Reactions of group 3 metal alkyls in the gas phase. Part 8.—Homogeneous thermal unimolecular elimination of ethylene from triethylaluminium

Abstract

The kinetics of the homogeneous gas-phase elimination of ethylene from triethylaluminium have been studied in the presence of excess propene in the temperature range 426–488 K. The observed first-order rate constants k₀ based on the loss of ethyl groups fit the Arrhenius relationship, log k₀/s^–1= 10.93 ± 0.13 –(30.05 ± 0.27)/θ, where θ= 4.58 × 10^–3T in units of kcal mol^–1[= 10.93 ± 0.13 –(125.7 ± 1.1)/19.15 × 10^–3T in units of kJ mol^–1]. These activation parameters are compatible with the 4-centre polar transition state proposed for olefin eliminations from other Group 3 metal alkyls, and the effect of β-substitution on the rate of elimination is additive. The activation energy for the back-addition of ethylene to dialkylaluminium hydride is estimated to be 4.9 kcal mol^–1.