Lack of kinetic hydrogen isotope effect in acetylene pyrolysis

✍ Scribed by R. P. Durán; V. T. Amorebieta; A. J. Colussi

• Publisher: John Wiley and Sons
• Year: 1989
• Tongue: English
• Weight: 636 KB
• Volume: 21
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.550210909

✦ Synopsis

Second order rate constants for C2H2 or C2D2 polymerizations into vinylacetylene and higher C,H, products have been measured in a static reactor by dynamic mass spectrometry between"770-980 K. They are nearly identical within experimental error (-+50%). It is shown that these results are consistent with the participation of thermally equilibrated vinylidene H2C = C: as a reactive intermediate:

since this assumption only introduces a modest reverse equilibrium isotope effect (K,,/K, ca. 0.48 in this range) into overall rate constants. At the same time they seem to discriminate in general against alternative mechanisms in which the required H-atom transfers take place in rate determining steps. Present evidence, in conjunction with an updated analysis of relevant issues such as experimental and theoretical vs. termochemical estimates of the heat of formation of H2C=C:, the nature of the transition states of singlet vinylidene addition reactions and the likelihood of discrete biradical intermediates in C2H, dimerization, seems to lend further support to the notion that acetylene behaves as a singlet carbene at high temperatures.