Thermal decomposition of propargyl bromide and the subsequent formation of benzene

Mixtures of 3% C3H3Br, 3% C3H3Br + 5% H2, and 3% C3H3Br + 5% D2, all three containing neon diluent, were analyzed behind reflected shock waves by time-of-flight mass spectrometry to investigate the role of propargyl radical (C3H3) as precursor to benzene formation at high temperatures. The first mixture yields significant concentrations of benzene over the range 1310–1470 K; benzene yield is observed to increase twofold in the second mixture at comparable temperatures. The third mixture reveals a temporal ratio of [HBr]/([DBr] + [Br]) not, vert, similar 1, which is interpreted as evidence of an equal contribution from each of the two initiation reactions: (1) C3H3Br → c-C3H2 + HBr, where c-C3H2 is singlet cyclopropenylidene; and (2) C3H3Br → C3H3 + Br. In the first two mixtures, the major products are C2H2, C4H2, C6H2, C6H6 and HBr. A 2% propyne + neon mixture was also studied over the temperature range 1750–2620 K. The reaction profiles for C3H4, C2H2, C4H2, and C6H6 are modeled satisfactorily with a mechanism in which the dominant channel for propyne dissociation produces c-C3H2 + H2. Reaction pathways involving c-C3H2 insertion into C---H bonds are presented along with an analysis for the important disproportionation step, c-C3H2 + C3H4 → 2C3H3. Inclusion of six steps describing the reactions of bromine containing species to the core propyne decomposition mechanism results in satisfactory fits to the C3H3Br and C3H3Br + H2 experiments. It is concluded that benzene production in these mixtures is best explained by a sequence of reactions initiated by the dimerization of propargyl radicals.