Phenyllithium is an organometallic agent with the empirical formula C₆H₅Li. It is most commonly used as a metalating agent in organic syntheses and a substitute for Grignard reagents 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.

Structure and properties

Phenyllithium is an organolithium compound that forms monoclinic crystals. Solid phenyllithium can be described as consisting of dimeric Li₂Ph₂ 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 Li₂C₂ ring. Additional intermolecular bonding occurs between these phenyllithium dimers and the π-electrons of the phenyl groups in the adjacent dimers.

In solution, it takes a variety of structures dependent on the organic solvent. In tetrahydrofuran

Tetrahydrofuran

Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...

, 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. Phenyl groups are at the faces of the tetrahedron and bind to three of the nearest Li atoms.

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 [(PhLi • Et₂O)₄] complex instead becomes [(PhLi • Et₂O)₃ • LiBr). The Li atom of LiBr occupies one of the lithium sites in the cubane-like frame, and Br atom sits in an adjacent carbon site.

Preparation

Phenyllithium was first produced by the reaction of lithium metal with diphenylmercury

Diphenylmercury

Diphenylmercury is a colourless, crystalline organomercury compound with the chemical formula C12H10Hg. It can be synthesised by the reaction of a 2:1 molar ratio of mercury chloride and methyltriphenyltin in ethanol...

:
₂Ηg + 2Li → 2C₆Η₅Li + Ηg

The synthesis was improved soon afterward by directly reacting lithium

Lithium

Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

with phenyl halides.

C₆H₅X + 2Li → C₆H₅Li + 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 single bonds by nucleophilic addition and substitution reactions:

PhLi + R₂C=O → PhR₂COLi