Chapter 15 Electrophilic Aromatic Substitution...When benzene is treated with a mixture of nitric...

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abdollahi@yazd.ac.ir 1

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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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.

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

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

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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

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.

Starting with benzene and using any other necessary reagents of your choice, design

a synthesis of the following compound:

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Chapter 15 Electrophilic Aromatic Substitution...When benzene is treated with a mixture of nitric...

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