N-Butyllithium

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The correct title of this article is n-Butyllithium. The initial letter is shown capitalized due to technical restrictions.
n-Butyllithium
3D ball-and-stick model of 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 (pKb) >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

4
3
2
 
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, εr, 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)
Infobox 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

[edit] 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: Butyllithium represented as ions

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, either as a cubane structure with Li and CH2R groups alternating at the vertices, or as a Li4 tetrahedron 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.

[edit] 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.

[edit] Reactions

Schlosser's base is a superbase produced from butyllithium and potassium tert-butoxide. It is kinetically more reactive than butyllithium. The butoxide anion complexes the lithium and frees it from the butyl anion, breaking the butyllithium oligomers and producing extremely reactive "naked" butyl anions.

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{\leftarrow} 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{\rightarrow} LiH + CH3CH2CH=CH2

[edit] 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.

[edit] References

[edit] External link

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