Dissociation of 3-methylpenta-1, 4-diene and the resonance energy of the pentadienyl radical

Abstract

From measurements of the initial rate of formation of methane in the pyrolysis of 3-methylpenta-1, 4-diene at temperatures over the range 653–716 K and pressures between 15 and 200 Torr, methane has been shown to be formed by an homogeneous, first-order process. Examination of the other products of pyrolysis shows the most reasonable mode of formation of primary methane to be [graphic omitted] followed by [graphic omitted]. The rate expression for reaction (i) is log(k_i/s^–1)=(15.36 ± 0.1)–(65040 ± 300)/θ where θ= 2.303 RT/cal mol^–1(1 cal = 4.18 J) and this leads to D[H—CH(C₂H₃)₂]= 76.4 kcal mol^–1, which by comparison with the value of 95.0 for the secondary C—H bond dissociation energy in alkanes leads to a figure of 18.6 ± 1.5 kcal mol^–1 for the resonance stabilisation energy of the pentadienyl radical.