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Electrophilic Aromatic
Substitution
Chapter 15
Introduction
Based on its structure and properties, what kinds of reactions should benzene
undergo? Are any of its bonds particularly weak? Does it have electron-rich or
electron-deficient atoms?
As a result, the characteristic reaction of benzene is electrophilic aromatic
substitution—a hydrogen atom is replaced by an electrophile.
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Examples of
electrophilic
aromatic
substitution
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General Mechanism—Electrophilic Aromatic Substitution
Nitration
When benzene is treated with a mixture of nitric acid and sulfuric acid, a
nitration reaction occurs in which nitrobenzene is formed.
This reaction proceeds via an electrophilic aromatic substitution in which a
nitronium ion (NO2+) is believed to be the electrophile.
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Mechanism of Nitration
Sulfonation
When benzene is treated with fuming sulfuric acid, a sulfonation reaction
occurs and benzene-sulfonic acid is obtained.
he inefficient over-lap of these orbitals suggests
that we should consider the bond to be a single
bond that exhibits charge separation (S+ and O-),
rather than a double bond.
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Friedel-Crafts Alkylation
The Friedel-Crafts alkylation, discovered by Charles Friedel and James Crafts in 1877,
makes possible the installation of an alkyl group on an aromatic ring.
Mechanism o Friedel-Crafts Alkylation
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Primary alkyl halides are not converted into carbocations, since primary
carbocations are extremely high in energy.
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Deactivating Groups
A nitro group is inductively electron withdrawing, because a positively
charged nitrogen atom is extremely electronegative.
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Determining the Directing Effects of a Substituent
The activators are ortho-para directors, while the deactivators, except for the halo-
gens, are meta directors.
Activators
Strong activators are characterized by the presence of a lone pair immediately
adjacent to the aromatic ring.
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Deactivators
Halogens are weak deactivators.
Moderate deactivators are groups that exhibit a p bond to an electronegative
atom.
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An exception: halogen substituents, although deactivating, direct ortho and para
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Limitations:
1. Nitration cannot be performed on a ring that contains an amino group.
2. A Friedel-Crafts reaction (either alkylation or acylation) cannot be
accomplished on rings that are either moderately or strongly deactivated.
Starting with benzene and using any other necessary reagents of your choice, design
a synthesis of the following compound:
