Electrophilic Aromatic
Substitution
Dr. Marwa Eid
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No addition reactions
Aromatic substitution:
One of the H atoms is repalecd by some groups.
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A Resonance Picture of Bonding in Benzene
resonance
6 -electron delocalized
over 6 carbon atoms
The Stability of Benzene
benzene exhibits special stability due to resonance
delocalization of  -electrons.
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In chemistry, an electrophile (electron-lover)
is
a
reagent
attracted
to
electrons
that
participates in a chemical reaction by accepting
an electron pair in order to bond to a nuclephile.
Because electrophiles accept electrons, they
are Lewis acids.
4`j
• Benzene does not undergo addition reactions like
other unsaturated hydrocarbons, because addition
would yield a product that is not aromatic.
• Substitution of a hydrogen keeps the aromatic ring
intact.
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• The characteristic reaction of benzene is electrophilic aromatic
substitution —a hydrogen atom is replaced by an electrophile.
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Electrophilic Aromatic Substitution
H
E
d+ d–
+E
+H
Y
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Y
Electrophilic Aromatic Substitution
Nitration
Sulfonation
Halogenation
Friedel-Crafts alkylation
Friedel-Crafts Acylation
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9
• Nitration and sulfonation introduce two different functional groups
into the aromatic ring.
• Nitration is especially useful because the nitro group can be
reduced to an NH2 group.
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H
+ HONO2
H2SO4
NO2
+ H2O
Nitrobenzene
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H
heat
+ HOSO2OH
SO2OH
+ H2O
Benzenesulfonic acid
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• In halogenation, benzene reacts with Cl2 or Br2
in the presence of a Lewis acid catalyst
(electron pair acceptor), such as FeCl3 or
FeBr3,
to
give
chlorobenzene
or
bromobenzene, respectively.
• Halogenation reactions with I2 and F2 are not synthetically useful
because I2 is too un-reactive and F2 reacts too violently.
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 Friedel - Crafts alkylation
ArH + RCl/AlCl3
Ar-R + HCl
 Friedel - Crafts acylation
ArH + RCOCl/AlCl3
Ar-COR + HCl
H
+ (CH3)3CCl
AlCl3
C(CH3)3
+ HCl
2-chloro, 2-methyl propane
tert-Butylbenzene
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Friedel-Crafts Alkylation of benzene
• In Friedel-Crafts alkylation, treatment of benzene
with an alkyl halide and a Lewis acid (AlCl3) forms
an alkyl benzene.
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O
O
H
AlCl3
+ CH3CH2CCl
CCH2CH3
+ HCl
1-Phenyl-1-propanone
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18
E+
H
H
H
H
H
H E
H
H
H
+
H
H
H
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this step restores aromaticity of ring
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