Phenyl Lithium
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Phenyllithium 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 . It is most commonly used as a metalating agent in organic syntheses and a substitute for
Grignard reagents Grignard reagents or Grignard compounds are chemical compounds 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: : Reaction of a phenyl halide with lithium metal produces phenyllithium: : Phenyllithium can also be synthesized with a metal-halogen exchange reaction: : 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: : 2-Phenylpyridine is prepared by the reaction of phenyl lithium with pyridine, a process that entails an addition-elimination pathway: :


Structure and properties

Phenyllithium is an
organolithium In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...
compound that forms monoclinic crystals. Solid phenyl­lithium can be described as consisting of dimeric 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 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. 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 ...
, 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 complex instead becomes . The Li atom of LiBr occupies one of the lithium sites in the
cubane-type cluster A cubane-type cluster is an arrangement of atoms in a molecular structure that forms a cube. In the simplest case, the eight vertices are symmetry equivalent and the species has Oh symmetry group, symmetry. Such structure occurs in the hydrocarbon ...
and Br atom sits in an adjacent carbon site.


References

{{Lithium compounds Organolithium compounds Phenyl compounds