Chapter 17 Handouts

Chapter 17 Handouts - Chapter 17 Reactions of Aromatic...

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Chapter 17 Reactions of Aromatic Compounds
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Chapter 17 2 Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. =>
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Chapter 17 3 Mechanism (Important!) => (Base is usually The counteranion of E + )
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Chapter 17 4 Bromination of Benzene • Requires a stronger electrophile than Br 2 . • Use a Lewis acid catalyst, FeBr 3 . Br Br FeBr 3 Br FeBr 3 3 H H H H H H H H H H H H + + FeBr 4 _ HBr + + - =>
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Chapter 17 5 Energy Diagram for Bromination => TS resembles cation intermediate. Factors that stabilize (destabilize) intermediate have same effect on TS - IMPORTANT
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Chapter 17 6 Chlorination and Iodination • Chlorination is similar to bromination. Use AlCl 3 as the Lewis acid catalyst. • Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion. H + HNO 3 I 2 1 / 2 I + NO 2 H 2 O + + ++ =>
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Chapter 17 7 Nitration of Benzene Use sulfuric acid with nitric acid to form the nitronium ion electrophile. =>
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Chapter 17 8 Sulfonation Sulfur trioxide, SO 3 , in fuming sulfuric acid is the electrophile. S O OO S O S O S O + ++ _ _ _ S O O O H S O O O H + _ S HO O O benzenesulfonic acid =>
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Chapter 17 9 Desulfonation • All steps are reversible, so sulfonic acid group can be removed by heating in dilute sulfuric acid. • A similar process is used to place deuterium in place of hydrogen on benzene ring. D + is the electrophile in this case. Benzene- d 6 => D D D D D D D 2 SO 4 /D 2 O large excess H H H H H H
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Chapter 17 10 What’s the mechanism? CH 3 NO 2 SO 3 H H + , H 2 O, heat CH 3 NO 2 excess D 2 SO 4 D D D D D D
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Chapter 17 11 Nitration of Toluene • Toluene reacts 25 times faster than benzene. So the methyl group is an activator . • The product mix contains mostly ortho and para substituted molecules. =>
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Chapter 17 12 Sigma Complex Intermediate is more stable if nitration occurs at the ortho or para position. =>
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Chapter 17 13 Energy Diagram =>
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Chapter 17 14 Activating, O -, P - Directing Substituents • Alkyl groups stabilize the sigma complex by induction , donating electron density through the sigma bond. • Substituents with a lone pair of electrons stabilize the sigma complex by resonance . OCH 3 H NO 2 + 3 H 2 + =>
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Chapter 17 15 Substitution on Anisole =>
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Chapter 17 16 The Amino Group Aniline reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the HBr that’s also formed. NH 2 Br 2 3 H 2 O, NaHCO 3 2 => Why?
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Chapter 17 17 Summary of Activators =>
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Chapter 17 18 Deactivating Meta- Directing Substituents • Electrophilic substitution reactions for nitrobenzene are 100,000 times slower than for benzene. • The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers. • Meta-directors deactivate all positions on the ring, but the meta position is less deactivated.
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This note was uploaded on 04/08/2008 for the course CHEM 224 taught by Professor Pearson during the Spring '08 term at Case Western.

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Chapter 17 Handouts - Chapter 17 Reactions of Aromatic...

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