Organic Chemistry,6th Edition L.G.Wade,Jr. Chapter 17 Reactions of Aromatic Compounds Jo Blackburn Richland College,Dallas,TX Dallas County Community College District ©2006，Prentice Hall
Chapter 17 Reactions of Aromatic Compounds Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2006, Prentice Hall Organic Chemistry, 6th Edition L. G. Wade, Jr
Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. B: sigma complex substituted Copyright 2005 Pearson Prentice Hall,Inc. => Chapter 17 2
Chapter 17 2 Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. =>
Mechanism Step 1:Attack on the electrophile forms the sigma complex. H H H H H H〈 H《 E+ H H H sigma complex (arenium ion) Step 2:Loss of a proton gives the substitution product. H H H base: +base-H H H H H > Copyright 2005 Pearson Prentice Hall,Inc. Chapter 17 3
Chapter 17 3 Mechanism Step 1: Attack on the electrophile forms the sigma complex. => Step 2: Loss of a proton gives the substitution product
Bromination of Benzene Requires a stronger electrophile than Br2. Use a strong Lewis acid catalyst,FeBr3. :Br-Br:FeBr3 Br-Br一feBr3 H H H H H Br-Br-FeBr3 +FeBr4 H H H H H B + HBr Chapter 17
Chapter 17 4 Bromination of Benzene • Requires a stronger electrophile than Br2 . • Use a strong Lewis acid catalyst, FeBr3 . Br + HBr Br Br FeBr 3 Br Br FeBr 3 + - Br Br FeBr3 H H H H H H H H H H H H Br + + FeBr4 + - _ =>
Comparison with Alkenes Cyclohexene adds Br2,AH =-121 kJ Addition to benzene is endothermic,not normally seen. Substitution of Br for H retains aromaticity,AH =-45 kJ. Formation of sigma complex is rate- limiting. > Chapter 17 5
Chapter 17 5 Comparison with Alkenes • Cyclohexene adds Br2 , H = -121 kJ • Addition to benzene is endothermic, not normally seen. • Substitution of Br for H retains aromaticity, H = -45 kJ. • Formation of sigma complex is ratelimiting. =>