N-Butyllithium

Encyclopedia : N : NB : NBU : N-Butyllithium

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n-Butyllithium

General
Systematic name	n-Butyllithium
Other names	NBL, BuLi, 1-lithiobutane
Molecular formula	C₄H₉Li
SMILES	CCCCLi
Molar mass	64.05 g/mol
Appearance	colorless crystals unstable usually obtained as soln
CAS number	[109-72-8]
Properties
Density and phase	0.68 g/cm³, solvent defined
Solubility in water	reacts violently
Solubility in cyclohexane	soluble
Solubility in diethyl ether	soluble
Melting point	-76 °C (<273 K)
Boiling point	decomposes
Basicity (pK_b)	>35
Structure
Molecular shape	tetrameric in solution
Dipole moment	0 D
Hazards
MSDS	External MSDS
Main hazards	inflames in air, decomposes to corrosive LiOH
NFPA 704
R/S statement	R: R11, R14/15, R17, R34, R48/10, R51/53, R62, R65 S: S16, S26, S36/37/39, S45, S61, S7/8
RTECS number	?
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other cations	?
Related organolithium reagents	sec-butyllithium tert-butyllithium hexyllithium methyllithium
Related compounds	lithium hydroxide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) [Chemical infoboxInfobox disclaimer and references]

The chemical compound n-butyllithium is the most prominent organolithium reagent. It enjoys wide use as a polymerisation initiator in the production of elastomers such as polybutadiene or styrene-butadiene-styrene (SBS). Also, it is broadly employed as a strong base in organic synthesis. Annual worldwide production and consumption of butyllithium and other organolithium compounds is estimated at 1800 metric tonnes.

Contents

1 Chemical and physical properties
2 Preparation
3 Reactions
4 Safety precautions
5 References
6 External link

Chemical and physical properties

Due to the pyrophoric character of BuLi and its solutions, determination of the chemical and physical properties requires great care to protect such solutions from air. It reacts violently with water.

C₄H₉Li + H₂O → C₄H₁₀ + LiOH

BuLi also reacts with CO₂ to give lithium pentanoate:

C₄H₉Li + CO ₂ → C₄H₉CO₂Li

Due to the large difference between the electronegativities of carbon (2.55) and lithium (0.98), the C-Li bond is highly polarized, although it is not ionic. Although the precise charge separation is not known, it has been estimated to be 55-95%. Nonetheless, n-BuLi can often be considered to react as the butyl anion, n-Bu⁻, and a lithium cation, Li⁺ as depicted:

This model, however, is incorrect: n-BuLi is not ionic. As a solid, and even in solution, n-BuLi exists as a cluster, as do most organolithium compounds, consisting of covalent bonds between lithium and carbon. In the case of n-BuLi, the clusters are tetrameric (in ether) or hexameric (in cyclohexane). The Li---C interactions are not 2-center/2-electron bonds. There are simply not enough electrons to fill the entire valence shell of Li. The tetrahedral clusters can be viewed in either of two equivalent ways. A cubane with Li and CH₂R groups alternating at the vertices. The alternative view, a Li₄ tetrahedron in interpenetrated with a tetrahedron [CH₂R]₄ packing Li atoms. Such solid state structures are maintained in solutions of nonpolar solvents. By associating a number of negatively charged organic chains around the tetrahedral Li clusters, a 2 electron/4 center bond stabilizes the Li. This same property of Li to coordinate multiple hydrocarbon chains using its unoccupied orbitals also allows n-butyllithium to coordinate other σ-donors in solution.

Preparation

The standard preparation for n-BuLi is reaction of butyl-bromide or butyl-chloride with Li metal:

2 Li + C₄H₉X → C₄H₉Li + LiX

where X = Cl, Br

This reaction is accelerated by the presence of 1% Na in the Li. 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 LiBu. BuLi forms a weaker complex with LiCl, so that the reaction of BuCl with Li produces a precipitate of LiCl.

Reactions

BuLi exchanges with halocarbons, typically bromides, to give new organolithium compounds.

C₄H₉Li + RBr → C₄H₉Br + RLi

The newly produced RLi reagents, which are rarely isolated, also contain highly nucleophilic carbon centres. These reactions are usually conducted in diethyl ether at -78 °C.

A similar category of reactions is transmetalation wherein two organometallic compounds exchange their metals. Many examples of such reactions involve Li exchange with Sn:

C₄H₉Li + Me₃SnAr → C₄H₉SnMe₃ + LiAr

where Ar is aryl and Me is methyl

One of the most useful chemical properties of n-BuLi is its basicity. t-butyllithium and s-butyllithium are more basic still. n-BuLi can deprotonate almost any hydrocarbon where the conjugate base is somewhat stabilized by electron delocalization. Examples include acetylenes (H-C2R), methylphosphines (H-CH₂PR₂), and ferrocene (Fe(H-C₅H₄)(C₅H₅). The stability and volatility of the butane resulting from such deprotonation reactions is convenient. The kinetic basicity of n-BuLi is affected by the reaction solvent.

LiC₄H₉ + R-H [\overrightarrow] C₄H₁₀ + R-Li

Ligands that complex Li⁺ such as tetramethylethylenediamine (TMEDA) and 1,4-diazabicyclo[2.2.2]octane (DABCO) polarize the Li-C bond and accelerate the lithiations. Such additives also aid in the isolation of the lithiated product, a famous example of which is dilithioferrocene.

Fe(C₅H₅)₂ + 2 LiC₄H₉ + 2 TMEDA → C₄H₁₀ + Fe(C₅H₄Li)₂(TMEDA)₂.

Organolithium reagents, including n-BuLi can be 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

Organolithium reagents can also be used in the synthesis of alkenes. When heated, organolithium compounds undergo β-hydride elimination to produce an alkene and LiH:

C₄H₉Li + Δ[] LiH + CH₃CH₂CH=CH₂

Safety precautions

Butyllithium is extremely reactive toward air and moisture, often inflaming upon exposure to the atmosphere. It must be stored and handled in sealed systems under inert gas to prevent loss of activity.

References

[ChemExper Chemical Directory]
[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 Encyclopedia of Reagents for Organic Synthesis "n-butyllithium." Wiley and sons. 2006. DOI: [link]
Elschenbroich, C.; Salzer, A. Organometallics: a Concise Introduction 1st ed. 1989: VHC publishers, New York.
Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed. 1997: Butterworth-Heinemann, Boston.
Brandsma, L.; Verkraijsse, H. D. "Preparative Polar Organometallic Chemistry I"; Springer-Verlag: Berlin, 1987.

External link

[Computational Chemistry Wiki]

For a full list of external links to MSDSs, spectroscopic data, commercial chemicals suppliers etc. for this compound, see [Chemical sources].

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