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 Close-up of the delocalized bonds between butyl and lithium | |||
| Names | |||
|---|---|---|---|
| IUPAC name butyllithium, tetra-μ3-butyl-tetralithium | |||
| Other names NBL, BuLi, 1-lithiobutane | |||
| Identifiers | |||
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3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
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| ECHA InfoCard | 100.003.363 | ||
| UNII | |||
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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 asTHF, hydrocarbons | ||
| Acidity (pKa) | 50 (of the conjugate acid)[1] | ||
| Structure | |||
| tetrameric in solution | |||
| 0D | |||
| Hazards | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards | Pyrophoric (spontaneously combusts in air), decomposes to corrosiveLiOH | ||
| NFPA 704 (fire diamond) | |||
| Related compounds | |||
Relatedorganolithium reagents | sec-butyllithium tert-butyllithium hexyllithium methyllithium | ||
Related compounds | lithium hydroxide | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
n-Butyllithium C4H9Li (abbreviatedn-BuLi) is anorganolithium reagent. It is widely used as apolymerization initiator in the production ofelastomers such aspolybutadiene orstyrene-butadiene-styrene (SBS). Also, it is broadly employed as a strongbase (superbase) in thesynthesis of organic compounds as in the pharmaceutical industry.
Butyllithium is commercially available as solutions (15%, 25%, 1.5 M, 2 M, 2.5 M, 10 M, etc.) inalkanes such aspentane,hexanes, andheptanes. Solutions indiethyl ether andTHF can be prepared, but are not stable enough for storage. Annual worldwide production and consumption of butyllithium and other organolithium compounds is estimated at 2000 to 3000 tonnes.[2]
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]
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.[5] In the case ofn-BuLi, the clusters are tetrameric (in ether) or hexameric (incyclohexane). The cluster is a distortedcubane-type cluster with Li andCH2R groups at alternating vertices. An equivalent description describes the tetramer as a Li4tetrahedron interpenetrated with a tetrahedron [CH2R]4. Bonding within the cluster is related to that used to describe diborane, but more complex since eight atoms are involved. Reflecting its electron-rich character,n-butyllithium is highly reactive towardLewis acids.
Due to the large difference between theelectronegativities ofcarbon (2.55) andlithium (0.98), the C−Li bond is highly polarized. The charge separation has been estimated to be 55–95%. For practical purposes,n-BuLi can often be considered to react as the butylanion,n-Bu−, and a lithiumcation, Li+.
The standard preparation forn-BuLi is reaction of1-bromobutane or1-chlorobutane with Li metal:[3]
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 includebenzene, 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 ofoctane. BuLi forms a weaker complex with LiCl, so that the reaction of BuCl with Li produces a precipitate ofLiCl.
Solutions of butyllithium, which are susceptible to degradation by air, are standardized bytitration. A popular weak acid isbiphenyl-4-methanol, which gives a deeply colored dilithio derivative at the end point.[6]
Butyllithium is principally valued as an initiator for the anionicpolymerization ofdienes, such asbutadiene.[7] The reaction is called "carbolithiation":
Isoprene can be polymerized stereospecifically in this way. Also of commercial importance is the use of butyllithium for the production ofstyrene-butadiene polymers. Evenethylene will insert into BuLi.[8]
Butyllithium is a strong base (pKb ≈ -36), but it is also a powerfulnucleophile andreductant, depending on the other reactants. Furthermore, in addition to being a strong nucleophile,n-BuLi binds to aprotic Lewis bases, such as ethers and tertiaryamines, which partially disaggregate the clusters by binding to the lithium centers. Its use as a strongbase is referred to asmetalation. Reactions are typically conducted intetrahydrofuran anddiethyl ether, which are good solvents for the resulting organolithium derivatives (see below).
One of the most useful chemical properties ofn-BuLi is its ability to deprotonate a wide range of weakBrønsted acids.t-Butyllithium ands-butyllithium are more basic.n-BuLi can deprotonate (that is, metalate) many types of C−H bonds, especially where theconjugate base is stabilized by electrondelocalization or one or more heteroatoms (non-carbon atoms). Examples include acetylenes (H−CC−R), methyl sulfides (H−CH2SR), thioacetals (H−CH(SR)2, e.g.dithiane), methylphosphines (H−CH2PR2),furans,thiophenes andferrocene (Fe(H−C5H4)(C5H5)).[9] In addition to these, it will also deprotonate all more acidic compounds such as alcohols, amines,enolizable carbonyl compounds, and any overtly acidic compounds, to produce alkoxides, amides, enolates and other salts of lithium, respectively. The stability andvolatility of thebutane resulting from suchdeprotonation reactions is convenient, but can also be a problem for large-scale reactions because of the volume of a flammable gas produced.
The kinetic basicity ofn-BuLi is affected by the solvent or cosolvent. Ligands that complex Li+ such astetrahydrofuran (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.
Schlosser's base is asuperbase produced by treating butyllithium withpotassiumt-butoxide. It is kinetically more reactive than butyllithium and is often used to accomplish difficultmetalations. While somen-butylpotassium is present and is a stronger base thann-BuLi, the reactivity of the mixture is not exactly the same as isolatedn-butylpotassium.[10]
An example of the use ofn-butyllithium as a base is the addition of an amine to methyl carbonate to form a methylcarbamate, wheren-butyllithium serves to deprotonate the amine:
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:
Thislithium–halogen exchange reaction is useful for preparation of several types of RLi compounds, particularlyaryllithium and somevinyllithium reagents. The utility of this method is significantly limited, however, by the presence in the reaction mixture ofn-BuBr orn-BuI, which can react with the RLi reagent formed, and by competingdehydrohalogenation reactions, in whichn-BuLi serves as a base:
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, andt-BuLi rather thann-BuLi is usually used, since the formedt-BuI is immediately destroyed by thet-BuLi in a dehydrohalogenation reaction (thus requiring two equivalents oft-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]
A related family of reactions are thetransmetalations, wherein two organometallic compounds exchange their metals. Many examples of such reactions involve lithium exchange withtin:
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). Othermetals andmetalloids which undergo such exchange reactions are organic compounds ofmercury,selenium, andtellurium.
Organolithium reagents, includingn-BuLi are used in synthesis of specificaldehydes andketones. One such synthetic pathway is the reaction of an organolithium reagent with disubstitutedamides:
THF is deprotonated by butyllithium, especially in the presence ofTMEDA, by loss of one of four protons adjacent to oxygen. This process, which consumes butyllithium to generate butane, induces a ring opening to give enolate ofacetaldehyde andethylene.[11] Therefore, reactions of BuLi in THF are typically conducted at low temperatures, such as –78 °C, as is conveniently produced by afreezing bath ofdry ice and acetone. Higher temperatures (−25 °C or even −15 °C) are also used.
When heated,n-BuLi, analogously to other alkyllithium reagents with "β-hydrogens", undergoesβ-hydride elimination to produce1-butene andlithium hydride (LiH):
Alkyl-lithium compounds are stored under inert gas to prevent loss of activity and for reasons of safety.n-BuLi reacts violently with water:
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:
