Triethylaluminium

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Triethylaluminium
Al2Et6wedge.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 C12H30Al2
Molar mass 228.33 g mol−1
Appearance colorless liquid
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
Hazards
R-phrases R14 R17 R34
S-phrases S16 S43 S45
Main hazards pyrophoric
NFPA 704
Flammability code 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g., propane Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity code 3: Capable of detonation or explosive decomposition but requires a strong initiating source, must be heated under confinement before initiation, reacts explosively with water, or will detonate if severely shocked. E.g., fluorine Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Flash point −18 °C (0 °F; 255 K)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Triethylaluminium (TEA) 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. 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[edit]

Monomeric form found in the gas phase

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.[1]

Synthesis and reactions[edit]

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[edit]

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[edit]

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[edit]

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[edit]

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[edit]

References[edit]

  1. ^ Gábor Vass, György Tarczay, Gábor Magyarfalvi, András Bödi, and László Szepes “HeI Photoelectron Spectroscopy of Trialkylaluminum and Dialkylaluminum Hydride Compounds and Their Oligomers” Organometallics, 2002, volume 21, pp. 2751–2757. doi:10.1021/om010994h
  2. ^ a b Michael J. Krause, Frank Orlandi, Alfred T. Saurage, Joseph R. Zietz Jr. “Aluminum Compounds, Organic” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a01_543
  3. ^ Krause, M. J; Orlandi, F; Saurage, A T.; Zietz, J R, "Organic Aluminum Compounds" Wiley-Science 2002.
  4. ^ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-527-29390-2
  5. ^ Wataru Nagata and Yoshioka Mitsuru (1988), "Diethylaluminum Cyanides", Org. Synth. ; Coll. Vol. 6: 436 
  6. ^ TEA Material Safety Data Sheet, accessed March 27, 2007
  7. ^ 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."
  8. ^ M202A1 Flame Assault Shoulder Weapon (Flash), inetres.com
  9. ^ Encyclopedia of Explosives and Related Items, Vol.8, US Army