1. D.A.V PUBLIC SCHOOL
,SEC 14 , FARIDABAD
TOPIC - Electrophilic Substitution reactions in Benzene
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2. What is Electrophilic Substitution of
Benzene?
• Electrophilic substitution of
benzene is the one where an
electrophile substitutes the
hydrogen atom of benzene.
As the aromaticity of benzene
is not disturbed in the
reaction, these reactions are
highly spontaneous in nature.
3. The
Mechanism
for
Electrophilic
Substitution
of Benzene
Generation of electrophile : Due to the
presence of Lewis acid, generation of
electrophile takes place. As the Lewis acid
accepts the electron pair from the attacking
reagent.
Formation of arenium ion : The electrophile
generated attacks on the benzene ring to form a
positively charged cyclohexadienyl cation better
called an arenium ion containing one
sp3 hybridized carbon atom. The positive charge is
effectively distributed over three carbon atoms by
resonance which makes it partially stable.
Removal of positive charge from the
carbocation intermediate : The arenium ion
finally loses its proton from sp3 hybridized carbon
to a Lewis base restoring the aromaticity.
5. Nitration of Benzene • Benzene reacts with nitric acid at 323-333k in
presence of sulphuric acid to form nitrobenzene.
This reaction is known as nitration of Benzene.
6. Sulfonation of
Benzene
• Sulfonation of benzene is a process of heating
benzene with fuming sulphuric acid
(H2SO4 +SO3) to produce benzenesulfonic acid.
The reaction is reversible in nature.
7. Halogenation of
Benzene
• Benzene reacts with halogens in the presence of
Lewis acid like FeCl3, FeBr3 to form aryl halides.
This reaction is termed as halogenation of
benzene.
8. Structure of Benzyne
• Benzyne is a highly distorted alkyne
• The triple bond uses sp-hybridized carbons, not the usual sp
• The triple bond has one x bond formed by p-p overlap and
another by weak sp²-sp² overlap.
9. Origins of
Substituent
Effects
• An interplay of inductive effects
and resonance effects
• Inductive effect - withdrawal or
donation of electrons through a o
bond = Polar Covalent Bonds
• Resonance effect - withdrawal or
donation of electrons through a
bond due to the overlap of a p
orbital on the substituent with a p
orbital on the aromatic ring.
10. Inductive
Effects
• Controlled by electronegativity and
the polarity of bonds in functional
groups
• Halogens, C=O, CN, and NO,
withdraw electrons through a bond
connected to ring
• Alkyl groups donate electrons
11. Explanation of Substituent Effects
Activating groups donate
electrons to the ring.
stabilizing the carbocation
intermediate
1
Deactivating groups
withdraw electrons from the
ring, destabilizing
carbocation intermediate.
2
