Lack of kinetic hydrogen isotope effect in acetylene pyrolysis

Abstract

Second order rate constants for C_2H_2 or C2_D_2 polymerizations into vinylacetylene and higher C_nH_n 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 H_2C=C: as a reactive intermediate: (i) H-C≡C_H ⇌ H_2C=C: since this assumption only introduces a modest reverse equilibrium isotope effect (K_(iH)/K_(iD) 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. thermochemical estimates of the heat of formation of H_2C=C:, the nature of the transition states of singlet vinylidene addition reactions and the likelihood of discrete biradical intermediates in C_2H_2 dimerization, seems to lend further support to the notion that acetylene behaves as a singlet carbene at high temperatures.