Phenyl Group - Preparation, Occurrence, and Applications

Preparation, Occurrence, and Applications

Phenyl groups are usually introduced using reagents that behave as sources of the phenyl anion or the phenyl cation. Representative reagents include phenyllithium (C₆H₅Li) and phenylmagnesium bromide (C₆H₅MgBr). Electrophiles attack benzene to give phenyl derivatives:

C₆H₆ + E+ → C₆H₅E + H+

where E+ (the "electrophile") = Cl+, NO₂+, SO₃. These reactions are called electrophilic aromatic substitutions.

• Representative compounds containing phenyl groups

• Lipitor, a blockbuster drug featuring two phenyl and one p-fluorophenyl groups.

• Allegra, another blockbuster drug, which features a diphenylmethyl group as well as a p-phenylene (C₆H₄) group.

• Phenylalanine, a common amino acid.

• biphenyl, consisting of two phenyl groups. The two rings tend not to be coplanar.

• chlorobenzene (or phenyl chloride), a solvent.

Phenyl groups are found in many organic compounds, both natural and synthetic (see figure). Most common among natural products is the amino acid phenylalanine, which contains a phenyl group. A major product of the petrochemical industry is "BTX" consisting of benzene, toluene, and xylene - all of which are building blocks for phenyl compounds. The polymer polystyrene is derived from a phenyl-containing monomer and owes its properties to the rigidity and hydrophobicity of the phenyl groups. Many drugs as well as many pollutants contain phenyl rings.

One of the simplest phenyl-containing compounds is phenol, C₆H₅OH. It is often said the resonance stability of phenol makes it a stronger acid than that of aliphatic alcohols such as ethanol (pK_a = 10 vs. 16–18). However, a significant contribution is the greater electronegativity of the sp2 alpha carbon in phenol compared to the sp3 alpha carbon in aliphatic alcohols.