Chapter 17 - Reactions of aromatic compounds. In this chapter, we will study several of these reactions and then consider how substituents on the ring influence its reactivity toward electrophilic aromatic substitution and the regiochemistry seen in the products. We will also study other reactions of aromatic compounds, including nucleophilic aromatic substitution, addition reactions, reactions of side chains, and special reactions of phenols. | Chapter 17 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Reactions of Aromatic Compounds Chapter 17 Electrophilic Aromatic Substitution Although benzene’s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. Aromaticity is regained by loss of a proton. Chapter 17 Mechanism of Electrophilic Aromatic Substitution Chapter 17 Bromination of Benzene Chapter 17 Mechanism for the Bromination of Benzene: Step 1 Before the electrophilic aromatic substitution can take place, the electrophile must be activated. A strong Lewis acid catalyst, such as FeBr3, should be used. B r B r F e B r 3 B r B r F e B r 3 + - (stronger electrophile than Br2) Chapter 17 Step 2: Electrophilic attack and formation of the sigma . | Chapter 17 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Reactions of Aromatic Compounds Chapter 17 Electrophilic Aromatic Substitution Although benzene’s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. Aromaticity is regained by loss of a proton. Chapter 17 Mechanism of Electrophilic Aromatic Substitution Chapter 17 Bromination of Benzene Chapter 17 Mechanism for the Bromination of Benzene: Step 1 Before the electrophilic aromatic substitution can take place, the electrophile must be activated. A strong Lewis acid catalyst, such as FeBr3, should be used. B r B r F e B r 3 B r B r F e B r 3 + - (stronger electrophile than Br2) Chapter 17 Step 2: Electrophilic attack and formation of the sigma complex. Step 3: Loss of a proton to give the products. Mechanism for the Bromination of Benzene: Steps 2 and 3 Chapter 17 Energy Diagram for Bromination Chapter 17 Chlorination and Iodination Chlorination is similar to bromination. AlCl3 is most often used as catalyst, but FeCl3 will also work. Iodination requires an acidic oxidizing agent, like nitric acid, to produce iodide cation. H+ + HNO3 + ½ I2 I+ + NO2 + H2O Chapter 17 Predict the major product(s) of bromination of p-chloroacetanilide. The amide group (–NHCOCH3) is a strong activating and directing group because the nitrogen atom with its nonbonding pair of electrons is bonded to the aromatic ring. The amide group is a stronger director than the chlorine atom, and substitution occurs mostly at the positions ortho to the amide. Like an alkoxyl group, the amide is a particularly strong activating group, and the reaction gives some of the dibrominated product. Solved Problem 1 Solution Copyright © 2006 .
