Abbreviations TEA Density 832 kg/m³ Appearance colorless liquid Melting point −46 °C (−51 °F; 227 K) | Formula (C2H5)3Al Main hazards pyrophoric Molar mass 228.34 g·mol−1 | |
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Boiling point 128 to 130 °C (262 to 266 °F; 401 to 403 K) at 50 mmHg | ||
Triethylaluminium (TEAL) 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. TEAL is mainly used as a co-catalyst in the industrial production of polyethylene, polypropylene and for the production of medium chain alcohols.
Contents
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 → Al2Et6Because 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 NaClReactivity
The Al-C bond is polarized such that triethylaluminium is easily protonated, releasing ethane:
Al2Et6 + 6 HX → 2 AlX3 + 6 EtHFor 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 LAlEt3Applications
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)6Subsequently, 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
TEAL has niche uses as a precursor to other organoaluminium compounds, such as diethylaluminium cyanide:
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.