Richard F. Daley and Sally J. Daley Organic

Richard F. Daley and Sally J. Daley



Organic

Chemistry

Chapter 18

Aromatic Substitution Reactions

18.1 Mechanism of Aromatic Electrophilic Substitution 914

18.2 The Nitration of Benzene

917

18.3 Halogenation and Sulfonation of Benzene

920

18.4 Friedel-Crafts Alkylation and Acylation

924

18.5 Effects of Monosubstituted Arenes on Substitution 928

18.6 Rate Effects with Monosubstituted Arenes

932

18.7 Classification of Substituents

935

18.8 Friedel-Crafts Acylation

943

Synthesis of o-Benzoylbenzoic Acid

947

18.9 Multiple Substituent Effects

948

18.10 Substitution on Polycyclic Arenes

951

18.11 Diazotization

954

Synthesis of Methyl Orange

957

Sidebar - Sulfa Drugs

958

18.12 Other Diazonium Salt Reactions

961

18.13 Nucleophilic Aromatic Substitution

963

18.14 Benzyne

965

Synthesis of Trypticene

968

18.15 Synthesis Examples

969

Key Ideas from Chapter 18

975

Organic Chemistry - Ch 18

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Daley & Daley

Copyright 1996-2005 by Richard F. Daley & Sally J. Daley All Rights Reserved.

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5 July 2005

Organic Chemistry - Ch 18

913

Daley & Daley

Chapter 18

Aromatic Substitution Reactions

Chapter Outline

18.1 18.2 18.3

18.4 18.5

18.6

18.7

18.8 18.9

18.10 18.11 18.12 18.13 18.14 18.15

Mechanism of Electrophilic Aromatic Substitution

The mechanism of electrophilic substitution of benzene

The Nitration of Benzene

A case study of aromatic electrophilic substitution

Halogenation and Sulfonation of Benzene

The mechanism of chlorination, bromination, and sulfonation of benzene

Friedel-Crafts Alkylation

Formation of alkyl benzenes

Effects of Monosubstituted Arenes on Substitution

The effects of one substituent on the position of reaction by a second substituent

Rate Effects with Monosubstituted Arenes

The effect of one substituent on the rate of reaction by a second substituent

Classification of Substituents

A listing of common substituents showing their directive and rate controlling effects

Friedel-Crafts Acylation

Formation of acyl benzenes

Multiple Substituent Effects

Predicting the position of substitution when two or more substituents are on the ring

Substitution on Polycyclic Arenes

Aromatic electrophilic substitution on polycyclic aromatic compounds

Diazotization

Diazotization and the use of the diazonium ion as an electrophile

Other Diazonium Salt Reactions

Replacement of the diazonium ion with a variety of groups

Nucleophilic Aromatic Substitution

Nucleophilic substitution on an aromatic ring

Benzyne

The formation and reaction of the reactive benzyne intermediate

Synthesis Examples

Organic synthesis using aromatic electrophilic substitution reactions



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Objectives

Understand the mechanism for aromatic electrophilic substitution reactions

Recognize appropriate electrophiles that will substitute on an aromatic ring

Be able to predict the position of a new substitution on an aromatic ring with one or more existing substituents

Know how the structure of one substituent affects the rate of reaction for adding a second substituent on the ring

Know the diazotization reaction and how the diazonium salts react Understand the nucleophilic substitution reaction and its

mechanism Be able to use the reactions in this chapter in synthesis

`Tis true; there's magic in the web of it. . . --Shakespeare

C hapter 17 presents the characteristics that make a compound aromatic. Understanding those characteristics is the foundation for this chapter, as it examines the various types of reactions that occur with aromatic hydrocarbons. The chapter first discusses electrophilic aromatic substitution--a major aromatic hydrocarbon mechanistic type. Electrophilic substitution allows you to directly introduce a variety of functional groups onto the aromatic ring. The chapter then looks at several examples of electrophilic substitutions on benzene and its derivatives. Much of the rest of the chapter discusses how the substituents already on the ring affect the placement of additional substituents.

18.1 Mechanism of Electrophilic Aromatic Substitution

Chapter 14 discusses electrophilic addition reactions to the bond of an alkene. The result of an electrophilic addition reaction is



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Daley & Daley

that a new atom or group of atoms adds to both carbons involved in the double bond. The mechanism proceeds in two steps: The electron cloud of the bond reacts with the electrophile to form an intermediate carbocation. A nucleophile adds to the intermediate carbocation, thus replacing the bond with two new bonds.

E

E Nu:

Nu

E

Benzenoid aromatic compounds contain benzene rings or fused benzene rings.

The AdE2 mechanism.

Benzenoid aromatic compounds also have an electron-rich bond cloud that is susceptible to attack by an electrophile. As with an alkene, the electron cloud of the benzenoid aromatic compound reacts with the electrophile and adds the electrophile to one of the carbons in the ring. This reaction produces a carbocation intermediate.

Step 1

A complex is a resonance-stabilized carbocation intermediate.

The complex formed by the attack of the electrophile

The carbocation intermediate, called a complex, is not aromatic. The complex is written as follows to show the delocalization of the positive charge:

H

+

E



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