Trimethylaluminium

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Trimethylaluminium
Trimethylaluminium-3D-balls.png
Identifiers
CAS number 75-24-1 YesY
PubChem 16682925
ChemSpider 10606585 YesY
Jmol-3D images Image 1
Properties
Molecular 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) [1][2]
Solubility in water reacts
Vapor pressure 1.2 kPa (20 °C)
9.24 kPa (60 °C)[1]
Viscosity 1.12 cP (20 °C)
0.9 cP (30 °C)
Thermochemistry
Specific
heat capacity
C
155.6 J/mol·K[2]
Std molar
entropy
So298
209.4 J/mol·K[2]
Std enthalpy of
formation
ΔfHo298
−136.4 kJ/mol[2]
Gibbs free energy ΔG -9.9 kJ/mol[2]
Hazards
GHS pictograms The flame pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)[1]
GHS signal word Danger
GHS hazard statements H250, H260, H314[1]
GHS precautionary statements P222, P223, P231+232, P280, P370+378, P422[1]
EU classification Highly Flammable FCorrosive C
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 −17.0 °C (1.4 °F; 256.1 K) [1]
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

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[edit]

Al2Me6 exists mostly as a dimer at room temperature and pressure,[3] 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.[4] 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.[3] VSEPR Theory predicts and electron diffraction confirms[5] that it has a trigonal planar (threefold) symmetry, as observed in BMe3.

Synthesis and applications[edit]

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.[6]

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.[7]

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[edit]

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.

References[edit]

  1. ^ a b c d e f Sigma-Aldrich Co., Trimethylaluminum. Retrieved on 2014-05-05.
  2. ^ a b c d e http://chemister.ru/Database/properties-en.php?dbid=1&id=3290
  3. ^ a b Carlsson, J.; Gorbatkin, S.; Lubben, D.; Greene, J. E. (1991). "Thermodynamics of the homogeneous and heterogeneous decomposition of trimethylaluminum, monomethylaluminum, and dimethylaluminumhydride: Effects of scavengers and ultraviolet-laser photolysis". Journal of Vacuum Science and Technology B 9 (6): 2759–2770. doi:10.1116/1.585642. 
  4. ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5. 
  5. ^ Almennin, A.; Halvorse, S.; Haaland, A. (2005). "Gas Phase Electron Diffraction Investigation of Molecular Structures of Trimethylaluminum Monomer and Dimer". Acta Chemica Scandinavica 44 (15): 2232–2234. doi:10.3891/acta.chem.scand.25-1937. 
  6. ^ Biswas, K.; Prieto, O.; Goldsmith, P. J.; Woodward, S. (2005). "Remarkably Stable (Me3Al)2 · DABCO and Stereoselective Nickel-Catalyzed AlR3 (R = Me, Et) Additions to Aldehydes". Angewandte Chemie International Edition 44 (15): 2232–2234. doi:10.1002/anie.200462569. PMID 15768433. 
  7. ^ Negishi, E.; Matsushita, H. (1984), "Palladium-Catalyzed Synthesis of 1,4-Dienes by Allylation of Alkenyalane: α-Farnesene [1,3,6,10-Dodecatetraene, 3,7,11-trimethyl-]", Org. Synth. 62: 31 ; Coll. Vol. 7: 245 

External links[edit]