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Electrophilic Aromatic
Substitution
Chapter 15
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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
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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
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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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Mechanism of Sulfonation
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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.
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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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Halogenation
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Mechanism of Bromination
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Orientation and Recativity
Activating Groups
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Nitration of Anisole
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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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Multiple Substituents
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Steric effects
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Synthesis Strategies
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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.
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Starting with benzene and using any other necessary reagents of your choice, design
a synthesis of the following compound: