sec-Butyllithium Solution: An Overview
sec-Butyllithium (sec-BuLi) solution is a highly reactive organolithium reagent widely used in organic synthesis, particularly in deprotonation, metalation, and initiation of polymerization reactions. It consists of a lithium atom bonded to a secondary butyl group (CH₃CHLiCH₂CH₃), and is typically supplied as a solution in hydrocarbon solvents such as hexanes or cyclohexane due to its extreme reactivity and pyrophoric nature.
Chemical Identity
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Chemical Formula: C₄H₉Li
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Structure: CH₃–CHLi–CH₂–CH₃ (secondary butyl group attached to lithium)
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Molar Mass: 64.06 g/mol
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Appearance: Colorless to pale yellow liquid (in solution)
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CAS Number: 598-30-1
Because of its air- and moisture-sensitive nature, sec-butyllithium is never isolated as a pure solid and is always handled as a solution, typically in concentrations ranging from 1.3M to 2.5M in hydrocarbon solvents.
Physical and Chemical Properties
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State: Liquid solution (in hydrocarbons)
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Boiling Point: Depends on the solvent (hexane: ~69°C)
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Solubility: Insoluble in water; soluble in nonpolar organic solvents
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Reactivity: Extremely reactive with air, water, alcohols, CO₂
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Flammability: Pyrophoric — ignites spontaneously on exposure to air
As a strong base and nucleophile, sec-butyllithium is more reactive than n-butyllithium due to the increased steric strain and higher accessibility of its lithium-carbon bond.
Major Applications
1. Strong Base in Organic Synthesis
sec-Butyllithium is commonly used to deprotonate weakly acidic hydrogens, such as in:
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Aromatic lithiation (metalation of aromatic rings)
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Formation of enolates and carbanions
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Directed ortho-lithiation for regioselective functionalization of arenes
2. Organolithium Synthesis
It is used to prepare other organolithium reagents through halogen-lithium exchange, especially with aryl or alkyl halides.
3. Polymerization Initiator
sec-BuLi serves as an initiator in the anionic polymerization of dienes (e.g., butadiene, isoprene), particularly for producing synthetic rubbers like SBS (styrene-butadiene-styrene) block copolymers.
4. Fine Chemical and Pharmaceutical Intermediates
It plays a role in multi-step organic syntheses, helping build complex molecules via precise, base-driven transformations.
Safety and Handling
sec-Butyllithium is extremely dangerous and must be handled under strictly controlled conditions.
Hazards:
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Pyrophoric: Ignites spontaneously in air
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Corrosive: Causes severe burns to skin and eyes
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Toxic: Harmful if inhaled or ingested
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Reacts violently with water, acids, alcohols, and carbon dioxide
Safety Measures:
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Handle under an inert gas (nitrogen or argon) in a glove box or using Schlenk techniques
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Use flame-resistant lab coats, face shields, and heavy-duty gloves
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Avoid glassware with stress points or damage to prevent breakage under pressure
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Store and transport in sealed, labeled containers with secondary containment
In case of accidental exposure or spill, do not use water. Cover with dry sand or a Class D fire extinguisher material and ventilate area. Seek immediate medical attention if contact occurs.
Storage Guidelines
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Store in an inert atmosphere (under nitrogen or argon)
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Keep in tightly sealed containers made of compatible materials (e.g., stainless steel or specialty glass)
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Store away from heat sources, oxidizing agents, acids, and moisture
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Keep cool and in a flameproof storage area
Environmental Considerations
sec-Butyllithium is not environmentally safe and must never be released into drains or the environment. Waste must be neutralized under inert conditions with a suitable alcohol (e.g., isopropanol) and disposed of as hazardous chemical waste in accordance with regulatory guidelines.
Conclusion
sec-Butyllithium solution is a powerful and indispensable reagent in modern synthetic chemistry, especially valued for its strength as a base and its ability to initiate complex chemical transformations. However, its pyrophoric and corrosive nature makes it one of the most hazardous laboratory reagents in common use. When handled properly with the right precautions and expertise, sec-BuLi plays a vital role in advancing high-precision chemistry in pharmaceuticals, polymers, and materials science.
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