Triethylaluminium

Structure and bonding

The compound is a dimer of triethylaluminium. One pair of ethyl groups bridge the two Al centers, and four are terminal ligands. The two bridging carbon centres are five-coordinate. The bonding is reminiscent of that of diborane, involving 3-centred, 2-electron bonds. As in trimethylaluminium, triethylaluminium is structurally fluctional resulting in rapid interchange of the terminal and bridging ethyl groups. At higher temperatures, the dimer cracks into monomeric AlEt₃.

Synthesis and reactions

TEAL can be formed via several routes. The discovery of an efficient route was significant technologically. The multistep process can be summarized in the following reaction:

2 Al + 3 H₂ + 6 C₂H₄ → Al₂Et₆

Because of this efficient synthesis, triethylaluminium is one of the most available organoaluminium compounds.

TEAL 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 TEA:

3 Al₂Cl₃Et₃ + 9 Na → 2 Al₂Et₆ + 2 Al + 9 NaCl

Reactivity

The Al-C bond is polarized such that triethylaluminium 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 bases (L) to give adducts with the formula AlEt₃L:

Al₂Et₆ + 2 L → 2 LAlEt₃

Applications

TEAL 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 – the famed 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)₆Al₂(OC₈H₁₇)₆ + 6 H₂O → 6 C₈H₁₇OH + 2 "Al(OH)₃"

Reagent in organic and organometallic chemistry

TEAL has niche uses as a precursor to other organoaluminium compounds, such as diethylaluminium cyanide:

1 2 Al 2 Et 6 + HCN ⟶   1 n [ Et 2 AlCN ] n + C 2 H 6

Pyrophoric agent

TEAL ignites on contact with air and will ignite and/or decompose on contact with water, and with any other oxidizer. TEA is one of the few substances pyrophoric enough to ignite on contact with cryogenic liquid oxygen. 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 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 TEAL and the hexane vapors.