 | |
| Names | |
|---|---|
| IUPAC name Methyllithium | |
| Other names Lithium methanide | |
| Identifiers | |
| |
3D model (JSmol) | |
| 3587162 | |
| ChEBI | |
| ChemSpider |
|
| ECHA InfoCard | 100.011.843 |
| EC Number |
|
| 288 | |
| |
| |
| Properties | |
| CH3Li | |
| Molar mass | 21.98 g·mol−1 |
| Reacts | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | pyrophoric |
| NFPA 704 (fire diamond) | |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Methyllithium is the simplestorganolithium reagent, with the empirical formula LiCH3. Thiss-blockorganometallic compound adopts anoligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used in solution with anether as the solvent, is areagent inorganic synthesis as well asorganometallic chemistry. Operations involving methyllithium require anhydrous conditions, because the compound is highly reactive towardswater.Oxygen andcarbon dioxide are also incompatible with MeLi. Methyllithium is usually not prepared, but purchased as a solution in various ethers.
In the direct synthesis,methyl bromide is treated with a suspension oflithium indiethyl ether.
Thelithium bromide forms a complex with the methyllithium. Most commercially available methyllithium consists of this complex. "Low-halide" methyllithium is prepared frommethyl chloride.[1]Lithium chloride precipitates from the diethyl ether since it does not form a strong complex with methyllithium. The filtrate consists of fairly pure methyllithium. Alternatively, commercial methyllithium can be treated withdioxane to precipitate LiBr(dioxane), which can be removed by filtration.[2] The use of halide-free vs LiBr-MeLi has a decisive effect on some syntheses.[3]
Methyllithium is both stronglybasic and highlynucleophilic due to the partial negative charge oncarbon and is therefore particularly reactive towards electron acceptors and proton donors. In contrast ton-BuLi, MeLi reacts only very slowly withTHF at room temperature, and solutions inether are indefinitely stable. Water and alcohols react violently. Most reactions involving methyllithium are conducted below room temperature. Although MeLi can be used for deprotonations,n-butyllithium is more commonly employed since it is less expensive and more reactive.
Methyllithium is mainly used as the synthetic equivalent of the methyl anionsynthon. For example, ketones react to give tertiary alcohols in a two-step process:
Nonmetal halides are converted to methyl compounds with methyllithium:
Such reactions more commonly employ theGrignard reagents methylmagnesium halides, which are often equally effective, and less expensive or more easily prepared in situ.
It also reacts withcarbon dioxide to giveLithium acetate:
Transition metal methyl compounds can be prepared by reaction of MeLi with metal halides. Especially important are the formation oforganocopper compounds (Gilman reagents), of which the most useful islithium dimethylcuprate. This reagent is widely used for nucleophilic substitutions ofepoxides,alkyl halides andalkyl sulfonates, as well as for conjugate additions to α,β-unsaturated carbonyl compounds by methyl anion.[4] Many other transition metal methyl compounds have been prepared.[5]
Two structures have been verified by single crystalX-ray crystallography as well as by6Li,7Li, and13CNMR spectroscopy. The tetrameric structure is a distortedcubane-type cluster, with carbon and lithium atoms at alternate corners. The Li---Li distances are 2.68 Å, almost identical with the Li-Li bond in gaseousdilithium. The C-Li distances are 2.31 Å. Carbon is bonded to three hydrogen atoms and three Li atoms. The nonvolatility of (MeLi)4 and its insolubility in alkanes results from the fact that the clusters interact via further inter-clusteragostic interactions. In contrast the bulkier cluster (tertiary-butylLi)4, where intercluster interactions are precluded by steric effects, is volatile as well as soluble in alkanes.[6]
Colour code: Li- purple C- black H- white
The hexameric form features hexagonal prisms with Li and C atoms again at alternate corners.
Colour code: Li- purple C- black H- white
The degree of aggregation, "n" for (MeLi)n, depends upon the solvent and the presence of additives (such as lithium bromide). Hydrocarbon solvents such as benzene[7] favour formation of the hexamer, whereasethereal solvents favour the tetramer.
These clusters are considered "electron-deficient," that is, they do not follow theoctet rule because the molecules lack sufficient electrons to form four 2-centered, 2-electron bonds around each carbon atom, in contrast to mostorganic compounds. The hexamer is a 30 electron compound (30 valence electrons.) If one allocates 18 electrons for the strong C-H bonds, 12 electrons remain for Li-C and Li-Li bonding. There are six electrons for six metal-metal bonds and one electron per methyl-η3 lithium interaction.
The strength of the C-Li bond has been estimated at around 57kcal/mol fromIR spectroscopic measurements.[7]
