Cyclic chemical group (?C?H?)
In
organic chemistry
, the
phenyl group
, or
phenyl ring
, is a
cyclic
group of atoms with the formula
C
6
H
5
, and is often represented by the symbol
Ph
(archaically
φ
). The phenyl group is closely related to
benzene
and can be viewed as a benzene ring, minus a hydrogen, 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
.
[1]
Although often depicted with alternating double and single bonds, the phenyl group is
chemically aromatic
and has equal bond lengths between carbon atoms in the ring.
[1]
[2]
Nomenclature
[
edit
]
Usually, a "phenyl group" is synonymous with
C
6
H
5
?
and is
represented by the symbol
Ph or, archaically,
Φ
. Benzene is sometimes denoted as PhH. Phenyl groups are generally attached to other atoms or groups. For example,
triphenylmethane
(
Ph
3
CH
) 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
C
6
H
5
Cl
is normally called
chlorobenzene
, although it could be called phenyl chloride. In special (and rare) cases, isolated phenyl groups are detected: the phenyl anion (
C
6
H
?
5
), the phenyl cation (
C
6
H
+
5
), and the phenyl
radical
(
C
6
H
?
5
).
Although Ph and phenyl uniquely denote
C
6
H
5
?
, substituted derivatives also are described using the phenyl terminology. For example,
C
6
H
4
NO
2
?
is nitrophenyl, and
C
6
F
5
?
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
[
edit
]
Phenyl is derived from
French
phenyle
, which in turn derived from
Greek
φα?νω
(phaino)
'shining', as the first phenyl compounds named were byproducts of
making and refining
various
gases used for lighting
.
[3]
According to McMurry, "The word is derived from
Greek
pheno
'I bear light', commemorating the discovery of benzene by
Michael Faraday
in 1825 from the oily residue left by the illuminating gas used in London street lamps."
[4]
Structure, bonding, and characterization
[
edit
]
Phenyl compounds are derived from
benzene
(
C
6
H
6
), 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 sp
2
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 is
hydrophobic
. Phenyl groups tend to resist oxidation and reduction. Phenyl groups (like all aromatic compounds) have enhanced stability in comparison to equivalent bonding in
aliphatic
(non-aromatic) groups. This increased stability is due to the unique properties of aromatic
molecular orbitals
.
[2]
The bond lengths between carbon atoms in a phenyl group are approximately 1.4
A
.
[6]
In
1
H-
NMR
spectroscopy, protons of a phenyl group typically have
chemical shifts
around 7.27 ppm. These chemical shifts are influenced by
aromatic ring current
and may change depending on substituents.
Preparation, occurrence, and applications
[
edit
]
Phenyl groups are usually introduced using reagents that behave as sources of the phenyl anion or the phenyl cation. Representative reagents include
phenyllithium
(
C
6
H
5
Li
) and
phenylmagnesium bromide
(
C
6
H
5
MgBr
). Electrophiles are attacked by benzene to give phenyl derivatives:
where
E
+
(the "electrophile") =
Cl
+
, NO
+
2
, SO
3
. These reactions are called
electrophilic aromatic substitutions
.
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
,
C
6
H
5
OH
. It is often said the
resonance stability
of phenol makes it a stronger
acid
than that of
aliphatic
alcohols
such as
ethanol
(
p
K
a
= 10 vs. 16?18). However, a significant contribution is the greater
electronegativity
of the
sp
2
alpha carbon
in phenol compared to the
sp
3
alpha carbon in aliphatic alcohols.
[7]
References
[
edit
]
- ^
a
b
March, Jerry (1992).
Advanced organic chemistry: reactions, mechanisms, and structure
(4th ed.). New York: Wiley.
ISBN
978-0-471-60180-7
.
- ^
a
b
"Aromaticity. Benzene and Other Aromatic Compounds"
.
Virtual Textbook of Organic Chemistry
. Michigan State University.
- ^
"phenyl"
.
English by Lexico Dictionaries
. Archived from
the original
on February 16, 2013
. Retrieved
24 July
2019
.
- ^
McMurry, John E. (2009).
Organic Chemistry, Enhanced Edition
. Cengage Learning. p. 518.
ISBN
9781111790042
.
- ^
Hansch, Corwin.; Leo, A.; Taft, R. W. (1991-03-01). "A survey of Hammett substituent constants and resonance and field parameters".
Chemical Reviews
.
91
(2): 165?195.
doi
:
10.1021/cr00002a004
.
ISSN
0009-2665
.
- ^
Hameka, Hendrik F. (1987). "Computation of the structures of the phenyl and benzyl radicals with the UHF method".
The Journal of Organic Chemistry
.
52
(22): 5025?5026.
doi
:
10.1021/jo00231a035
.
ISSN
0022-3263
.
- ^
Silva, Pedro Jorge (2009). "Inductive and Resonance Effects on the Acidities of Phenol, Enols, and Carbonyl α-Hydrogens".
The Journal of Organic Chemistry
.
74
(2): 914?916.
doi
:
10.1021/jo8018736
.
hdl
:
10284/3294
.
ISSN
0022-3263
.
PMID
19053615
.
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