Very low‐pressure pyrolysis (VLPP) of pentynes. I. Kinetics of the retro‐ene decomposition of pent‐1‐yne

Abstract

AbstractThe thermal unimolecular decomposition of pent‐1‐yne has been investigated over the temperature range of 923–1154 K using the technique of very low‐pressure pyrolysis (VLPP). Under the experimental conditions the reaction proceeds predominantly via a molecular retro‐ene pathway to yield allene and ethylene. There was some evidence for the occurrence of the higher energy C3C4 bond fission pathway at the high end of the temperature range. Interpretation of the data with the aid of RRKM theory and taking into account a decrease in gas‐wall collision efficiency with temperature yields the following high‐pressure rate constant expression for the retro‐ene pathway: at 1100 K where θ = 2.303 RT kcal/mol and the A factor was assigned from the results of shock‐tube studies of similar molecules. These rate parameters are independent of the inclusion of the bond fission pathway in the RRKM calculations. The results are compared with previous data on the retro‐ene decomposition of alkynes.