Methyl lithium
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| Surname | Methyl lithium | |||||||||||||||
| Molecular formula | CH 3 Li | |||||||||||||||
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| Molar mass | 21.98 g mol −1 | |||||||||||||||
| Physical state |
firmly |
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| solubility |
violent reaction with water |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||||||||
Methyl lithium is a chemical compound from the group of organolithium compounds with the empirical formula CH 3 Li. It is a highly reactive compound that can only be used in aprotic solvents such as diethyl ether , tetrahydrofuran or 1,2-dimethoxyethane . It is used as a reagent in organic and organometallic syntheses . The compound adopts an oligomeric structure, both in solution and in the solid state. Reactions of methyllithium require anhydrous conditions because the compound reacts very strongly with water. This also applies to the presence of oxygen and carbon dioxide .
history
The first synthesis of organolithium compounds (including methyllithium) succeeded Wilhelm Schlenk in 1917 .
Extraction and presentation
In the direct synthesis, methyl bromide is mixed with a suspension of lithium in diethyl ether.
Lithium bromide forms a complex compound with methyllithium. Most of the commercially available methyl lithium consists of such a complex compound. "Halogen-free" methyl lithium is obtained by reacting lithium with methyl chloride. The lithium chloride precipitates in the reaction because it does not form such a strong complex with methyllithium. The filtrate consists of relatively pure methyl lithium.
properties
Physical Properties
By X-ray crystallography and NMR spectroscopy , two different structures could be detected methyl lithium. On the one hand, tetrameric associations with a (LiCH 3 ) 4 - heterocubane structure are formed. The tetramers here have ideal Td symmetry and form a body-centered cubic lattice with a distance between the lithium atoms of 268 pm and a C-Li distance of 231 pm. The approximate position of the H atoms could be obtained by refining the structure. The proof of pyramidal methyl groups (C 3 v symmetry) was also successful . The carbon atom of a methyl group has short distances to the Li atoms of its own tetramer, but also only a 5–10 pm longer distance to the tetramer adjacent on a spatial diagonal of the body-centered cubic unit cell. This leads to the coordination number 7 for carbon. The extremely low volatility and infusibility of methyllithium is a direct result of this three-dimensional crosslinking.
Chemical properties
Methyl lithium is both strongly basic and very nucleophilic because the carbon carries the negative charge. It is therefore particularly reactive with electron and proton suppliers. Thus, THF , typically a chemically inert solvent , at least attacked at room temperature of MeLi. Water and alcohols react violently. Most reactions involving methyllithium are carried out at temperatures below room temperature. Although MeLi can be used for deprotonation, n -butyllithium is more commonly used because it is cheaper, more reactive and less dangerous. A pyrolysis of methyllithium above 200 ° C leads to the main products methane and lithium hydride , with further hydrocarbons such as ethene , ethane , propene and isobutene are formed.
use
Methyl lithium is used as a reagent for alkylation and metalation.
It is to be regarded as the synthetic equivalent of a methyl anion. For example, ketones react in a two-step process to form tertiary alcohols:
Non-metal halides can be converted to methyl compounds with MeLi:
In such reactions, however, methyl magnesium halides ( Grignard reagents ) are more often used, which are less dangerous than MeLi and can be prepared just as efficiently and conveniently.
Individual evidence
- ↑ Data sheet methyllithium, 1-2M in ether from AlfaAesar, accessed on March 26, 2010 ( PDF )(JavaScript required) .
- ↑ a b Data sheet Methyllithium solution (1.6 M in diethyl ether) from Sigma-Aldrich , accessed on April 10, 2011 ( PDF ).
- ↑ Parts of the labeling of hazardous substances relate to the hazards caused by the solvent.
- ↑ W. Schlenk, Johanna Holtz: About the simplest organometallic alkali compounds. In: Reports of the German Chemical Society. 50, 1917, pp. 262-274, doi : 10.1002 / cber.19170500142 .
- ↑ Entry on methyllithium. In: Römpp Online . Georg Thieme Verlag, accessed on October 10, 2017.
- ↑ MJ Lusch, MV Phillips, WV Sieloff, GS Nomura, HO House: Preparation of Low-Halide Methyllithium In: Organic Syntheses . 62, 1984, p. 101, doi : 10.15227 / orgsyn.062.0101 ; Coll. Vol. 7, 1990, p. 346 ( PDF ).
- ^ C. Elschenbroich: Organometallchemie , 2006, Wiley-VCH: Weinheim. ISBN 978-3-527-29390-2 .
- ↑ Baran, JR; Lagow, RJ: Some observations on the pyrolysis of methyllithium in J. Organomet. Chem. 427 (1992) 1-7.
- ^ E-EROS Encyclopedia of Reagents for Organic Synthesis , 1999-2013, John Wiley and Sons, Inc., entry for Methyllithium, accessed October 10, 2017 .