Shock Tube Study on the Mechanism of Hydrogenation and Pyrolysis of Acetylene

Mixtures of acetylene and deuterium have been subjected to shock heating in a single-pulse shock tube in the temperature range 1000–1600 K. The reaction products were quite the same as those obtained from the pyrolysis of acetylene alone, with a considerable increase in the yields of the minor products. The predominance of 1-buten-3-yne formation and no appreciable dependence of its rate on the concentration of deuterium suggest that the initiation step is the same as that proposed in the pyrolysis of acetylene by itself, viz. 2C2H2→C4H3+H, the initiation step of the bimolecular reaction of C2H2 with D2 not being important. A complementary analysis of the isotopic distributions of 1-buten-3-yne, ethylene, and acetylene shows that hydrogenation also occurs by a free-radical chain mechanism, and the C4H3 radical and H atom generated by the initiation step are mainly responsible for the formation of 1-buten-3-yne and ethylene, respectively. The ethylene formation was of first-order in both acetylene and deuterium, and the second-order rate constant was obtained as k(cm^3 mol^-1 s^-1)=(4.9±1.3)×10^11exp((-34900±600)/RT).