N-Butyllithium

Key takeaways

N-Butyllithium belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect N-Butyllithium to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of N-Butyllithium from memory before moving on to harder problems.

Reference excerpt

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, such as in the pharmaceutical industry. Butyllithium is commercially available as solutions (15%, 25%, 1.5 M, 2 M, 2.5 M, 10 M, etc.) in alkanes such as pentane, hexanes, and heptanes. Solutions in diethyl ether and THF 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. Although butyllithium is colorless, n-butyllithium is usually encountered as a pale yellow solution in alkanes. Such solutions are stable indefinitely if properly stored, but in practice, they degrade upon aging, where a fine white precipitate (lithium hydride) is deposited and the color changes to orange.

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. In the case of n-BuLi, the clusters are tetrameric (in ether) or hexameric (in cyclohexane). The cluster is a distorted cubane-type cluster with Li and CH2R groups at alternating vertices. An equivalent description describes the tetramer as a Li4 tetrahedron 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 toward Lewis acids. Due to the large difference between the electronegativities of carbon (2.55) and lithium (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 butyl anion, n-Bu−, and a lithium cation, Li+.

Preparation The standard preparation for n-BuLi is reaction of 1-bromobutane or 1-chlorobutane with Li metal:

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.

Applications Butyllithium is principally valued as an initiator for the anionic polymerization of dienes, such as butadiene. 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.

Reactions Butyllithium is a strong base (pKb ≈ −36), but it is also a powerful nucleophile and reductant, depending on the other reactants. Furthermore, in addition to being a strong nucleophile, n-BuLi binds to aprotic Lewis bases, such as ethers and tertiary amines, which partially disaggregate the clusters by binding to the lithium centers. Its use as a strong base is referred to as metalation. Reactions are typically conducted in tetrahydrofuran and diethyl ether, which are good solvents for the resulting organolithium derivatives (see below).

Metalation

One of the most useful chemical properties of n-BuLi is its ability to deprotonate a wide range of weak Brønsted acids. t-Butyllithium and s-butyllithium are more basic. n-BuLi can deprotonate (that is, metalate) many types of C−H bonds, especially where the conjugate base is stabilized by electron delocalization 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 and ferrocene (Fe(H−C5H4)(C5H5)). 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 and volatility of the butane resulting from such deprotonation reactions is convenient, but can also be a problem for large-scale reactions because of the volume of a flammable gas produced.

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. 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:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with N-Butyllithium

Start with the simplest possible case. Write down what N-Butyllithium claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to N-Butyllithium before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about N-Butyllithium ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of N-Butyllithium

In research: N-Butyllithium appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses N-Butyllithium in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: N-Butyllithium is common in secondary-school and first-year university syllabi. It links to neighbouring topics Butyl compounds, Organolithium compounds, Pyrophoric materials, so understanding it makes those chapters shorter.
In everyday life: Look for N-Butyllithium outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study N-Butyllithium in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what N-Butyllithium means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain N-Butyllithium out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is N-Butyllithium in simple terms?

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).

Why does N-Butyllithium matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study N-Butyllithium?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on N-Butyllithium.