Very low‐pressure pyrolysis (VLPP) of pentynes. III. Pent‐2‐yne. Heat of formation and resonance stabilization energy of the 3‐methylpropargyl radical

Abstract

AbstractThe thermal unimolecular decomposition of pent‐2‐yne has been studied over the temperature range of 988–1234 K using the technique of very low‐pressure pyrolysis (VLPP). The main reaction pathway is C4C5 bond fission producing the resonance‐stabilized 3‐methylpropargyl radical. There is a concurrent process producing molecular hydrogen and penta‐1,2,4‐triene presumably via the intermediate formation of cis‐penta‐1,3‐diene. The 1,4‐hydrogen elimination from cis‐penta‐1,3‐diene is the rate‐determining step in the molecular pathway. This is supported by an independent VLPP study of cis‐ and trans‐penta‐1,3‐diene. RRKM calculations show that the experimental rate constants for CC bond fission are consistent with the following high‐pressure rate expression at 1100 K: where θ = 2.303RT kcal/mol and the A factor was assigned from the results of shock‐tube studies of related alkynes. The activation energy leads to ΔH[CH3CCĊH2] = 70.3 and DH[CH3CCCH2H] = 87.4 kcal/mol. The resonance stabilization energy of the 3‐methylpropargyl radical is 10.6 ± 2.5 kcal/mol, which is consistent with previous results for this and other propargylic radicals.