Benzene has 6π electrons delocalized in 6p orbitals that overlap above and below the plane of the ring. Because benzene’s six pie electrons satisfy Huckel’s rule, benzene is especially stable. Reaction that keep the aromatic ring intact are therefore favoured

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Mohd Kamil Hussain (Ph.D)
Asst. Professor Govt Raza PG College
Rampur
Aromatic Substitution

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Electrophilic aromatic substitution :A hydrogen atom is replaced by an electrophile
Nucleophilic aromatic substitution :A leaving group replaced by a nucleophile
Benzene has 6π electrons delocalized in 6p
orbitals that overlap above and below the plane
of the ring. Because benzene’s six pie electrons
satisfy Huckel’s rule, benzene is especially
stable. Reaction that keep the aromatic ring
intact are therefore favoured
Aromatic Substitution
Dr. Mohd Kamil Hussain

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Electrophilic aromatic substitution
 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.
 There are five main examples of electrophilic aromatic substitution

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Step 1: Formation of arenium ion: The initial step is the attack of an electrophile
creating a resonance stabilized carbocation called arenium ion, which is also known as the
Wheland Intermediate
Step 2: Formation of product and regeneration of Aromaticity: In the second step the
H atom departs. This leads to regeneration of aromatic stabilization. The second step is
nearly always faster than the first, making the first rate determining, and the reaction is
second order.

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Directing and activating effects

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Some Examples of Aromatic Electrophilic Substitution

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Aromatic Nucleophilic Substitution
Involve bond formation between aromatic carbon atom and nucleophile along with
bond breaking to the leaving group.
Nucleophilic substitution reactions require:
Addition/elimination mechanism

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Elimination/addition mechanism (The benzyne mechanism)

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Conclusions
Arenium ion/Wheland intermediate/ σ-complex is the name of the intermediate involved
in the electrophilic substitution of aromatic compounds .
The group already on the ring determines which position the new group will take and
whether the reaction will be slower or faster than with benzene. Groups that increase the
reaction rate are called activating (o-/p- directing) and those that slow it are deactivating
(m- directing).
Orientation in other ring systems including hetero-aromatic systems can be predicted on
similar lines by considering the resonating structures of the σ-complexes.
Aromatic nucleophilic substitution reactions involve attack of nucleophile on
substituted aromatic rings and thus are relatively rare.
Aromatic nucleophilic substitutions are favored by electron withdrawing substituents on
benzene ring and good leaving groups.
Benzyne mechanism gives rise to regio-selectivity in product formation.
For unimolecular aromatic nucleophilic substitution diazonium salts are good
substrates as di-nitrogen is a good leaving group.
A number of useful name reactions such as Chichibabin reaction and Von Ritcher
rearrangement involve aromatic nucleophilic substitution mechanism