|Jmol-3D images||Image 1|
|Molar mass||144.18 g/mol|
|Melting point||15 °C|
|Boiling point||125 °C|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Trimethylaluminum is the chemical compound with the formula Al2(CH3)6, abbreviated as Al2Me6, (AlMe3)2 or the abbreviation TMA. This pyrophoric, colorless liquid is an industrially important organoaluminum compound. It evolves white smoke (aluminum oxides) when the vapor is released into the air.
Structure and bonding
Al2Me6 exists mostly as a dimer at room temperature and pressure, analogous in structure and bonding to diborane. As with diborane, the molecules are connected by 2 3-center-2-electron bonds: the shared methyl groups bridge between the two aluminum atoms. The Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14 Å, respectively. The carbon atoms of the bridging methyl groups are each surrounded by five neighbors: three hydrogen atoms and two aluminum atoms. The methyl groups interchange readily intramolecularly and intermolecularly.
3-Centered-2-electron bonds are an utterance of "electron-deficient" molecules and tend to undergo reactions with Lewis bases that would give products consisting of 2-centered-2-electron bonds. For example upon treatment with amines gives adducts R3N-AlMe3. Another reaction that gives products that follow the octet rule is the reaction of Al2Me6 with aluminum trichloride to give (AlMe2Cl)2.
The monomer species AlMe3, which has an aluminum atom bonded to three methyl groups, occurs at high temperature and low pressure. VSEPR Theory predicts and electron diffraction confirms that it has a trigonal planar (threefold) symmetry, as observed in BMe3.
Synthesis and applications
TMA is prepared via a two-step process that can be summarized as follows:
- 2 Al + 6 CH3Cl + 6 Na → Al2(CH3)6 + 6 NaCl
TMA is mainly used for the production of methylaluminoxane, an activator for Ziegler-Natta catalysts for olefin polymerisation. TMA is also employed as a methylation agent. Tebbe's reagent, which is used for the methylenation of esters and ketones, is prepared from TMA. TMA is often released from sounding rockets as a tracer in studies of upper atmospheric wind patterns.
In combination with Cp2ZrCl2 (zirconocene dichloride), the (CH3)2Al-CH3 adds "across" alkynes to give vinyl aluminum species that are useful in organic synthesis in a reaction known as carboalumination.
Semiconductor grade TMA
TMA is the preferred metalorganic source for metalorganic vapour phase epitaxy (MOVPE) of aluminum-containing compound semiconductors, such as AlAs, AlN, AlP, AlSb, AlGaAs, AlInGaAs, AlInGaP, AlGaN, AlInGaN, AlInGaNP, etc. Criteria for TMA quality focus on (a) elemental impurites, (b) oxygenated and organic impurities.
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