Triethylaluminium

Triethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name the compound has the formula Al₂( C₂H₅)₆ (abbreviated as Al₂Et₆ or TEA). This colorless liquid is pyrophoric. It is an industrially important compound, closely related to trimethylaluminium.

Structure and bonding

The structure and bonding in Al₂R₆ and diborane are analogous (R = alkyl). Referring to Al₂Me₆, the Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14 Å, respectively. The Al center is tetrahedral. The carbon atoms of the bridging ethyl groups are each surrounded by five neighbors: carbon, two hydrogen atoms and two aluminium atoms. The ethyl groups interchange readily intramolecularly. At higher temperatures, the dimer cracks into monomeric AlEt₃.

Synthesis and reactions

Triethylaluminium can be formed via several routes. The discovery of an efficient route was a significant technological achievement. The multistep process uses aluminium, hydrogen gas, and ethylene, summarized as follows:
:2 Al + 3 H₂ + 6 C₂H₄ → Al₂Et₆
Because of this efficient synthesis, triethylaluminium is one of the most available organoaluminium compounds.

Triethylaluminium can also be generated from ethylaluminium sesquichloride (Al₂Cl₃Et₃), which arises by treating aluminium powder with chloroethane. Reduction of ethylaluminium sesquichloride with an alkali metal such as sodium gives triethylaluminium:
:6 Al₂Cl₃Et₃ + 18 Na → 3 Al₂Et₆ + 6 Al + 18 NaCl

Reactivity

The Al–C bonds of triethylaluminium are polarized to such an extent that the carbon is easily protonated, releasing ethane:
:Al₂Et₆ + 6 HX → 2 AlX₃ + 6 EtH
For this reaction, even weak acids can be employed such as terminal acetylenes and alcohols.

The linkage between the pair of aluminium centres is relatively weak and can be cleaved by Lewis bases (L) to give adducts with the formula AlEt₃L:
:Al₂Et₆ + 2 L → 2 LAlEt₃

Applications

Precursors to fatty alcohols

Triethylaluminium is used industrially as an intermediate in the production of fatty alcohols, which are converted to detergents. The first step involves the oligomerization of ethylene by the ''Aufbau'' reaction, which gives a mixture of trialkylaluminium compounds (simplified here as octyl groups):
:Al₂(C₂H₅)₆ + 18 C₂H₄ → Al₂(C₈H₁₇)₆
Subsequently, these trialkyl compounds are oxidized to aluminium alkoxides, which are then hydrolysed:
:Al₂(C₈H₁₇)₆ + 3 O₂ → Al₂(OC₈H₁₇)₆
:Al₂(OC₈H₁₇)₆ + 6 H₂O → 6 C₈H₁₇OH + 2 Al(OH)₃

Co-catalysts in olefin polymerization

A large amount of TEAL and related aluminium alkyls are used in Ziegler-Natta catalysis. They serve to activate the transition metal catalyst both as a reducing agent and an alkylating agent. TEAL also functions to scavenge water and oxygen.

Reagent in organic and organometallic chemistry

Triethylaluminium has niche uses as a precursor to other organoaluminium compounds, such as diethylaluminium cyanide:
:$\ce\ \tfrac 1 n \ce_n + \ce$

Pyrophoric agent

Triethylaluminium ignites on contact with air and will ignite and/or decompose on contact with water, and with any other oxidizer—it is one of the few substances sufficiently pyrophoric to ignite on contact with cryogenic liquid oxygen. The enthalpy of combustion, Δ_cH°, is (–22.36 kJ/ g). Its easy ignition makes it particularly desirable as a rocket engine ignitor. The SpaceX Falcon 9 rocket uses a triethylaluminium- triethylborane mixture as a first-stage ignitor.

Triethylaluminium thickened with polyisobutylene is used as an incendiary weapon, as a pyrophoric alternative to napalm; e.g., in the M74 clip holding four rockets for the M202A1 launchers. In this application it is known as TPA, for ''thickened pyrotechnic agent'' or ''thickened pyrophoric agent''. The usual amount of the thickener is 6%. The amount of thickener can be decreased to 1% if other diluents are added. For example, ''n''-hexane, can be used with increased safety by rendering the compound non-pyrophoric until the diluent evaporates, at which point a combined fireball results from both the triethylaluminium and the hexane vapors.''Encyclopedia of Explosives and Related Items'', Vol.8, US Army The M202 was withdrawn from service in the mid-1980s owing to safety, transport, and storage issues. Some saw limited use in the Afghanistan War against caves and fortified compounds.

See also

* Triethylborane, used as an ignitor in the Pratt & Whitney J58 turbojet/ramjet engines.
* Trimethylaluminium

References

{{Aluminium compounds

Organoaluminium compounds
Rocket propellants
Rocket fuels
Incendiary weapons
Pyrophoric materials
Ethyl compounds