Unimolecular dissociation of vinylacetylene: A molecular reaction

The thermal decomposition of vinylacetylene (C/sub 4/H/sub 4/) has been studied in the shock tube with two time-resolved diagnostics, laser-schlieren (2% and 4% C/sub 4/H/sub 4/-Kr, 1650-2500 K, 110-427 Torr) and time-of-flight mass spectrometry (2% C/sub 4/H/sub 4/-Ne 1500-2000 K, 150-300 Torr). The time-of-flight mass spectra show dominant products C/sub 2/H/sub 2/ and C/sub 4/H/sub 2/ with a very consistent 5:1 ratio of C/sub 2/H/sub 2/ to C/sub 4/H/sub 2/. The laser-schlieren semilog density gradient profiles the effective heat of reaction between 30 and 50 kcal/mol. Rate constants calculated from the zero-time gradients assuming ..delta..H/sup 0//sub 298/ = 40 kcal/mol are fit with a routine RRKM model which indicates a barrier E/sub 0/ = 79.5 +/- 3 kcal/mol. Detailed balance rate constants for 2C/sub 2/H/sub 2/ ..-->.. C/sub 4/H/sub 4/ are in good agreement with the extensive previous data on the second-order acetylene reaction, confirming that vinylacetylene is a direct and dominant product of C/sub 2/H/sub 2/ dimerization for moderate temperatures. Above 1500 K, the C/sub 4/H/sub 4/ dissociation shows significant falloff even for high pressures, and the now dominant C/sub 4/H/sub 2/ in C/sub 2/H/sub 2/ pyrolysis may then be formed in part through 2C/sub 2/H/sub 2/ ..-->.. C/sub 4/H/sub 2/ + H/sub 2/. It is proposed that vinylacetylene dissociates as a substituted ethylene, either by 1,1-elimination of molecular hydrogen, leaving vinylidene acetylene which rapidly isomerizes to C/sub 4/H/sub 2/, or by 2,2-elimination of C/sub 2/H/sub 2/, followed by rapid isomerization of the vinylidene to a second C/sub 2/H/sub 2/.