Reactions of group III metalalkyls in the gas phase. Part 3.—Kinetics of the intramolecular elimination of but-1-ene from di-n-alkyl-n-butyl aluminium

Abstract

The gas phase kinetics and the mechanistics of the unimolecular elimination of but-1-ene from a mixture of methyldi-n-butylaluminium and n-butyldimethylaluminium have been studied in the presence of excess ethylene in the temperature range 401–478 K. The observed rate constants for the apparently homogeneous elimination reaction yield (with standard errors) log k(s^–1)=(10.85 ± 0.18)–(27.80±0.36)/θ, where θ equals 4.58 × 10^–3T in kcal mol^–1[=(10.85 ± 0.18)–(116.3±1.5)/19.15 × 10^–3T in kJ mol^–1] resulting in a negative intrinsic entropy of activation of 13.0 cal K^–1 mol^–1[54.4 JK^–1 mol^–1]. These data when compared with the corresponding activation parameters reported earlier for tri-isobutylaluminium substantiate the concept of a tight polar four-centre cyclic transition state. The apparent stabilizing effect of a methyl group on the positively charged carbon centre in the transition state is only 1.2 kcal mol^–1. Available thermodynamic data are used to derive intrinsic ΔH^°_f-increment values for this class of compounds, resulting in an estimated activation energy for the back reaction, the addition of 1-butene to di-n-alkylaluminium hydride, of ∼5.2 kcal mol^–1.