaction rate. The reactions demonstrated the first order kinetics under both uninhibited and inhibited conditions and could be represented by the expressions $$k_{un}/s^{-1}\,=\,(3.45\pm1.50)\times10^{11}\exp[(-48100\pm2000)/RT]$$ $$k_{inh}/s^{-1}\,=\,(2.98\pm1.40)\times10^{12}\exp[(-55800\pm1800)RT]$$ where $k_{un}$ is the observed rate constant of cis-1-bromopropene(1-$BP_c$) to trans-1-bromopropene(1-$BP_t$) under uninhibited condition at initial pressure of 50 Torr and $k_{inh}$, that under maximal inhibition by cyclohexene at 3 Torr initial pressure of 1-$BP_c$. The rate constants that bromine atom abstrated the allylic hydrogen of reactant(1-BP) and ihibitors,i.e., propene and cyclohexene, w...