Dissociation of 1,3-hexadiene and the resonance energy of the pentadienyl radical

Abstract

Measurements of the rate at which methane is formed in the pyrolysis of 1,3-hexadiene at temperatures over the range 694–759 K and pressures between 25 and 200 Torr show that the dissociation reaction CH₃CH₂CHCHCHCH₂→ CH₃·+·CH₂CHCHCHCH₂(1) is a homogeneous, first-order reaction under these conditions and the rate expression is log (k₁/s^–1)=(15.92 ± 0.17)–(66 390 ± 840)/θ where θ= 2.303RT/cal mol^–1(1 cal = 4.18 J).

The experimental activation energy yields for the pentadienyl radical, ΔH^°_f= 45.9 ± 1.0 kcal mol^–1 at 298 K and a resonance energy of 18.5 ± 1.5 kcal mol^–1, assuming a value of 98.2 kcal mol^–1 for the primary C—H bond dissociation energy in alkanes.