Aluminum alkyl reactivity

H. G. Wissink
Akzo Nobel, Netherlands

Chemical & Engineering News (3 Mar 1997) Vol. 75, No. 9, pp. 6.

Metal alkyls and their solutions are used in a variety of industrial chemical processes such as polymerization (Ziegler-Natta process), oligomerization (Ziegler chemistry), alkylation, and stereochemical synthesis.

Aluminum alkyls are highly reactive materials. They are compatible and miscible in all proportions with saturated aliphatic and aromatic hydrocarbons such as pentane, hexane, heptane, and toluene. However, they combine rapidly with compounds containing oxygen. Reaction of dialkylaluminum hydrides and ethers is particularly hazardous. In "Organoaluminum Compounds" by J. Mole and E. A. Jeffery, this was shown to result in ether cleavage accompanied by gas evolution. Tetrahydrofuran (THF) is sometimes used as a solvent to dilute metal alkyls such as diisobutylaluminum hydride (DIBAL-H) and triethylaluminum (TEAL). TEAL may contain small amounts of diethylaluminum hydride (DEAL-H) as a by-product. The Akzo Nobel safety research department in Deventer, Netherlands, has investigated the safety aspects of DIBAL-H/THF and TEAL/THF.

Calorimeter tests proved that mixing DIBAL-H and THF results in an exothermic complexation risking substantial heating of the mixture, which can be followed by a fast thermal decomposition when the temperature is too high. Therefore, it is essential to have sufficient cooling capacity during the preparation of these mixtures.

Furthermore, thermal analysis studies showed that DIBAL-H/THF solutions undergo thermal decomposition with gas formation even at ambient conditions. Because of pressure buildup, storage of these mixtures is not recommended.

Solutions should be used promptly and unused material destroyed. Even storage in a glass bottle is hazardous; during tests at 55 C, the pressure developed in a few weeks by a 10% solution of DIBAL-H in THF in a metal cylinder was more than sufficient to shatter a sealed glass bottle.

Solutions of TEAL, containing various amounts of diethylaluminum hydride (up to 1.8% expressed as AlH3), in THF are thermally stable up to 200 C.

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