CHE 242 Unit VI The Study of Conjugated Systems, Aromaticity and Reactions of Aromatic Compounds CHAPTER SEVENTEEN Terrence P. Sherlock Burlington County College 2004
Chapter 172 Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. =>
Chapter 173 Mechanism =>
Chapter 174 Bromination of Benzene Requires a stronger electrophile than Br 2. Use a strong Lewis acid catalyst, FeBr 3. Animation =>
Chapter 175 Energy Diagram for Bromination =>
Chapter 176 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. =>
Chapter 177 Nitration of Benzene Use sulfuric acid with nitric acid to form the nitronium ion electrophile. NO 2 + then forms a sigma complex with benzene, loses H + to form nitrobenzene. =>
Chapter 178 Sulfonation Sulfur trioxide, SO 3, in fuming sulfuric acid is the electrophile. =>
Chapter 179 Nitration of Toluene Toluene reacts 25 times faster than benzene. The methyl group is an activator. The product mix contains mostly ortho and para substituted molecules. =>
Chapter 1710 Sigma Complex Intermediate is more stable if nitration occurs at the ortho or para position. =>
Chapter 1711 Energy Diagram =>
Chapter 1712 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. =>
Chapter 1713 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. =>
Chapter 1714 Summary of Activators =>
Chapter 1715 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. =>
Chapter 1716 Ortho Substitution on Nitrobenzene =>
Chapter 1717 Para Substitution on Nitrobenzene =>
Chapter 1718 Meta Substitution on Nitrobenzene =>
Chapter 1719 Energy Diagram =>
Chapter 1720 Structure of Meta- Directing Deactivators The atom attached to the aromatic ring will have a partial positive charge. Electron density is withdrawn inductively along the sigma bond, so the ring is less electron-rich than benzene. =>
Chapter 1721 Summary of Deactivators =>
Chapter 1722 More Deactivators =>
Chapter 1723 Halobenzenes Halogens are deactivating toward electrophilic substitution, but are ortho, para-directing! Since halogens are very electronegative, they withdraw electron density from the ring inductively along the sigma bond. But halogens have lone pairs of electrons that can stabilize the sigma complex by resonance. =>
Chapter 1724 Sigma Complex for Bromobenzene Ortho and para attacks produce a bromonium ion and other resonance structures. => No bromonium ion possible with meta attack.
Chapter 1725 Multiple Substituents The most strongly activating substituent will determine the position of the next substitution. May have mixtures. =>
Chapter 1726 Friedel-Crafts Alkylation Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl 3. Reactions of alkyl halide with Lewis acid produces a carbocation which is the electrophile. Other sources of carbocations: alkenes + HF or alcohols + BF 3. =>
Chapter 1727 Examples of Carbocation Formation =>
Chapter 1728 Formation of Alkyl Benzene => + -
Chapter 1729 Limitations of Friedel-Crafts Reaction fails if benzene has a substituent that is more deactivating than halogen. Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl 3 produces isopropylbenzene. The alkylbenzene product is more reactive than benzene, so polyalkylation occurs. =>
Chapter 1730 Friedel-Crafts Acylation Acyl chloride is used in place of alkyl chloride. The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation. The product is a phenyl ketone that is less reactive than benzene. =>
Chapter 1731 Clemmensen Reduction Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc. =>
Chapter 1732 Gatterman-Koch Formylation Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst. Product is benzaldehyde. =>
Chapter 1733 Benzyne Mechanism Reactant is halobenzene with no electron- withdrawing groups on the ring. Use a very strong base like NaNH 2. =>
Chapter 1734 Benzyne Intermediate =>
Chapter 1735 Chlorination of Benzene Addition to the benzene ring may occur with high heat and pressure (or light). The first Cl 2 addition is difficult, but the next 2 moles add rapidly. The product, benzene hexachloride, is an insecticide. =>
Chapter 1736 Catalytic Hydrogenation Elevated heat and pressure is required. Possible catalysts: Pt, Pd, Ni, Ru, Rh. Reduction cannot be stopped at an intermediate stage. =>
Chapter 1737 Side-Chain Oxidation Alkylbenzenes are oxidized to benzoic acid by hot KMnO 4 or Na 2 Cr 2 O 7 /H 2 SO 4. =>
Chapter 1738 Side-Chain Halogenation Benzylic position is the most reactive. Chlorination is not as selective as bromination, results in mixtures. Br 2 reacts only at the benzylic position. =>
Chapter 1739 S N 1 Reactions Benzylic carbocations are resonance- stabilized, easily formed. Benzyl halides undergo S N 1 reactions. =>
Chapter 1740 S N 2 Reactions Benzylic halides are 100 times more reactive than primary halides via S N 2. Transition state is stabilized by ring. =>
Chapter 1741 Reactions of Phenols Some reactions like aliphatic alcohols: phenol + carboxylic acid ester phenol + aq. NaOH phenoxide ion Oxidation to quinones: 1,4-diketones. =>
Chapter 1742 Electrophilic Substitution of Phenols Phenols and phenoxides are highly reactive. Only a weak catalyst (HF) required for Friedel-Crafts reaction. Tribromination occurs without catalyst. Even reacts with CO 2. =>
Chapter 1743 POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5 TH EDITION” L.G. WADE ALL MATERIALS USED WITH PERMISSION OF AUTHOR PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE ORGANIC CHEMISTRY COURSE BY: ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN