Trimethylaluminium
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| Trimethylaluminium | |
|---|---|
| Chemical name | Trimethylaluminium |
| Chemical formula | C6H18Al2 |
| Molecular mass | 144.18 g/mol |
| Melting point | 15.0 °C |
| Boiling point | 125.0 °C |
| Density | 0.752 g/cm3 |
| CAS number | 75-24-1 |
| SMILES | |
| Disclaimer and references | |
Trimethylaluminium(sometimes called trimethyl aluminium) is a pyrophoric colorless liquid. It give a white smoke when the vapor is allowed to contact air. The liquid spontaneously bursts into flames if exposed to air and reacts explosively with water. The molecule is dimeric, with the formula (Al(CH3)3)2, abbreviated (AMe3)2 or TMA.
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[edit] Structure and bonding
Aluminium atom bonded to three methyl groups, each via a pair of electrons, is surrounded by six electrons. As suggested by the octet rule, aluminium is much more stable when surrounded by eight electrons. Consequently, the molecule is dimeric with the Al centers joined together by sharing two methyl groups. (AMe3)2 is analogous to diborane with respect to bonding. As with diborane, the constituents of the dimer are held together by a 3-center-2-electron bond: the shared methyl groups create a bridge between the two aluminium atoms, with each bridging methyl group contributing two electrons to the bridge bond. The Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14A, respectively.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref>
The carbon atoms of the bridging methyl groups are each surrounded by five neighbors, three hydrogen atoms and two aluminium atoms. The methyl groups interchange readily intramolecularly and intermolecularly. This electron-deficient situation causes TMA to be very reactive toward any substance towards Lewis bases, i.e. any substance that provide basic electron-pairs. For example upon treatment with AlCl3, TMA forms (AMe2Cl)2.
[edit] Synthesis and applications
TMA is prepared via a two-step process that can be summarized as follows:
- 2Al + 6 CH3Cl + 6 Na → (Al(CH3)3)2 + 6 NaCl
TMA is mainly used for the production of methylaluminoxane, an activator for alkene polymerization catalysts. TMA is also employed as a methylation agent and for growing semiconductors crystals. In the laboratory it is used for the preparation of Tebbe's reagent, which is itself used for the methylenation of the carbonyl group of esters and ketones. TMA is often released from sounding rockets as a tracer in studies of upper atmospheric wind patterns.
TMA forms a complex with the tertiary amine DABCO [1], which is safer to handle that TMA itself.<ref>Biswas, K.; Prieto, O.; Goldsmith, P. J.Woodward, S. Remarkably Stable (Me3Al)2DABCO and Stereoselective Nickel-Catalyzed AlR3 (R = Me, Et) Additions to Aldehydes Angewandte Chemie International Edition Volume 44, Pages 2232 - 2234 (2005).</ref>
In combination with (C5H5)2ZrCl2, the (CH3)2Al-CH3 adds "across" alkynes to give vinyl aluminum species that are useful in organic synthesis.<ref>* Negishi, E.; Matsushita, H. Palladium-Catalyzed Synthesis of 1,4-Dienes by Allylation of Alkenyalane: α-Farnesene [1,3,6,10-Dodecatetraene, 3,7,11-trimethyl-]" Organic Syntheses, Collected Volume 7, p.245; Vol. 62, p.31.</ref>
[edit] References
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