Triethylaluminium

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Triethylaluminium is an organoaluminium compound. Despite its name, the formula for this compound is Al₂(C₂H₅)₆, shortened to Al₂Et₆ (Et = ethyl). This volatile, colorless liquid is highly pyrophoric, igniting immediately upon exposure to air. It is normally stored in stainless steel containers either as a pure liquid or as a solution in hydrocarbon solvents such as hexane, heptane, or toluene. Triethylaluminium is mainly used as a co-catalyst in the industrial production of polyethylene, polypropylene and for the production of medium chain alcohols.

Video 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.

Maps Triethylaluminium

Synthesis and reactions

Triethylaluminium can be formed via several routes. The discovery of an efficient route was significant technologically. The multistep process uses aluminium metal, 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:

3 Al₂Cl₃Et₃ + 9 Na -> 2 Al₂Et₆ + 2 Al + 9 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

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Applications

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 - 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)₆

Al₂(OC₈H₁₇)₆ + 6 H₂O -> 6 C₈H₁₇OH + 2 "Al(OH)₃"

Reagent in organic and organometallic chemistry

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

${\displaystyle {\ce {{1/2Al2Et6}+ HCN ->}}\ {\tfrac {1}{n}}{\ce {[Et2AlCN]}}_{n}+{\ce {C2H6}}}$

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 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 triethylaluminium and the hexane vapors.

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See also

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

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References

Source of the article : Wikipedia