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

Abstract

T h e thermal unimolecular decomposition of pent-1 -yne has been investigated over the temperature range of 923-1 154 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 C:1-C4 bond fission pathway a t 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: log k (see-') = (12.8 f 0.4) -(57.0 f 2.0)/0 a t 1100 K where 0 = 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 KRKM calculations. T h e results are compared with previous data on the retro-ene decomposition of alkynes.