Reactions of group III metal alkyls in the gas phase. Part V. The kinetics of the intramolecular elimination of isobutene from tri-isobutylgallium

Abstract

The gas-phase kinetics and mechanistics of the concerted unimolecular elimination of isobutene from tri-isobutylgallium have been studied in presence of excess of ethylene. For temperatures between 416 and 500 K the unimolecular rate constant k₁ for the 4-centre elimination process can be represented (with standard errors) by the Arrhenius relationship: log k₁/s^–1=(11·57 ± 0·29)–(30·40 ± 0·58)/θ where θ= 2·303 × 10^–3RT with T in K and R in kcal mol^–1[=(11·57 ± 0·29)–(127·2 ± 2·4)/θ in kJ mol^–1]. The activation energy for the back reaction, the addition of isobutene to di-isobutylgallium hydride is estimated at 10 ± 2 kcal mol^–1(41·8 kJ mol^–1), based on an estimated enthalpy change of 20·5 kcal mol^–1 for the title reaction. The reaction is apparently homogeneous and essentially free from side reactions. These data when compared with the activation parameters for the similar reactions with AlBuⁱ₃, AlBuⁿ₃, and BBuⁱ₃ further substantiate the suggested concept of a reasonably tight, quadrupolar four-centre cyclic transition state involved in these reactions.