Phenyllithium or lithobenzene is an
organometallic
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
agent with the empirical formula C
6H
5Li. It is most commonly used as a metalating agent in organic syntheses and a substitute for
Grignard reagents
A Grignard reagent or Grignard compound is a chemical compound with the general formula , where X is a halogen and R is an organic group, normally an alkyl or aryl. Two typical examples are methylmagnesium chloride and phenylmagnesium bromide . ...
for introducing phenyl groups in organic syntheses. Crystalline phenyllithium is colorless; however, solutions of phenyllithium are various shades of brown or red depending on the solvent used and the impurities present in the solute.
Preparation
Phenyllithium was first produced by the reaction of lithium metal with
diphenylmercury
Diphenylmercury is the organomercury compound with the formula Hg(C6H5)2. It is a white solid. The compound is of historic interest as a particularly stable organometallic compound but it finds few uses because of its high toxicity.
Preparation
...
:
:(C
6Η
5)
2Ηg + 2Li → 2C
6Η
5Li + Ηg
Reaction of a phenyl halide with lithium metal produces phenyllithium:
:X-Ph + 2Li → Ph-Li + LiX
Phenyllithium can also be synthesized with a metal-halogen exchange reaction:
:n-BuLi + X-Ph → n-BuX + Ph-Li
The predominant method of producing phenyllithium today are the latter two syntheses.
Reactions
The primary use of PhLi is to facilitate formation of carbon-carbon bonds by nucleophilic addition and substitution reactions:
:PhLi + R
2C=O → PhR
2COLi
2-Phenylpyridine is prepared by the reaction of phenyl lithium with pyridine, a process that entails an addition-elimination pathway:
:C
6H
5Li + C
5H
5N → C
6H
5-C
5H
4N + LiH
Structure and properties
Phenyllithium is an
organolithium
In organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, ...
compound that forms monoclinic crystals. Solid phenyllithium can be described as consisting of dimeric Li
2Ph
2 subunits. The Li atoms and the ''
ipso'' carbons of the phenyl rings form a planar four-membered ring. The plane of the phenyl groups are perpendicular to the plane of this Li
2C
2 ring. Additional strong intermolecular bonding occurs between these phenyllithium dimers and the π-electrons of the phenyl groups in the adjacent dimers, resulting in an infinite polymeric ladder structure.
![Phenyllithium-chain-from-xtal-Mercury-3D-balls](https://upload.wikimedia.org/wikipedia/commons/a/a6/Phenyllithium-chain-from-xtal-Mercury-3D-balls.png)
In solution, it takes a variety of structures dependent on the organic solvent. In
tetrahydrofuran
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...
, it equilibrates between monomer and dimer states. In ether, as it is commonly sold, phenyllithium exists as a tetramer. Four Li atoms and four ipso carbon centers occupy alternating vertices of a distorted cube. Ph
![Phenyllithium-etherate-tetramer-from-xtal-Mercury-3D-sticks](https://upload.wikimedia.org/wikipedia/commons/6/62/Phenyllithium-etherate-tetramer-from-xtal-Mercury-3D-sticks.png)
The C–Li bond lengths are an average of 2.33 Å. An ether molecule binds to each of the Li sites through its oxygen atom. In the presence of LiBr, a byproduct of directly reacting lithium with a phenyl halide, the
2O)4">PhLi•Et2O)4complex instead becomes
2O)3•LiBr). The Li atom of LiBr occupies one of the lithium sites in the cubane-type cluster">PhLi•Et2O)3•LiBr). The Li atom of LiBr occupies one of the lithium sites in the cubane-type cluster and Br atom sits in an adjacent carbon site.
References
{{Lithium compounds
Lithium compounds
Organolithium compounds
Phenyl compounds