Triethylaluminium is an organoaluminium compound. It is actually a dimer, so despite its name, the formula for this compound is Al26, shortened to Al2Et6. 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.
Structure and bonding
The compound is a dimer of triethylaluminium. One pair of ethyl groupsbridge the two Al centers, and the other four groups 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
Triethylaluminium can be formed via several routes. The discovery of an efficient route was a significant technological achievement. The multistep process uses aluminium metal, hydrogen gas, and ethylene, summarized as follows: Because of this efficient synthesis, triethylaluminium is one of the most available organoaluminium compounds. Triethylaluminium can also be generated from ethylaluminium sesquichloride, which arises by treating aluminium powder with chloroethane. Reduction of ethylaluminium sesquichloride with an alkali metal such as sodium gives triethylaluminium:
Reactivity
The Al–C bonds of triethylaluminium are polarized to such an extent that the carbon is easily protonated, releasing ethane: 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 to give adducts with the formula AlEt3L:
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 : Subsequently, these trialkyl compounds are oxidized to aluminium alkoxides, which are then hydrolysed:
Triethylaluminium has niche uses as a precursor to other organoaluminium compounds, such as diethylaluminium cyanide:
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, ΔcH°, is . Its easy ignition makes it particularly desirable as a rocket engineignitor. The SpaceXFalcon 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. 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.
