Inorganic chemistry, thephenyl group, orphenyl ring, is acyclic group of atoms with the formulaC6H5, and is often represented by the symbolPh (archaicallyφ) orØ. The phenyl group is closely related tobenzene and can be viewed as a benzene ring, minus a hydrogen, which may be replaced by some other element or compound to serve as afunctional group. A phenyl group has six carbon atoms bonded together in ahexagonal planar ring, five of which are bonded to individual hydrogen atoms, with the remaining carbon bonded to asubstituent. Phenyl groups are commonplace inorganic chemistry.[1] Although often depicted with alternating double and single bonds, the phenyl group ischemically aromatic and has equal bond lengths between carbon atoms in the ring.[1][2]
Usually, a "phenyl group" is synonymous withC6H5− and isrepresented 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 (Ph3CH) 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 derivativeC6H5Cl is normally calledchlorobenzene, although it could be called phenyl chloride. In special (and rare) cases, isolated phenyl groups are detected: the phenyl anion (C6H−5), the phenyl cation (C6H+5), and the phenylradical (C
6H•
5).
Although Ph and phenyl uniquely denoteC6H5−, substituted derivatives also are described using the phenyl terminology. For example,C6H4NO2− is nitrophenyl, andC6F5− is pentafluorophenyl. Monosubstituted phenyl groups (that is, disubstituted benzenes) are associated withelectrophilic aromatic substitution reactions and the products follow thearene substitution pattern. So, a given substituted phenyl compound has three isomers,ortho (1,2-disubstitution),meta (1,3-disubstitution) andpara (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.
Phenyl is derived from French phényle, which in turn derived from Greek φαίνω (phaino) 'shining', as the first phenyl compounds named were byproducts ofmaking and refining variousgases used for lighting.[3] According to McMurry, "The word is derived from Greek pheno 'I bear light', commemorating the discovery of benzene byMichael Faraday in 1825 from the oily residue left by the illuminating gas used in London street lamps."[4]
Phenyl compounds are derived frombenzene (C6H6), at least conceptually and often in terms of their production. In terms of its electronic properties, the phenyl group is related to avinyl 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.[5] The phenyl group ishydrophobic. Phenyl groups tend to resist oxidation and reduction. Phenyl groups (like all aromatic compounds) have enhanced stability in comparison to equivalent bonding inaliphatic (non-aromatic) groups. This increased stability is due to the unique properties of aromaticmolecular orbitals.[2]
The bond lengths between carbon atoms in a phenyl group are approximately 1.4 Å.[6]
In1H-NMR spectroscopy, protons of a phenyl group typically havechemical shifts around 7.27 ppm. These chemical shifts are influenced byaromatic ring current and may change depending on substituents.
Phenyl groups are usually introduced using reagents that behave as sources of the phenyl anion or the phenyl cation. Representative reagents includephenyllithium (C6H5Li) andphenylmagnesium bromide (C6H5MgBr). Electrophiles are attacked by benzene to give phenyl derivatives:
whereE+ (the "electrophile") =Cl+, NO+2, SO3. These reactions are calledelectrophilic aromatic substitutions.
Phenyl groups are found in many organic compounds, both natural and synthetic (see figure). Most common among natural products is theamino acidphenylalanine, which contains a phenyl group. A major product of thepetrochemical industry is "BTX" consisting of benzene, toluene, and xylene - all of which are building blocks for phenyl compounds. Thepolymerpolystyrene 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 isphenol,C6H5OH. It is often said theresonance stability of phenol makes it a strongeracid than that ofaliphaticalcohols such asethanol (pKa = 10 vs. 16–18). However, a significant contribution is the greaterelectronegativity of thesp2alpha carbon in phenol compared to thesp3 alpha carbon in aliphatic alcohols.[7]
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