Chemistry 20
Chapter 4
Benzene
Benzene: a molecule of benzene consists of a ring of six carbon atoms with onr hydrogen
atom attached to each carbon. It has the molecular formula C6H6. The real molecule of
benzene is a resonance hybrid of the two Lewis structures (a unique feature that makes
benzene chemically stable).
H
H
C
C
H
C
C
H
C
C
H
H
H
H
C
C
H
C
C
H
C
C
H
H
Aromatic compounds: they are unsaturated hydrocarbons that contain one or more benzene
rings in their structures. Because some of them have pleasant odors, they were called aromatic
compounds.
Arene: a compound containing one or more benzene-like rings.
Aryl group: a group derived by removal of an H atom from an arene is called an arel group
and given the symbol Ar- (phenyl C6H5-)
Naming aromatic compounds:
1. When one substituent group is attached to benzene, we write the name of the group in front
of benzene.
CH2 CH3
Cl +
NO2 +
Ch lorobenzen e
N itrob enzene
Eth ylb enzene
Note: The IUPAC system retains certain common names for several of the simpler
monosubstituted alkylbenzenes:
CH3
Toluen e
OH
Phen ol
OCH3
A nisole
NH2
A niline
O
C-H
Ben zaldehyde
O
C-OH
Benzoic acid
2. When two or more substituents are attached to benzene, the ring is numbered give the
lowest numbers to the substituents. The substituents are listed alphabetically.
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Chemistry 20
Mt SAC
Note: When two substituents occur on a benzene ring, three isomers are possible. We locate
the substituents either by numbering the atoms of the ring or by using the locators ortho (o),
meta (m), and para (p).
1
4
1
2
3
1
1,2- or
1,3- or meta
1,4- or para
ortho-
1
COOH
Br
2
1
CH2 CH3
CH3
4
3
2
3
2
CH3
2-Bromobenzoic acid
(o-Bromoben zoic acid)
1
Cl
1-Ch loro-4-ethylbenzen e
(p-Chloroeth ylb enzene)
1,3-D imethylben zene
(m-Xylen e)
Note: The substituent group derived by loss of an H atom from benzene is called a phenyl
group (C6H5-).
4
C6 H5
1-Phenylcyclohexene
3
2
1
4-Phenyl-1-butene
Resonance: although, benzene has three double bonds, it does not behave like an alkene. We
can draw two Lewis structures for benzene (with two different positions for its double bonds).
Each individual Lewis structure is called a contribution structure. The real molecule of
benzene is a resonance hybrid of these two Lewis structures. Therefore, the carbon-carbon
bonds are neither single nor double but rather something intermediate between the two
extremes. Whenever we find resonance, we find stability. Benzene ring is greatly stabilized
by resonance, which explains why it does not undergo the addition reactions typical of
alkenes.
H
H
C
C
H
C
C
H
C
C
H
H
H
H
C
C
H
C
C
H
C
C
H
H
Chemical properties of benzene and its derivatives: The most important characteristic
reaction of aromatic compounds is substitution at a ring carbon (aromatic substitution). We
can name for them these three reactions: 1.Halogenation 2. Nitration 3. Sulfonation.
Halogenation: in the present of an iron catalyst, chlorine reacts rapidly with benzene to give
chlorobenzene and HCl.
Dr. Behrang Madani
Chemistry 20
Mt SAC
FeCl3
H + Cl2
Cl + HCl
Ch lorobenzen e
Benzen e
Nitration: when we heat benzene or one of its derivatives with a mixture of concentrated
nitric and sulphuric acids, a nitro (-NO2) group replaces one of the hydrogen atoms bonded to
the ring. In this reaction, sulfuric acid is a catalyst and it is added to speed up the reaction.
H2 SO4
H + HNO3
NO2 + H2 O
N itrob enzene
Sulfonation: By heating an aromatic compound with concentrated sulfuric acid, one of the H
atom is replaced by sufonic acid (-SO3H) group (all of which are strong acids, comparable in
strength to sulfuric acid).
H + H2 SO4
SO3 H + H2 O
Benzenes ulfon ic acid
Phenol: a compound that contains an hydroxyl group (-OH) bonded to a benzene ring.
Substituted phenols are named either as derivatives of phenol or by common names.
OH
OH
1
3
3 -M e th y l p h e n o l
(m
- C re s o l )
(m-Cresol)
OH 2
1 2
NO2
3
Cl
3-Chloro-2-nitrophenol
Phenol
Note: Phenols are widely distributed in nature. Many of them are used as antiseptics and
disinfectants. Phenols are weak acids, with pKa values of approximately 10. Most phenols are
insoluble in water, but they react with strong bases, such as NaOH and KOH, to form watersoluble salts.
OH + NaOH
Phenol
H2 O
O- Na + + H2 O
S od ium phenoxide
(a w ater-soluble salt)
Autoxidation: foods and other materials that contain C=C can be oxidized only in presence
of oxygen (no other reactant). This oxidation is called Attoxidation. In this reaction, an R-H
group is converted to an R-O-O-H (hydroperoxide) group.
H
CH2 CH=CH-CH
Section of a fatty
acid h yd rocarbon chain
Dr. Behrang Madani
+
light
O2
or h eat
Oxygen
Chemistry 20
O-O-H
CH2 CH=CH-CH-CH2
A hydroperoxid e
Mt SAC
Note: Phenols are antioxidants and they can prevent autoxidation. Therefore, they are added
to foods to “retard spoilage”. For example, Vitamin E as a phenol is an antioxidant.
O
H
3
HO
Vitamin E
Dr. Behrang Madani
Chemistry 20
Mt SAC