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Thickened pyrophoric agent

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Thickened pyrophoric agent

Triethylaluminium
File:Al2Et6.png
Identifiers
Abbreviations TEA
CAS number 97-93-8 YesY
PubChem 16682930
ChemSpider 10179159 YesY
EC number 202-619-3
Jmol-3D images Image 1
Properties
Molecular formula C6H15Al
Molar mass 114.16 g mol−1
Appearance colorless liquid
Density 0.8324 g/mL at 25 °C
Melting point

-46 °C, 227 K, -51 °F

Boiling point

128-130 °C, 401-403 K, 262-266 °F (at 50 mm Hg)

Hazards
R-phrases R14 R17 R34
S-phrases S16 S43 S45
Main hazards pyrophoric
NFPA 704
4
3
3
W
Flash point −18 °C
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Triethylaluminium[pronunciation?] (TEA) is an organoaluminium compound. 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. TEA is mainly used as a co-catalyst in the industrial production of polyethylene and for the production of medium chain alcohols.

Structure and bonding

Although called triethylaluminium, the compound has a dimeric structure with the formula Al2Et6, where Et is ethyl, CH2CH3. One pair of ethyl groups are bridging 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.[1]

Synthesis and reactions

TEA 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]

2 Al + 3 H2 + 6 C2H4 → Al2Et6

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

TEA 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]

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:[4]

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

TEA 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):[2]

Al2(C2H5)6 + 18 C2H4 → Al2(C8H17)6

Subsequently, these trialkyl compounds are oxidized to aluminium alkoxides, which are then hydrolysed:

Al2(C8H17)6 + 3/2 O2 → Al2(OC8H17)6
Al2(OC8H17)6 + 3/2 H2O → 6 C8H17OH + 2 "Al(OH)3"

Reagent in organic and organometallic chemistry

TEA has niches uses as a precursor to other organoaluminium compounds, such as diethylaluminium cyanide:[5]

0.5 Al2Et6 + HCN → 1/n [Et2AlCN]n + C2H6

Pyrophoric agent

TEA ignites on contact with air and will ignite and/or decompose on contact with water, and with any other oxidizer.[6] 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 heavy-lift rocket uses a triethylaluminium-triethylborane mixture as a first-stage ignitor.[7]

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.[8] 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 TEA and the hexane vapors.[9]

See also

References

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