Arenes
Introduction
1. Hydrocarbon that contain rings which
stabilised by electron delocalisation are
called aromatic hydrocarbon or arenes
2. The simplest yet most important member of
the aromatic hydrocarbons is benzene
Naming Aromatic Hydrocarbon
• Monosubstituted alkylbenzene is named as
derivatives of benzene. For example:
CH3
methylbenzene
(Toluene)
CHO
Benzaldehyde
CH2CH3
ethylbenzene
NH2
Aniline
(Phenylamine)
OH
Phenol
CH=CH2
Phenylethene
(Styrene)
COOH
Benzeoic acid
• The IUPAC system retains certain common names for
some simpler alkylbenzenes. For example: Aniline,
styrene, toluene.
• When two or more substituents on a benzene ring, three
structural isomers are possible.
a) Location of two substituents may be indicated by the
prefixs ortho, meta or para
– 1,2 is equivalent to ortho, o– 1,3 is equivalent to meta, m– 1,4 is equivalent to para, pb) The order of decreasing priorities of common substituents
is:
COOH > SO3H> CHO> CN > C=O> OH> NH2> R>
NO2> X
Where R= alkyl group ;
X= halogen
CH3
1
CH3
OH
COOH
CH3
2
6
3
5
4
1,2-dimethylbenzene
o-xylene
1,4-dimethylbenzene
p-xylene
CH3
Benzeoic acid
Phenol
CH3
CH3
CH3
1
Br
6
2
3
5
4
Br
Br
2-bromomethylbenzene 3-bromomethylbenzene 4-bromomethylbenzene
p-bromotoluene
m-bromotoluene
o-bromotoluene
OH
OH
1
NO2
6
2
5
3
4
NO2
2-nitrophenol
o-nitrophenol
4-nitrophenol
p-nitrophenol
• When three or more groups are on the
benzene ring, a numbering system must be
used to locate them. For example,
CH3
CH3
1
NO2
6
2
6
3
5
1
4
NO2
2,4-dinitromethylbenzene
Br
2
3
5
4
NO2
2-bromo-4-nitrophenol
• In more complex molecules, the benzene ring
is named as a substituent on a parent chain. In
this case, the C6H5- group is called a phenyl
group. For example
1 2
3 4
2 3 4 5
CH2CH
CHCH3
CHCH2CH2CH3
1
CH3
1-phenylbut-2-ene
2-phenylpentane
Exercise
• Name the following aromatic compounds
CH2CH3
NO2
Cl
CH3
Cl
CH3
CH3
COOH
COOH
O2N
H3C
NO2
NO2
HO
Reaction of benzene
1. Nitration
• Nitration is the substitution of a nitro group,
-NO2, for one of the hydrogen atoms on the
benzene ring
• Benzene reacts with a mixture of
concentrated nitric acid and concentrated
sulphuric acid
• The product is nitrobenzene, a pale yellow
liquid
NO2
+
HNO3
concentrated H2SO4
o
<55 C
refulx
+ H2O
2. Halogenation
– This reaction involves substitution of a halogen
atom for a hydrogen atom on the aromatic ring
– In the presence of a catalyst or halogen carrier
such as aluminium chloride, AlCl3, benzene is
substituted by chlorine or bromine at room
temperature.
Cl
AlCl3
+ Cl2
room temp
+ HCl
• The catalyst aluminium chloride, AlCl3, an
electron-deficient compound which has only
six electrons in its outer shell, accepts a lone
pair of electrons from one of the chlorine
atoms, which induces polarisation in the
chlorine molecule.

Cl
Cl
AlCl3
• The positively charged end of the chlorine
molecule is now an electrophile and attacks
the benzene ring.
3. Alkylation: Friedel-Crafts reaction
• This reaction substitutes an alkyl group for a
hydrogen atom on an aromatic ring
• When benzene is warmed with
chloromethane, CH3Cl and aluminium
chloride (catalyst) under anhydrous
conditions, a substitution reaction occurs and
methylbenzene is formed
CH3
+
anhydrous AlCl3
+ HCl
CH3Cl
warm
– AlCl3 polarise the CH3 Cl molecule


H3C
Cl
AlCl3
– The positively charged methyl group attacks the
benzene ring and electrophilic substitution occurs


H3C
Cl
AlCl3
H
CH3
CH3+ + AlCl4CH3
CH3
+
H+
4. Acylation
• Involves substitution of an acyl group, RCO,
for a hydrogen atom on the aromatic ring.
• When benzene is added to ethanoyl chloride
and AlCl3, under anhydrous conditions, a
substitution reaction occurs and
phenylethanone is formed.
O
O
C
AlCl3
+ H3C
C
CH3
+ HCl
Cl
heat
Exercise
• Write the structural formula for the producta
formed from Friedal Crafts alkylation or
acylation of benzene with
(a) CH3CH2CH2Cl
(b)
CH3CH2CH2COCl
5. Sulphonation
• Involves substitution of an –SO3H group for a
hydrogen atom on the aromatic ring.
• If benzene and [H2SO4] are reflux together for
several hours, benzenesulphonic acid is
formed:
+ concentrated H2SO4
SO3H
reflux
+ H2O
Benzenesulphonic
acid
Reaction of methylbenzene
1. Reaction with chlorine
• When chlorine, Cl2, is bubbled through
boiling methylbenzene in strong sunlight or
uv light, substitution takes place on the
methyl group:
CH2Cl
CH3
Cl2, boil
uv light
CHCl2
CCl3
• If Cl2 is bubbled through methylbenzene in the absence of uv
light but in the presence of AlCl3, the ring is substitutes. A
mixture of two isomers is obtained.
• White fumes of HCl are given off
• Electron density of the benzene ring more strongly at the 2and 4- positions
CH3
CH3
Cl
+ Cl2
AlCl3
+ HCl
room temperature
roo
m
AlC
l3
tem
per
atu
re
CH3
+ HCl
Cl
2. Reaction with HNO3
• When methyl benzene is added to HNO3 and
concentrated H2SO4, at room temperature, a
mixture of 2- and 4-nitromethylbenzene
CH3
CH3
NO2
+ HNO3
H2SO4
room temperature
roo
m
HS
2
tem O4
per
atu
re
+ H2O
CH3
+ H2O
NO2
• If the temperature is raised, two or three nitro
groups, NO2 are introduced
CH3
CH3
NO2 O N
2
NO2
2,4-diphenylmethylbenzene
NO2
NO2
2,4,6-triphenylmethylbenzene
3. Reaction with CH3I (Friedal craft alkylation)
• When methlbenzene is added to CH3I in
AlCl3 at 20 oC, a mixture of 1,2dimethylbenzene and 1,4-dimethylbenzene is
CH
formed CH
3
3
CH3
+ CH3I
AlCl3
room temperature
roo
m
AlC
tem l3
per
atu
re
+ HI
CH3
+ HI
CH3
4. Reaction with CH3COCl (FriedalCraft Acylation)
•
•
The reaction of an acyl chloride with an aromatic
hydrocarbon is known as FriedalCraft acylation.
Involves substitution of an acyl group, RCO-, for a
hydrogen atom on the aromatic ring in the presence of
CH
CH
AlCl3.
O
3
3
O
+ H3C
C
C
Cl
ethanoyl
chloride
AlCl3
80oC
CH3
+ HCl
Al
80 o Cl3
C
CH3
+ HCl
C
O
CH3
Exercise
• Write the structural formula for the products
formed by Friedal Crafts acylation or alkylation
of methylbenzene with
(a) C6H5COCl
(b) CH3CH2Br
5. Reaction with SO3
• When methylbenzene is added to fuming
H2SO4 containing SO3 at 0oC, a mixture of 2and 4-methylbenzenesulphonic is obtained.
CH3
CH3
2
1
H2SO4
+ SO3
+ H+
o
0 C
2-methylbenzenesulphonic acid
H
0
SO3H
2 SO
4
o
C
CH3
4
+ H+
1
SO3H
4-methylbenzenesulphonic acid
6. Oxidation of alkylbenzene
• Alkylbenzenes are oxidised to benzoic acid on
prolonged reflux with KMnO4 or K2Cr2O7
R
KMnO4 / K2Cr2O7
COOH
+ H2O + CO2
• Example
CH3
+ 3 [O] KMnO4 / K2Cr2O7
reflux
CH2CH3 + 6 [O]
KMnO4 / K2Cr2O7
reflux
COOH
COOH
+ H2O
+ 2H2O + CO2
Exercise
• Predict the products from treatment of each
of the following compounds with KMnO4 /
H2SO4 under reflux.
CH3
(a)
CH3
(b)
CH3
(c)
CH2CH2CH3
CH3
CH3
Exercise
• Name compound P, Q, R and S below
CH3
CH3
CH3
R
AlCl3
P+Q
NO2
fuming
H2SO4
4-methylbenzenesulphonic acid
(S)
Br
Exercise
• Draw the structural formulae of the organic
products formed when methylbenzene reacts
with
(a) Concentrated nitric acid and concentrated
sulphuric acid at room temp
(b) Chlorine in the presence of anhydrous
aluminium chloride
(c) Chloromethane in the presence of aluminium
chloride