Methyllithium Solution: A Key Reagent in Organolithium Chemistry

Methyllithium (MeLi) is a highly reactive organolithium compound commonly used in organic synthesis. Supplied as a solution in diethyl ether or tetrahydrofuran (THF), methyllithium acts as a strong base and nucleophile, making it invaluable for a range of synthetic transformations, particularly in forming carbon–carbon bonds.

Chemical Identity

• Chemical Formula: CH₃Li

• Molecular Weight: 22.0 g/mol

• CAS Number: 917-54-4

• Other Names: MeLi, methyl lithium

Methyllithium consists of a methyl group (CH₃⁻) bonded to lithium (Li⁺). It exists as a tetramer or hexamer in solution depending on concentration and solvent, with the lithium centers often associated with solvent molecules.

Physical and Chemical Properties

• Appearance: Colorless to slightly yellow solution

• Solvent: Typically in diethyl ether or THF

• Boiling Point of Solution: Depends on solvent

• Reactivity: Extremely reactive with air, water, and protic solvents

• Basicity: Very strong (conjugate acid pKa ≈ 50)

• Nucleophilicity: High, due to the carbanion-like methyl group

Methyllithium solutions are pyrophoric, igniting spontaneously on contact with air or moisture, and require careful handling under inert conditions.

Key Applications

1. Nucleophilic Addition to Electrophiles
Methyllithium readily adds to carbonyl compounds (e.g., aldehydes, ketones, esters), forming alcohols after aqueous workup. This is a common method for building carbon chains.

2. Formation of Organometallic Complexes
It is used to synthesize organolithium intermediates or transmetallation reagents in organometallic chemistry.

3. Strong Base for Deprotonation
MeLi acts as a powerful base, capable of deprotonating very weak acids including terminal alkynes and some aromatic compounds.

4. Methylation Reagent
It provides a methyl group in methylation reactions, particularly where other methylating agents might be ineffective or incompatible.

Handling and Safety

Hazards:

• Pyrophoric: Can ignite spontaneously in air

• Reacts violently with water, acids, and alcohols

• Causes severe burns to skin, eyes, and respiratory tract

• Vapors and solutions are toxic if inhaled or ingested

Precautions:

• Always handle under dry inert atmosphere (glovebox or Schlenk line)

• Use dry, oxygen-free solvents and glassware

• Wear chemical-resistant gloves, goggles, and lab coat

• Keep suitable fire extinguishing materials (Class D) nearby

Storage Guidelines

• Store in tightly sealed containers under an inert gas such as nitrogen or argon

• Keep away from heat sources, moisture, and oxidizers

• Avoid opening containers in air

• Store in a cool, dry, and well-ventilated area

Waste Disposal

Methyllithium must be carefully quenched before disposal. The process generally involves slow addition of isopropanol or another alcohol under an inert atmosphere until gas evolution ceases, followed by dilution and neutralization. Disposal must comply with local hazardous chemical waste regulations.

Conclusion

Methyllithium solution is an essential reagent for modern organic and organometallic synthesis, offering both strong basic and nucleophilic properties. Its ability to form carbon–carbon bonds efficiently makes it especially valuable in constructing complex molecular frameworks. However, its extreme reactivity necessitates rigorous safety practices, including air-free handling and appropriate personal protection. When used properly, methyllithium is a powerful and reliable tool for chemists working in advanced synthesis.

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