Trimethyl aluminum

Structural formula
General
Surname	Trimethyl aluminum
Molecular formula	C 3 H 9 Al; C 6 H 18 Al 2 ( dimer );
Brief description	pyrophoric, colorless liquid
External identifiers / databases
EC number	200-853-0
ECHA InfoCard	100,000,776
PubChem	16682925
Wikidata	Q416321
properties
Molar mass	72.08 g mol −1 ; 144.18 g mol −1 (dimer);
Physical state	liquid
density	0.75 g cm −3
Melting point	15 ° C
boiling point	125 ° C
Vapor pressure	0.3 hPa (20 ° C); 0.7 hPa (30 ° C); 60 hPa (50 ° C); 115 hPa (65 ° C);
solubility	violent reaction with water; soluble in toluene , hexane and heptane ;
safety instructions
H and P phrases	H: 250-260-314
	EUH: 014
	P: 210-231 + 232-280-305 + 351 + 338-370 + 378-422
Thermodynamic properties
ΔH f 0	−136.4 kJ / mol
	As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Trimethylaluminum ( TMA or TMAl ) is a pyrophoric colorless liquid and belongs to the aluminum alkyls . TMA has a boiling point of 127 ° C and a very high vapor pressure . In the air it forms white smoke, ignites spontaneously and reacts explosively with water . Therefore this compound must be stored and handled under argon or nitrogen . A solution in toluene , hexane or heptane is often used as the reagent .

properties

Chemical properties

Since the aluminum atom is only surrounded by three methyl groups, it has only six valence electrons . In order to fulfill the octet rule and to achieve a noble gas configuration , the dimer of the TMA is formed in which the two aluminum cores are bridged by two methyl groups . The structure is similar to that of the diborane molecule, with the aluminum atoms corresponding to the boron atoms and the methyl groups to the hydrogen atoms . Like the hydrogen nuclei in diborane, the bridging methyl groups form a 2-electron-3-center bond , to which each methyl group provides two electrons . Although the carbon atoms of the bridging methyl groups are each surrounded by five neighbors, they only have eight electrons in the outermost shell. This situation causes an electron deficit which makes TMA extremely reactive. TMA is a Lewis acid and reacts with a variety of substances. It very quickly exchanges methyl groups for other groups or atoms ( e.g. halogens ) that contain more electrons. This property makes TMA an excellent methylating agent .

use

TMA is used in the production of methylaluminoxane , a cocatalyst for Ziegler-Natta polymerizations, as an activator and as a methylating agent. In the semiconductor industry it is u. a. used in the MOVPE process for the production of aluminum-containing compound semiconductors such as AlGaAs and AlN .

The Tebbe reagent , which is used in the methylenation of esters and ketones , is also made from TMA . TMA reacts with titanium tetrachloride to form the Reetz reagent (according to Manfred T. Reetz ), with which it is possible to alkylate the α-carbon of a keto group to a tertiary carbon, even with sterically demanding residues .

safety instructions

TMA is very corrosive. The compound ignites spontaneously in air and reacts explosively with water. Therefore, work must always be carried out in a dry protective gas atmosphere.

accident

In the early morning of October 17, 2016, a truck loaded with trimethylaluminum crashed near Bornstedt on BAB 2 . A fire broke out and the motorway had to be completely closed for hours for extinguishing and rescue work.

Individual evidence

↑ ^a ^b Data sheet trimethylaluminum (PDF) from Strem, accessed on December 25, 2012.
↑ ^a ^b ^c ^d data sheet trimethylaluminum from Sigma-Aldrich , accessed on April 24, 2011 ( PDF ).
↑ ^a ^b ^c ^d ^e ^f ^g ^h Entry on trimethylaluminum in the GESTIS material database of the IFA , accessed on October 29, 2018(JavaScript required) .
↑ Not explicitly listed in Regulation (EC) No. 1272/2008 (CLP) , but with the specified labeling it falls under the group entry aluminum alkyls in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016 . marketer can use the harmonized classification and labeling expand .
↑ David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Standard Thermodynamic Properties of Chemical Substances, pp. 5-25.
↑ Manfred T. Reetz, Wilhelm F. Maier: " tert -Alkylierung von Ketonen und Aldehyden", in: Angew. Chem. , 1978 , 90 , p. 50; doi : 10.1002 / anie.19780900110 .
^ Serious accident on the A2 in Volksstimme. Accessed October 17, 2016.

[strem-1] Data sheet trimethylaluminum (PDF) from Strem, accessed on December 25, 2012.

[sigma-2] ta sheet trimethylaluminum from Sigma-Aldrich , accessed on April 24, 2011 ( PDF ).

[GESTIS-3] ↑ ^a ^b ^c ^d ^e ^f ^g ^h Entry on trimethylaluminum in the GESTIS material database of the IFA , accessed on October 29, 2018(JavaScript required) .

[CLP_100.240.763-4] Not explicitly listed in Regulation (EC) No. 1272/2008 (CLP) , but with the specified labeling it falls under the group entry aluminum alkyls in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016 . marketer can use the harmonized classification and labeling expand .

[CRC90_5_25-5] David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Standard Thermodynamic Properties of Chemical Substances, pp. 5-25.

[6] Manfred T. Reetz, Wilhelm F. Maier: " tert -Alkylierung von Ketonen und Aldehyden", in: Angew. Chem. , 1978 , 90 , p. 50; doi : 10.1002 / anie.19780900110 .

[7] Serious accident on the A2 in Volksstimme. Accessed October 17, 2016.