The Monomer—Dimer Equilibria of liquid Aluminum Alkyls : V. Tri-n-Propylaluminum, Tri-n-Butylaluminum and Tri-n-Octylaluminum: The effect of chain length on the equilibria

Abstract

The monomer—dimer equilibria of three liquid aluminum n-alkyls have been studied in n-hexadecane solution by a thermochemical method applied earlier to triethylaluminum (TEA) and triisobutylaluminum (TiBA). The values obtained for the heat and entropy of dissociation of 1 mole of dimer are: for tri-n-propylaluminum (TnPA), 15.41 ± 0.20 kcal and 33.48 ± 0.60 cal · K−1; for tri-n-octylaluminum (TnOA), 14.68 ± 0.12 kcal and 33.76 ± 0.34 cal · K−1. Equilibrium constants and degrees of dissociation of the neat alkyls are tabulated over wide temperature ranges, as are degrees of dissociation at various mole fractions in hexadecane. The effects of chain length on the monomer—dimer equilibria are discussed. Plots of ΔH0d and ΔS0d vs. chain length are used to obtain estimated values for certain other alkyls. Heats of complexation of monomeric R3Al with Et3N are calculated from literature values for the heats of reaction of (primarily dimeric) R3Al with Et3N and plotted against chain length. The monomer—dimer equilibria of the aluminum n-alkyls in mesitylene and in benzene are estimated from experimental data for TEA in mesitylene. The effect of solvent type on the degree of dissociation of R3Al in dilute solution is discussed. Heats of complexation of monomeric R3Al with aromatic hydrocarbons are estimated and compared with corresponding values for stronger bases. The experimental and estimated Kd values are compared with values derived from cryoscopic molecular weight determinations. Applications of degrees and heats of dissociation of aluminum alkyls are discussed.