n-Butyllithium
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Close-up of the delocalized bonds between butyl and lithium
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Names | |||
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IUPAC name
butyllithium, tetra-μ3-butyl-tetralithium
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Other names
NBL, BuLi,
1-lithiobutane | |||
Identifiers | |||
3D model (
JSmol ) |
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ChEBI | |||
ChemSpider | |||
ECHA InfoCard
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100.003.363 | ||
PubChem CID
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UNII | |||
CompTox Dashboard (EPA)
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Properties | |||
C4H9Li | |||
Molar mass | 64.06 g·mol−1 | ||
Appearance | colorless liquid unstable usually obtained as solution | ||
Density | 0.68 g/cm3, solvent defined | ||
Melting point | −76 °C (−105 °F; 197 K) (<273 K) | ||
Boiling point | 80 C | ||
Exothermic decomposition | |||
Solubility | Ethers such as THF, hydrocarbons | ||
Acidity (pKa) | 50 (of the conjugate acid)[1] | ||
Structure | |||
tetrameric in solution | |||
0 D | |||
Hazards | |||
Occupational safety and health (OHS/OSH): | |||
Main hazards
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Pyrophoric (spontaneously combusts in air), decomposes to corrosive LiOH | ||
NFPA 704 (fire diamond) | |||
Related compounds | |||
Related organolithium
reagents |
sec-butyllithium tert-butyllithium hexyllithium methyllithium | ||
Related compounds
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lithium hydroxide | ||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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n-Butyllithium C4H9Li (abbreviated n-BuLi) is an organolithium reagent. It is widely used as a polymerization initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene (SBS). Also, it is broadly employed as a strong base (superbase) in the synthesis of organic compounds as in the pharmaceutical industry.
Butyllithium is commercially available as solutions (15%, 25%, 1.5
Although butyllithium is colorless, n-butyllithium is usually encountered as a pale yellow solution in alkanes. Such solutions are stable indefinitely if properly stored,[3] but in practice, they degrade upon aging. Fine white precipitate (lithium hydride) is deposited and the color changes to orange.[3][4]
Structure and bonding
n-BuLi exists as a cluster both in the solid state and in a solution. The tendency to aggregate is common for organolithium compounds. The aggregates are held together by delocalized covalent bonds between lithium and the terminal carbon of the butyl chain.
Due to the large difference between the
Preparation
The standard preparation for n-BuLi is reaction of
- 2 Li + C4H9X → C4H9Li + LiX (X = Cl, Br)
If the lithium used for this reaction contains 1–3% sodium, the reaction proceeds more quickly than if pure lithium is used. Solvents used for this preparation include benzene, cyclohexane, and diethyl ether. When BuBr is the precursor, the product is a homogeneous solution, consisting of a mixed cluster containing both LiBr and BuLi, together with a small amount of octane. BuLi forms a weaker complex with LiCl, so that the reaction of BuCl with Li produces a precipitate of LiCl.
Solutions of butyllithium, which are susceptible to degradation by air, are standardized by titration. A popular weak acid is biphenyl-4-methanol, which gives a deeply colored dilithio derivative at the end point.[6]
Applications
Butyllithium is principally valued as an initiator for the anionic polymerization of dienes, such as butadiene.[7] The reaction is called "carbolithiation":
- C4H9Li + CH2=CH−CH=CH2 → C4H9−CH2−CH=CH−CH2Li
Isoprene can be polymerized stereospecifically in this way. Also of commercial importance is the use of butyllithium for the production of styrene-butadiene polymers. Even ethylene will insert into BuLi.[8]
Reactions
Butyllithium is a strong base (pKb ≈ -36), but it is also a powerful
Metalation
One of the most useful chemical properties of n-BuLi is its ability to deprotonate a wide range of weak
- LiC4H9 + RH → C4H10 + RLi
The kinetic basicity of n-BuLi is affected by the solvent or cosolvent. Ligands that complex Li+ such as tetrahydrofuran (THF), tetramethylethylenediamine (TMEDA), hexamethylphosphoramide (HMPA), and 1,4-diazabicyclo[2.2.2]octane (DABCO) further polarize the Li−C bond and accelerate the metalation. Such additives can also aid in the isolation of the lithiated product, a famous example of which is dilithioferrocene.
- Fe(C5H5)2 + 2 LiC4H9 + 2 TMEDA → 2 C4H10 + Fe(C5H4Li)2(TMEDA)2
Schlosser's base is a superbase produced by treating butyllithium with potassium t-butoxide. It is kinetically more reactive than butyllithium and is often used to accomplish difficult metalations. While some n-butylpotassium is present and is a stronger base than n-BuLi, the reactivity of the mixture is not exactly the same as isolated n-butylpotassium.[10]
An example of the use of n-butyllithium as a base is the addition of an amine to methyl carbonate to form a methyl carbamate, where n-butyllithium serves to deprotonate the amine:
- n-BuLi + R2NH + (MeO)2CO → R2NCO2Me + LiOMe + BuH
Halogen–lithium exchange
Butyllithium reacts with some organic bromides and iodides in an exchange reaction to form the corresponding organolithium derivative. The reaction usually fails with organic chlorides and fluorides:
- C4H9Li + RX → C4H9X + RLi (X = Br, I)
This
- 2 C4H9Br + RLi → 2 C4H9R + LiBr
- 2 C4H9Li + R′CH=CHBr → 2 C4H10 + R′C≡CLi + LiBr
These side reaction are significantly less important for RI than for RBr, since the iodine–lithium exchange is several orders of magnitude faster than the bromine–lithium exchange. For these reasons, aryl, vinyl and primary alkyl iodides are the preferred substrates, and t-BuLi rather than n-BuLi is usually used, since the formed t-BuI is immediately destroyed by the t-BuLi in a dehydrohalogenation reaction (thus requiring two equivalents of t-BuLi). Alternatively, vinyl lithium reagents can be generated by direct reaction of the vinyl halide (e.g. cyclohexenyl chloride) with lithium or by tin–lithium exchange (see next section).[3]
Transmetalations
A related family of reactions are the transmetalations, wherein two organometallic compounds exchange their metals. Many examples of such reactions involve lithium exchange with tin:
- C4H9Li + Me3SnAr → C4H9SnMe3 + LiAr (where Ar is aryl and Me is methyl)
The tin–lithium exchange reactions have one major advantage over the halogen–lithium exchanges for the preparation of organolithium reagents, in that the product tin compounds (C4H9SnMe3 in the example above) are much less reactive towards lithium reagents than are the halide products of the corresponding halogen–lithium exchanges (C4H9Br or C4H9Cl). Other
.Carbonyl additions
Organolithium reagents, including n-BuLi are used in synthesis of specific aldehydes and ketones. One such synthetic pathway is the reaction of an organolithium reagent with disubstituted amides:
- R1Li + R2CONMe2 → LiNMe2 + R2C(O)R1
Degradation of THF
THF is deprotonated by butyllithium, especially in the presence of
Thermal decomposition
When heated, n-BuLi, analogously to other alkyllithium reagents with "β-hydrogens", undergoes
- C4H9Li → LiH + CH3CH2CH=CH2
Safety
Alkyl-lithium compounds are stored under inert gas to prevent loss of activity and for reasons of safety. n-BuLi reacts violently with water:
- C4H9Li + H2O → C4H10 + LiOH
This is an exergonic and highly exothermic reaction. If oxygen is present the butane produced may ignite.
BuLi also reacts with CO2 to give lithium pentanoate:
- C4H9Li + CO2 → C4H9CO2Li
See also
- Propynyllithium, an organometallic compound.
References
- ^ Bernier, David. "Some useful pKa values". Org@Work. Archived from the original on 9 May 2017. Retrieved 26 May 2017.
- .
- ^ ISBN 3-540-16916-4..
- ^ "n-Butyllithium solution". sigmaaldrich.com. Retrieved 17 August 2023.
- ISBN 3-527-29390-6.
- .
- .
- .
- .
- .
- ISSN 1369-9261.
Further reading
- FMC Lithium manufacturer's product sheets
- Environmental Chemistry directory
- Weissenbacher, Anderson, Ishikawa, Organometallics, July 1998, p681.7002, Chemicals Economics Handbook SRI International
- HPV test plan, submitted by FMC Lithium to EPA
- Ovaska, T. V. e-EROS
- Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed. 1997: Butterworth-Heinemann, Boston.