Unimolecular Dissociation of Vinylacetylene: A Molecular Reaction

Article abstract of DOI:10.1021/j100314a021

The thermal decomposition of vinylacetylene (C4H4) has been studied in the shock tube with two time-resolved diagnostics, laser-schlieren (2percent and 4percent C4H4-Kr, 1650-2500 K, 110-427 Torr) and time-of-flight mass spectrometry (2percent C4H4-Ne, 1500-2000 K, 150-300 Torr).The time-of-flight mass spectra show dominant products C2H2 and C4H2 with a very consistent 5:1 ratio of C2H2 to C4H2, in essential agreement with earlier shock tube results.The laser-schlieren semilog density gradient profiles are all concave-upward, showing no trace of chain acceleration.Analysis of these profiles also sets the effective heat of reaction between 30 and 50 kcal/mol.Rate constants calculated from the zero-time gradients assuming ΔH⁰₂₉₈ = 40 kcal/mol are fit with a routine RRKM model which indicates a barrier E₀=79.5 +/- 3 kcal/mol.These observations and the time-of-flight product profiles are consistent with the molecular reactions C4H4 --> 2C2H2 (ΔH⁰₂₉₈ = 39 kcal/mol)and C4H4 --> C4H2+H2(ΔH⁰₂₉₈ = 42 kcal/mol).Detailed balance rate constants for 2C2H2 --> C4H4 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 C2H2 dimerization for moderate temperatures.Above 1500 K, the C4H4 dissociation shows significant falloff even for high pressures, and the now dominant C4H2 in C2H2 pyrolysis may then be formed in part through 2C2H2 --> C4H2 + H2.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 C4H2, or by 2,2-elimination of C2H2, followed by rapid isomerization of the vinylidene to a second C2H2.This mechanism is consistent with the observed barrier, provides a rationale for the constancy of the C2H2/C4H2 ratio, and offers a resonable explanation for the magnitude (A factor) of the observed rate constants.