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.