Trimethylaluminium

Trimethylaluminium

Names
IUPAC name Trimethylalane
Other names Trimethylaluminium; aluminium trimethyl; aluminium trimethyl
Identifiers
CAS Number	75-24-1 Y
ChemSpider	10606585 Y
Jmol 3D model	Interactive image
PubChem	16682925
InChI InChI=1S/3CH3.Al/h3*1H3; Y Key: JLTRXTDYQLMHGR-UHFFFAOYSA-N Y InChI=1/3CH3.Al/h3*1H3;/rC3H9Al/c1-4(2)3/h1-3H3 Key: JLTRXTDYQLMHGR-MZZUXTGEAJ
SMILES C[Al](C)C
Properties
Chemical formula	C6H18Al2
Molar mass	144.17 g/mol 72.09 g/mol (C3H9Al)
Appearance	Colorless liquid
Density	0.752 g/cm3
Melting point	15 °C (59 °F; 288 K)
Boiling point	125–130 °C (257–266 °F; 398–403 K)
Solubility in water	reacts
Vapor pressure	1.2 kPa (20 °C) 9.24 kPa (60 °C)
Viscosity	1.12 cP (20 °C) 0.9 cP (30 °C)
Thermochemistry
Specific heat capacity (C)	155.6 J/mol·K
Std molar entropy (So298)	209.4 J/mol·K
Std enthalpy of formation (ΔfHo298)	−136.4 kJ/mol
Gibbs free energy (ΔfG˚)	-9.9 kJ/mol
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y verify (what is YN ?)
Infobox references

Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al₂(CH₃)₆ (abbreviated as Al₂Me₆ or TMA) as it exists as a dimer. This colorless liquid is an industrially important compound but must be handled with care due to its pyrophoricity; it evolves white smoke (aluminium 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 aluminium 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 aluminium 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 aluminium trichloride to give (AlMe₂Cl)₂.

The monomer species AlMe₃, which has an aluminium 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.

TMA is also used in semiconductor fabrication to deposit thin film, high-k dielectrics such as Al₂O₃ via the processes of Chemical Vapor Deposition or Atomic Layer Deposition.

TMA forms a complex with the tertiary amine DABCO, which is safer to handle than TMA itself.

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.

The NASA ATREX mission (Anomalous Transport Rocket Experiment) employed the white smoke that TMA forms on air contact to study the high altitude jet stream.

Semiconductor grade TMA

TMA is the preferred metalorganic source for metalorganic vapour phase epitaxy (MOVPE) of aluminium-containing compound semiconductors, such as AlAs, AlN, AlP, AlSb, AlGaAs, AlInGaAs, AlInGaP, AlGaN, AlInGaN, AlInGaNP, etc. Criteria for TMA quality focus on (a) elemental impurities, (b) oxygenated and organic impurities.

External links

• NASA ATREX mission article.