Thermal reaction of the isopropyl radical with 2,3-dimethylbut-2-ene

Abstract

The CH₃ĊHCH₃ radical (iP˙) initiated homogeneous gas-phase thermal reaction of 2,3-dimethylbut-2-ene has been studied at 490–540 K by means of the thermal decomposition of azoisopropane (AIP). The products were identified using gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS).

The rate constant for the decomposition of AIP: AIP → 2 CH₃ĊHCH₃+ N₂(1) is log(k₁/s^–1)=(16.36 ± 0.49)–(201.5 ± 5.4) kJ mol^–1/θ where θ=RT ln 10.

The rate constant of the H-abstraction yielding the 1,1,2-trimethylallyl radical (TMA˙) relative to the corresponding self-combination was determined for the first time: CH₃ĊHCH₃+(CH₃)₂CC(CH₃)₂→ C₃H₈+(CH₃)₂CC(CH₃)ĊH₂(2), CH₃ĊHCH₃+ CH₃ĊHCH₃→(CH₃)₂CHCH(CH₃)₂(9) log[(k₂/k^1/2₉)/dm^3/2 mol^–1/2 s^–1/2]=(4.99 ± 0.49)–(57.0 ± 5.4) kJ mol^–1/θ. The TMA˙ formed in reaction (2) combines with iP˙ terminally and non-terminally. The ratio of the rate constants was found to be independent of temperature: k₁₀/k₁₁= 2.8 ± 0.1. iP˙ disproportionates with both iP˙ and TMA˙. The disproportionation–combination ratios determined are Δ(iP˙, iP˙)= 0.58 ± 0.06 and Δ(iP˙, TMA˙)= 0.03 ± 0.01.