The monomer−dimer equilibria of liquid aliminum alkyls: III. Trimethylaluminum: the monomer−dimer equilibria of liquid and gaseous trimethylaluminum and triethylaluminum

Abstract

There has been a dichotomy of opinion concerning the values for the heats of dissociation of trimethylaluminum (TMA) and triethylaluminum (TEA) in the liquid phase. It is believd that the result reported herein will enable these differences to be resolved. In the present study, the heat of dissociation (ΔHod of liquid TMA is evaluated as 19.40±0.30-kcal · (mole of dimer)−1 from the previously determined ΔH0d of liquid TEA and new heat of mixing data for TMA−TEA. Essentially the same result is derived from the established ΔH0d of gaseous TMA by application of a thermodynamic relationship between heats of dissociation and heats of vaporization (the “Dissociation−Vaporization Rule”). It is concluded that the experimental ΔH0d values for liquid TEA (16.93 kcal · (mole of dimer)−1) and gaseous TMA (20.40 are thermodynamically consistent. The ΔS0dof liquid TMA is evaluated as 29.3±0.3 cal · K−1 · (mole of dimer)−1. The ΔH0d valuesfor liquid adn gaseous TMA are shown to be thermodynamically consistent with vapor pressure data. Boiling points derived for pure TMA monomer and pure dimer are 8.1±2.1 and 131.95±0.02°C, respectively. Degrees of dissociation of TMA in the pure liquid state and at various mole fractions in aliphatic hydrocarbon solution are tabulated over a wide temperature range. The results indicate that the extent of dissociation of liquid TMA is 0.0047% at 20, 0.053% at 70, and 0.32% at 120°C.The ΔH0d and ΔS0d of gaseous TEA are derived as 18.17±0.30 kcal·(mole of dimer)−1. and 45.51±0.70 cal·K−1·(mole of dimer)−1 from the experimental values for liquid TEA. The ΔH0d adn ΔS0d values for liquid and gaseous TEA are shown to be thermodynamically consistent with vapor pressure data. It is also shown that the values ΔH0d(I)11±3 kcal·(mole of dimer)−1 and ΔH0á(g)10.2±1.0 kcal·(mole of dimer)−1 proposed in the literature are far too low. They are thermodynamically inconsistent with the established ΔH0d(g) for TMA and also with TEA vapor pressure data.