Chapter 17 Reactions of Aromatic Compounds

Transcription

1 rganic Chemistry, 6 th Edition L. G. Wade, Jr. Chapter 17 Reactions of Aromatic Compounds Jo Blackburn Richland College, Dallas, TX Dallas County Community College District 2006, Prentice all Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. Chapter

2 Mechanism Step 1: Attack on the electrophile forms the sigma complex. Step 2: Loss of a proton gives the substitution product. Chapter 17 3 Bromination of Benzene Requires a stronger electrophile than Br 2. Use a strong Lewis acid catalyst, FeBr 3. Br Br FeBr - 3 Br Br FeBr 3 - Br Br FeBr 3 Br Br _ FeBr 4 Br Chapter

3 Comparison with Alkenes Cyclohexene adds Br 2, = -121 kj Addition to benzene is endothermic, not normally seen. Substitution of Br for retains aromaticity, = -45 kj. Formation of sigma complex is ratelimiting. Chapter 17 5 Energy Diagram for Bromination Chapter

4 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. N 3 1/2 I 2 I N 2 2 Chapter 17 7 Nitration of Benzene Use sulfuric acid with nitric acid to form the nitronium ion electrophile. S N N S 4 _ N 2 N N 2 then forms a sigma complex with benzene, loses to form nitrobenzene. Chapter

5 Sulfonation Sulfur trioxide, S 3, in fuming sulfuric acid is the electrophile. _ S S S S S _ S S benzenesulfonic acid Chapter 17 9 Desulfonation All steps are reversible, so sulfonic acid group can be removed by heating in dilute sulfuric acid. This process is used to place deuterium in place of hydrogen on benzene ring. large excess D 2 S 4 /D 2 D D D D D Benzene-d 6 Chapter D 5

6 Nitration of Toluene Toluene reacts 25 times faster than benzene. The methyl group is an activating group. The product mix contains mostly ortho and para substituted molecules. Chapter Sigma Complex Intermediate is more stable if nitration occurs at the ortho or para position. Chapter

7 Energy Diagram Chapter Activating, -, 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. C 3 N 2 C 3 N 2 Chapter

8 Substitution on Anisole Chapter The Amino Group Aniline, like anisole, reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the Br that s also formed. Chapter

9 Summary of Activators Chapter 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

10 rtho Substitution on Nitrobenzene Chapter Para Substitution on Nitrobenzene Chapter

11 Meta Substitution on Nitrobenzene Chapter Energy Diagram Chapter

12 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 Summary of Deactivators Chapter

13 More Deactivators Chapter alobenzenes alogens 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

14 Sigma Complex for Bromobenzene rtho and para attacks produce a bromonium ion and other resonance structures. No bromonium ion possible with meta attack. Chapter Energy Diagram Chapter

15 Summary of Directing Effects Chapter Multiple Substituents The most strongly activating substituent will determine the position of the next substitution. May have mixtures. C 3 C 3 C 3 S 3 S 3 2 N 2 S 4 2 N 2 N S 3 Chapter

16 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. ther sources of carbocations: alkenes F, or alcohols BF 3. Chapter Examples of Carbocation Formation Cl C 3 C C 3 C 3 _ AlCl 3 C Cl AlCl 3 3 C 2 C C C 3 F _ F 3 C C C 3 3 C C C 3 BF 3 BF 3 3 C C C 3 3 C C C 3 _ BF3 Chapter

17 Formation of Alkyl Benzene C 3 C C 3 C(C 3 ) 2 F - F B C(C 3 ) 2 F C 3 C F C 3 F Chapter F B 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

18 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 Mechanism of Acylation C R C R _ Cl AlCl 3 C R Cl AlCl 3 Chapter

19 Clemmensen Reduction Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous Cl and amalgamated zinc. C 3 C 2 C Cl 1) AlCl 3 2) 2 C C 2 C 3 Zn(g) aq. Cl C 2 C 2 C 3 Chapter Gatterman-Koch Formylation Formyl chloride is unstable. Use a high pressure mixture of C, Cl, and catalyst. Product is benzaldehyde. C Cl C Cl AlCl 3 /CuCl C C C Chapter Cl AlCl 4 _ 19

20 Nucleophilic Aromatic Substitution A nucleophile replaces a leaving group on the aromatic ring. Electron-withdrawing substituents activate the ring for nucleophilic substitution. Chapter Examples of Nucleophilic Substitution Chapter

21 Addition-Elimination Mechanism Chapter