Pyrolysis of but-1-yne and the resonance energy of the propargyl and 3-methylpropargyl radicals

Abstract

The pyrolysis of but-1-yne has been studied at eight temperatures over the range 652–731 K and pressures between 50 and 1200 Torr. Measurements of the rate of formation of methane indicate that at the lower end of the temperature range studied, this product is formed mainly as a result of a bimolecular reaction. We propose the hydrogen transfer reaction 2CH₃CH₂CCH→CH₃CH₂ĊCH₂+ CH₃ĊHCCH (1) as the rate determining bimolecular step, this being followed by the radical decomposition reaction CH₃CH₂ĊCH₂→·CH₃+ CH₂CCH₂(1′) and hydrogen abstraction by the methyl radical so formed.

At higher temperatures the unimolecular dissociation reaction CH₃CH₂CCH →·CH₃+·CH₂CCH (2) contributes significantly to methane formation. The rate coefficient of this reaction is, under the experimental conditions employed, pressure-dependent.

The expressions log₁₀(k₁/dm³ mol^–1 s^–1)= 11.3 ± 0.2 –(47800 ± 600)/θ and log₁₀(k₂/s^–1)_∞= 17.2 ± 0.7 –(74800 ± 2100)/θ have been obtained for the rate constants of the bimolecular and unimolecular reactions, respectively, where θ= 2.303 RT/cal mol^–1. These parameters yield values of 9.2 ± 2.2 and 8.8 ± 2.2 kcal mol^–1 for the resonance energies of the propargyl and 3-methylpropargyl radicals, respectively.

Earlier results obtained from the pyrolysis of but-1-ene and its derivatives have been re-examined in the light of the present findings. Only in the case of 2-methylbut-1-ene is there any indication of the occurrence of a bimolecular hydrogen-transfer reaction leading to the formation of methane.