KOT 222 ORGANIC CHEMISTRY II CHAPTER 17. REACTIONS of AROMATIC COMPOUNDS

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1 KOT 222 ORGANIC CEMISTRY II CAPTER 17 REACTIONS of AROMATIC COMPOUNDS 1

2 Electrophilic Aromatic Substitution Substitution of an electrophile for a proton on the aromatic ring. benzene s pi electrons are available to attack a strong electrophile to give a carbocation (sigma complex). ighly endothermic Loss of a proton to regain aromaticity 2

3 Mechanism: Step 1: Attack on the electrophile forms the sigma complex. Regain the aromaticity 3

4 Bromination of Benzene Follows the mechanism for electrophilic aromatic substitution. Requires a stronger electrophile than Br 2. Use a strong Lewis acid catalyst, FeBr 3. Br Br + FeBr 3 Br Br FeBr Br Br FeBr 3 + Br _ + FeBr 4 4

5 Energy Diagram for Bromination The first step is strongly endothermic. Second step is strongly exothermic. Overall reaction is exothermic by 45 kj/mol. 5

6 Comparison with Alkenes Cyclohexene adds Br 2, Δ = -121 kj/mol. + Br 2 Similar addition to benzene is endothermic, not normally seen. Substitution of Br for retains aromaticity, Δ = -45 kj. Br Br 6

7 Chlorination and Iodination of Benzene Chlorination is similar to bromination. Use AlCl 3 as the Lewis acid catalyst. + Cl 2 AlCl 3 Cl + Cl Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion. + + NO 3 + 1/2 I 2 I + + NO O + 1/2 I 2 + NO 3 I + NO O 7

8 Nitration of Benzene Use sulfuric acid with nitric acid to form the nitronium ion electrophile. Protonation of -O Formation of nitronium ion Attack on the electrophile Loss of a proton gives nitrobenzene O N O 8

9 Reduction of nitro group R NO 3 2 SO 4 R NO 2 R NO 2 Zn, Sn, or Fe aq Cl R N 2 The best way to add an amino group to an aromatic ring. 9

10 Sulfonation of Benzene Electrophile = Sulfur trioxide, SO 3, in fuming sulfuric acid. O O O O O S O O S O O S O O S O benzenesulfonic acid 10

11 Desulfonation Sulfonation is reversible. sulfonic acid group can be removed by heating in dilute sulfuric acid. a proton adds to the ring (the electrophile) and loss of sulfur trioxide gives back benzene. SO O +, heat + 2 SO 4 11

12 Effects of Substituents The slow step of an electrophilic aromatic substitution reaction is the formation of the cation intermediate Electron-donating substituents increase the rate of the substitution reactions by stabilizing the carbocation intermediate and the transition state leading to its formation Electron-donating groups are activating substituents Electron-withdrawing groups are deactivating substituents 12

13 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. meta ortho para meta ortho Stability of the intermediate sigma complex determine the substitution pattern. 13

14 Sigma Complex Positive charge is delocalized onto the 3 o carbon atom Positive charge is not delocalized onto the 3 o carbon 14 atom

15 Energy Diagram Methyl group stabilize the sigma complex and the transition state leading to them. 15

16 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. 16

17 Substitution on Anisole All atoms (except ) have complete octets. Resonance stabilization is provided by a pi bond between the -OC 3 substituent and the ring. 17

18 The Amino Group Nitrogen atom with a pair of nonbonding electron serves as powerful activating group. Aniline reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the Br that s also formed. 18

19 Summary of Activators 19

20 Deactivating, Meta- Directing Substituents Substituent on a benzene ring has its greatest effect on the ortho and para positions. Nitrobenzene is about 100,000 less reactive than benzene in electrophilic substitution reactions. The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers. 20

21 Nitro group on benzene The nitrogen always has a formal positive charge. The positively charged nitrogen inductively withdraws elecron density from the ring. The benzene ring is deactivated toward reaction with electrophiles. 21

22 Sigma Complex 22

23 Energy Diagram 23

24 Effect of carbonyl group - Meta director 24

25 Summary of Deactivators The atom attached to the aromatic ring will have a positive / partial positive charge. 25

26 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. 26

27 Sigma Complex for Bromobenzene Ortho and para attacks produce a bromonium ion which provide extra resonance stability and other resonance structures. No bromonium ion possible with meta attack. 27

28 Energy Diagram 28

29 Effects of Multiple Substituents If the groups reinforce each other, the result is easy to predict. 29

30 A strongly activating substituent will win out over a weakly activating substituent or a deactivating substituent. 30

31 If the two substituents have similar activating properties, neither will dominate. 31

32 Friedel-Crafts Alkylation Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl 3. Lewis acid is used as a catalyst to generate the carbocation from the alkyl halide (2 or 3 ) or to activate the alkyl halide (1 or methyl halide) toward nucleophilic attack. Other sources of carbocations: alkenes + F, or alcohols + BF 3. 32

33 With 2 o and 3 o alkyl halides Step 1: Formation of carbocation Step 2: Electrophilic attack Step 3: Loss of proton 33

34 With 1 o alkyl halides 34

35 Protonation of Alkenes for Friedel-Crafts Alkylation 35

36 Alcohols with BF 3 for Friedel-Crafts Alkylation F (+) F B O (+) + F F F B O F The reaction is promoted by BF 3 36

37 Limitations of Friedel-Crafts 1. Reaction fails if benzene has a substituent that is more deactivating than halogen. C(C 3 ) 3 (C 3 ) 3 C-Cl AlCl 3 C(C 3 ) 3 C(C 3 ) 3 NO 3 2 SO 4 NO 2? NO 3 2 SO 4 NO 2 (C 3 ) 3 C-Cl AlCl 3 NO 2 Reaction fails 37

38 2. Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl 3 produces isopropylbenzene. No or low yield of n-propylbenzene 38

39 3. The alkylbenzene product is more reactive than benzene, so multiple alkylation occurs. To overcome carbocation rearrangement and polyalkylation, Friedel-Crafts acylation is used. 39

40 Friedel-Crafts Acylation Acyl chloride is used in place of alkyl chloride. benzoyl chloride The product is a phenyl ketone that is less reactive than benzene. 40

41 Mechanism of Acylation Step 1: formation of an acylium ion (the electrophilic species) by reaction of the acyl chloride with the catalyst. The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation. 41

42 Step 2: benzene attacks the acylium ion to form the sigma complex. Step 3: Loss of proton regenerates the aromatic system Step 4: Complexation of the product 2 O 42

43 Carbonyl group deactivate the benzene ring toward further substitution. When the aromatic substrate has an ortho, paradirecting group, para substitution will prevail. O C 3 C C C Cl 1. AlCl O C 3 C 2 O C C 3 43

44 Gatterman-Koch Formylation Formyl chloride is unstable, cannot be used in acylation. Use a high pressure mixture of CO, Cl, and catalyst. CO + Cl O C Cl AlCl 3 / CuCl C O AlCl 4 formyl chloride (unstable) formyl cation O + C O C + Cl benzaldehyde 44

45 Clemmensen Reduction Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous Cl and amalgamated zinc. 45

46 Nucleophilic Aromatic Substitution Aryl halide normally do not reacts with nucleophiles at standard condition. Electron-withdrawing groups (at ortho or para to the halide) activate the ring for nucleophilic substitution. 46

47 Nucleophilic aromatic substitution may involves either of the two mechanisms: 1. Addition-Elimination Mechanism 2.Birch Mechanism (Elimination-addition) 47

48 Addition-Elimination Mechanism Electron-withdrawing group at ortho or para-position stabilize the negatively charge sigma complex 48

49 Benzyne Mechanism Nucleophilic aromatic substitution without electron-withdrawing substituents. Require a strong base or high temperature. 49

50 Amide attacks at either end of the benzyne triple bond` 50

51 Chlorination of Benzene Addition of chlorine (excess) to the benzene ring may occur with high heat and pressure (or light). + 3 Cl 2 heat, pressure or hv Cl Cl Cl Cl Cl Cl First addition is difficult as it destroy the ring s aromaticity. Next two mole of Cl 2 add very rapidly. 51

52 Catalytic ydrogenation Transformation of benzene to cyclohexane. Take places at elevated temperature and pressure. Possible catalysts: Pt, Pd, Ni, Ru, Rh. 3 2, 1000 psi Pt 52

53 Birch Reduction Transformation of benzene to 1,4-cyclohexane. Involves twice adding a solvated electron, followed by a proton. 53

54 Mechanism: 54

55 Side-Chain Oxidation Alkylbenzenes are oxidized to benzoic acid by hot KMnO 4 or Na 2 Cr 2 O 7 / 2 SO 4 55

56 Side-Chain alogenation Free-radical halogenation is much more easily to happen at the benzylic position. 56

57 Chlorination is not as selective as bromination, results in mixtures. Br 2 reacts only at the benzylic position. α β α β α β C 2 C 3 Cl C C 3 C C 2 Cl Cl 2 hv + + dichlorinated products 56% 44% α β Br α β C 2 C α β 3 Br C C 3 Br C C 3 Br 2 or NBS + hv trace 57

58 Nucleophilic Substitution at the S N 1 Reactions Benzylic Position Benzylic carbocations are resonance-stabilized, easily formed. Benzyl halides undergo S N 1 reactions C 2 Br C 3 C 2 O, heat C 2 OC 2 C 3 58

59 S N 2 Reactions Benzylic halides are 100 times more reactive than primary halides via S N 2. Transition state is stabilized by conjugation with the pi electrons in the ring. 59

60 Reactions of Phenols Some reactions like aliphatic alcohols: phenol + carboxylic acid ester phenol + aq. NaO phenoxide ion Reactions that are peculiar to phenols are: Oxidation of phenols to quinones Electrophilic aromatic substitution of phenols 60

61 Oxidation of Phenols Phenols oxidized to quinones: 1,4-diketones. O O Na 2 Cr 2 O 7, 2 SO 4 C 3 O C 3 ydroquinone is used as a developer for film. It reacts with light-sensitized AgBr grains, converting it to black Ag. O O + 2 AgBr * + 2 Ag + 2 Br O O 61

62 Electrophilic Aromatic Substitution of Phenols Phenols and phenoxides are highly reactive. Only a weak catalyst (F) required for Friedel- Crafts reaction. Tribromination occurs without catalyst. Even reacts with CO 2, a weak electrophile. O O O O O CO 2, O - C O _ + C O salicylic acid 62

63 63

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