Molecular dissociation of vinylacetylene and its implications for acetylene pyrolysis

Abstract

Laser schlieren-shock tube measurements of the pyrolysis of vinylacetylene (C/sub 4/H/sub 4/, 1-buten-3-yne) show no trace of chain reaction. These also require a heat of reaction for the dissociation process near 40 kcal/mol, and an RRKM fit to the derived rate constants indicates a barrier of 80 +- 3 kcal/mol. Product distributions from this pyrolysis are strongly dominated by C/sub 2/H/sub 2/ and C/sub 4/H/sub 2/, with a remarkably consistent 5-10:1 ratio of C/sub 2/H/sub 2/ to C/sub 4/H/sub 2/. This all suggests the dominant dissociation path for vinylacetylene is the molecular process C/sub 4/H/sub 4/ ..-->.. 2C/sub 2/H/sub 2/(..delta..H/sub 298//sup 0/ approx. 39kcal/mol) with about 20% via C/sub 4/H/sub 4/ ..-->.. C/sub 4/H/sub 2/ + H/sub 2/ (..delta..H/sub 298//sup 0/ approx. 42kcal/mol). Detailed balance rate constants for 2C/sub 2/H/sup 2/ ..-->.. C/sub 4/H/sub 4/ are very close to literature rates for this reaction, confirming vinylacetylene as a major direct product of the homogeneous dimerization of C/sub 2/H/sub 2/ for low to moderate temperatures. At high temperatures/low pressures this will go over to the direct reaction 2C/sub 2/H/sub 2/ ..-->.. C/sub 4/H/sub 2/ + H/sub 2/, and to a much lesser extent 2C/sub 2/H/sub 2/ ..-->.. C/sub 4/H/sub 3/ + more » H. An inverse vinylidene insertion reaction is proposed as the mechanism for both vinylacetylene molecular dissociation channels. 40 refs., 6 figs. « less