Kinetics of the Gas-Phase Retroene Decomposition of Unsaturated Hydrocarbons. Very Low-Pressure Pyrolysis of Hepta- 1,6-diene and Hepta-1,6-diyne

Article abstract of DOI:10.1021/j100457a006

Studies of the thermal unimolecular decomposition of hepta-1,6-diene (HDE) and hepta-1,6-diyne (HDY) have been carried out at temperatures in the range 794-1225 K by using the technique of very low-pressure pyrolysis (VLPP).The reactions proceed essentially via retroene molecular elimination to yield propylene and buta-1,3-diene in the case of HDE, and allene and but-1-en-3-yne in the case of HDY.High-pressure rate constants, extracted from the VLPP data by means of RRKM theory, yield the following preferred Arrhenius expressions at = 1000 K: log(k/s^-1) = (11.3 +/- 0.4) - (47.1 +/- 1.0)/τ for HDE and log(k/s^-1) = (12.6 +/- 0.4) - (51.7 +/- 1.0)/τ for HDY, where τ = 2.303RT kcal/mol.The A factors were assigned from the results of a conventional static system study of HDE decomposition and a recent shock-tube study of the decomposition of some monoalkynes.The activation energies fall within the expected range, and the value for HDE is in excellent agreement with the static system result.Comparison with the rate data for similar compounds shows that the six-center retroene process is enhanced by allylic and propargylic weakening of the C-H bond which is involved in the 5 -> 1 hydrogen transfer.Alkynes react via the retroene pathway at rates slightly faster than their olefinic counterparts.