Triethylaluminium is an organoaluminium compound. It is actually a dimer, so 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.
3D model (JSmol)
|Abbreviations||TEA, TEAl, TEAL|
CompTox Dashboard (EPA)
|Molar mass||228.335 g·mol−1|
|Density||0.8324 g/mL at 25 °C|
|Melting point||−46 °C (−51 °F; 227 K)|
|Boiling point||128 to 130 °C (262 to 266 °F; 401 to 403 K) at 50 mmHg|
|R-phrases (outdated)||R14 R17 R34|
|S-phrases (outdated)||S16 S43 S45|
|Flash point||−18 °C (0 °F; 255 K)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Structure and bondingEdit
The compound is a dimer of triethylaluminium. One pair of ethyl groups bridge 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 reactionsEdit
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:
- 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
- Al2Et6 + 6 HX → 2 AlX3 + 6 EtH
For this reaction, even weak acids can be employed such as terminal acetylenes and alcohols.
- Al2Et6 + 2 L → 2 LAlEt3
Precursors to fatty alcoholsEdit
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 polymerizationEdit
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 chemistryEdit
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 –5105.70 ± 2.90 kJ/mol (–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. 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.
- Mission Status Center, June 2, 2010, 1905 GMT, SpaceflightNow, accessed 2010-06-02, Quotation: "The flanges will link the rocket with ground storage tanks containing liquid oxygen, kerosene fuel, helium, gaserous nitrogen and the first stage ignitor source called triethylaluminum-triethylborane, better known as TEA-TAB."
"Gulbrandsen Chemicals, Metal Alkyls: Triethylaluminum (TEAl)". Gulbrandsen. Retrieved December 12, 2017.
Triethylaluminum (TEAl) is a pyrophoric liquid...
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- M202A1 Flame Assault Shoulder Weapon (Flash), inetres.com
- Encyclopedia of Explosives and Related Items, Vol.8, US Army