In organic chemistry, the phenyl group, or phenyl ring, is a cyclic compound group of atoms with the formula , and is often represented by the symbol Ph (archaically φ) or Ø. The phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen atom, which may be replaced by some other element or compound to serve as a functional group. A phenyl group has six carbon atoms bonded together in a hexagonal planar ring, five of which are bonded to individual hydrogen atoms, with the remaining carbon bonded to a substituent. Phenyl groups are commonplace in organic chemistry.
Nomenclature
Usually, a "phenyl group" is synonymous with and is represented by the symbol Ph (archaically, Φ), or Ø. Benzene is sometimes denoted as PhH. Phenyl groups are generally attached to other atoms or groups. For example, triphenylmethane () has three phenyl groups attached to the same carbon center. Many or even most phenyl compounds are not described with the term "phenyl". For example, the chloro derivative is normally called chlorobenzene, although it could be called phenyl chloride. In special (and rare) cases, isolated phenyl groups are detected: the phenyl anion (), the phenyl cation (), and the phenyl radical ().
Although Ph and phenyl uniquely denote , substituted derivatives also are described using the phenyl terminology. For example, is nitrophenyl, and is pentafluorophenyl. Monosubstituted phenyl groups (that is, disubstituted benzenes) are associated with electrophilic aromatic substitution reactions and the products follow the arene substitution pattern. So, a given substituted phenyl compound has three isomers, ortho (1,2-disubstitution), meta (1,3-disubstitution) and para (1,4-disubstitution). A disubstituted phenyl compound (trisubstituted benzene) may be, for example, 1,3,5-trisubstituted or 1,2,3-trisubstituted. Higher degrees of substitution, of which the pentafluorophenyl group is an example, exist and are named according to IUPAC nomenclature.
Etymology
Phenyl is derived , which in turn derived , as the first phenyl compounds named were byproducts of making and refining various gas lighting.
Structure, bonding, and characterization
Phenyl compounds are derived from benzene (), at least conceptually and often in terms of their production. In terms of its electronic properties, the phenyl group is related to a vinyl group. It is generally considered an inductively withdrawing group (- I), because of the higher electronegativity of sp2 carbon atoms, and a resonance donating group (+ M), due to the ability of its π system to donate electron density when conjugation is possible.
The bond lengths between carbon atoms in a phenyl group are approximately 1.4 Å.
In 1H-NMR spectroscopy, protons of a phenyl group typically have around 7.27 ppm. These chemical shifts are influenced by aromatic ring current and may change depending on substituents.
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 () and phenylmagnesium bromide (). Electrophiles are attacked by benzene to give phenyl derivatives:
- C6H6 + E+ -> C6H5E + H+
where (the "electrophile") = . These reactions are called electrophilic aromatic substitutions.
File: Atorvastatin.svg|Atorvastatin, a blockbuster drug featuring two phenyl and one p-fluorophenyl groups. It is used to lower cholesterol in people with hypercholesterolaemia.
File: Fexofenadine2DCSD.svg|Fexofenadine, another blockbuster drug, which features a diphenylmethyl group as well as a p-phenylene () group. It is an antihistamine used to treat allergies.
File: Phenylalanin_-_Phenylalanine.svg|Phenylalanine, a common amino acid.
File: Bifenyl.svg|Biphenyl, consisting of two phenyl groups. The two rings tend not to be coplanar.
File: Chlorobenzene2.svg|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, . It is often said the resonance stability of phenol makes it a stronger acid than that of aliphatic alcohols such as ethanol (p Ka = 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.
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