Dissociation of 2-methylbut-1-ene and the resonance energy of the 2-methyl allyl radical

Abstract

The pyrolysis of 2-methylbut-1-ene has been investigated over the temperature range 671–722 K and pressures between 50 and 500 Torr. Values for the limiting high pressure rate constant of the dissociation reaction CH₃CH₂C(Me)CH₂→·CH₃+·CH₂C(Me)CH₂(1), have been obtained by monitoring the methane produced in the subsequent abstraction reaction ·CH₃+ C₅H₁₀→CH₄+·C₅H₉(2), and assuming that rates of the addition and the abstraction reaction between methyl and 2-methylbut-1-ene are the same. These yielded the rate expression log(k₁/s^–1)_∞=(16.6 ± 0.4)–(71000 ± 800)/θ where θ=2.303RT/cal mol^–1(1 cal = 4.18 J). Surface effects which influenced the rate of formation of methane at lower pressures became less important as the pressure was raised so that the effect on (k₁)_∞ was negligible. The experimental activation energy yields ΔH^°_f= 30.0 ± 1.0 kcal mol^–1 and a resonance energy of 12.6 ± 1.5 kcal mol^–1 for the 2-methyl allyl radical at 298 K. This value for the resonance energy agrees exactly with that obtained previously for the allyl radical itself indicating that the substituent on the central carbon atom of the radical has no measurable effect on the resonance energy. Results obtained previously for the 3,3-dimethyl allyl radical have been reviewed in the light of recent thermochemical data.