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Chapter 12 (Part a) Reactions of Arenes : Electrophilic Aromatic Substitution

H. E. d +. d –. +. +. E. Y. H. Y. Chapter 12 (Part a) Reactions of Arenes : Electrophilic Aromatic Substitution. Dr. Wolf's CHM 201 & 202. 12-1. H. E. d +. d –. +. +. E. Y. H. Y. Representative Electrophilic Aromatic Substitution Reactions of Benzene.

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Chapter 12 (Part a) Reactions of Arenes : Electrophilic Aromatic Substitution

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  1. H E d+ d– + + E Y H Y Chapter 12 (Part a)Reactions of Arenes:Electrophilic Aromatic Substitution Dr. Wolf's CHM 201 & 202 12-1

  2. H E d+ d– + + E Y H Y Representative Electrophilic Aromatic Substitution Reactions of Benzene Dr. Wolf's CHM 201 & 202 12-2

  3. H E d+ d– + + E Y H Y Electrophilic aromatic substitutions include: Nitration Sulfonation Halogenation Friedel-Crafts Alkylation Friedel-Crafts Acylation Dr. Wolf's CHM 201 & 202 12-3

  4. H NO2 Table 12.1: Nitration of Benzene H2SO4 + HONO2 + H2O Nitrobenzene(95%) Dr. Wolf's CHM 201 & 202 12-4

  5. H SO2OH Table 12.1: Sulfonation of Benzene heat + HOSO2OH + H2O Benzenesulfonic acid(100%) Dr. Wolf's CHM 201 & 202 12-5

  6. H Br Table 12.1: Halogenation of Benzene FeBr3 + Br2 + HBr Bromobenzene(65-75%) Dr. Wolf's CHM 201 & 202 12-6

  7. H C(CH3)3 Table 12.1: Friedel-Crafts Alkylation of Benzene AlCl3 + (CH3)3CCl + HCl tert-Butylbenzene(60%) Dr. Wolf's CHM 201 & 202 12-7

  8. O O H CCH2CH3 CH3CH2CCl Table 12.1: Friedel-Crafts Acylation of Benzene AlCl3 + + HCl 1-Phenyl-1-propanone(88%) Dr. Wolf's CHM 201 & 202 12-8

  9. Mechanistic PrinciplesofElectrophilic Aromatic Substitution Dr. Wolf's CHM 201 & 202 12-9

  10. E+ H H E H H H H H + H H H H H Step 1: attack of electrophileon p-electron system of aromatic ring highly endothermic carbocation is allylic, but not aromatic Dr. Wolf's CHM 201 & 202 12-10

  11. Step 2: loss of a proton from the carbocationintermediate highly exothermic this step restores aromaticity of ring H H E H H H H E + H H H H H H+ Dr. Wolf's CHM 201 & 202 12-11

  12. H H E H + H H H H H H H + E+ + H+ H H H E H H H H Dr. Wolf's CHM 201 & 202 12-12

  13. Based on this general mechanism: what remains is to identify the electrophile in nitration, sulfonation, halogenation, Friedel-Crafts alkylation, and Friedel-Crafts acylation to establish the mechanism of specific electrophilic aromatic substitutions Dr. Wolf's CHM 201 & 202 12-13

  14. Nitration of Benzene Dr. Wolf's CHM 201 & 202 12-14

  15. H NO2 + • • O N O • • •• •• Nitration of Benzene H2SO4 + HONO2 + H2O Electrophile isnitronium ion Dr. Wolf's CHM 201 & 202 12-15

  16. NO2+ H H NO2 H H H H H + H H H H H Step 1: attack of nitronium cationon p-electron system of aromatic ring Dr. Wolf's CHM 201 & 202 12-16

  17. Step 2: loss of a proton from the carbocationintermediate H H NO2 H H H H NO2 + H H H H H H+ Dr. Wolf's CHM 201 & 202 12-17

  18. – •• •• – •• •• • • O • O • O • O + • • + • •• •• N N + O • O • •• •• H H H •• + O • • O N O •• • • H H •• •• Where does nitronium ion come from? H2SO4 + Dr. Wolf's CHM 201 & 202 12-18

  19. Sulfonation of Benzene Dr. Wolf's CHM 201 & 202 12-19

  20. H SO2OH – •• •• • • O O • + • •• S • O • •• Sulfonation of Benzene heat + HOSO2OH + H2O Several electrophiles present: a major one is sulfur trioxide Dr. Wolf's CHM 201 & 202 12-20

  21. SO3 H H SO3– H H H H H + H H H H H Step 1: attack of sulfur trioxideon p-electron system of aromatic ring Dr. Wolf's CHM 201 & 202 12-21

  22. Step 2: loss of a proton from the carbocationintermediate H H SO3– H H H H SO3– + H H H H H H+ Dr. Wolf's CHM 201 & 202 12-22

  23. H H H H H SO3– H SO3H H H H H Step 3: protonation of benzenesulfonate ion H2SO4 Dr. Wolf's CHM 201 & 202 12-23

  24. Halogenation of Benzene Dr. Wolf's CHM 201 & 202 12-24

  25. H Br Halogenation of Benzene FeBr3 + Br2 + HBr Electrophile is a Lewis acid-Lewis basecomplex between FeBr3 and Br2. Dr. Wolf's CHM 201 & 202 12-25

  26. + – •• •• •• •• • • • Br Br Br Br FeBr3 • • • •• •• •• •• The Br2-FeBr3 Complex The Br2-FeBr3 complex is more electrophilic than Br2 alone. + FeBr3 Lewis base Lewis acid Complex Dr. Wolf's CHM 201 & 202 12-26

  27. H H Br H + H H H Step 1: attack of Br2-FeBr3 complex on p-electron system of aromatic ring + – Br Br FeBr3 H H H H H H + FeBr4– Dr. Wolf's CHM 201 & 202 12-27

  28. Step 2: loss of a proton from the carbocationintermediate H H Br H H H H Br + H H H H H H+ Dr. Wolf's CHM 201 & 202 12-28

  29. Friedel-Crafts Alkylation of Benzene Dr. Wolf's CHM 201 & 202 12-29

  30. H C(CH3)3 H3C + CH3 C H3C Friedel-Crafts Alkylation of Benzene AlCl3 + (CH3)3CCl + HCl Electrophile is tert-butyl cation Dr. Wolf's CHM 201 & 202 12-30

  31. + – •• • AlCl3 (CH3)3C Cl • •• Role of AlCl3 acts as a Lewis acid to promote ionizationof the alkyl halide •• + (CH3)3C Cl AlCl3 •• Dr. Wolf's CHM 201 & 202 12-31

  32. + – •• • AlCl3 (CH3)3C Cl • •• – •• • AlCl3 Cl • •• Role of AlCl3 acts as a Lewis acid to promote ionizationof the alkyl halide •• + (CH3)3C Cl AlCl3 •• + + (CH3)3C Dr. Wolf's CHM 201 & 202 12-32

  33. + C(CH3)3 H H C(CH3)3 H + H H H Step 1: attack of tert-butyl cationon p-electron system of aromatic ring H H H H H H Dr. Wolf's CHM 201 & 202 12-33

  34. Step 2: loss of a proton from the carbocationintermediate H H C(CH3)3 H H H H C(CH3)3 + H H H H H H+ Dr. Wolf's CHM 201 & 202 12-34

  35. H C(CH3)3 AlCl3 Rearrangements in Friedel-Crafts Alkylation Carbocations are intermediates. Therefore, rearrangements can occur + (CH3)2CHCH2Cl Isobutyl chloride tert-Butylbenzene(66%) Dr. Wolf's CHM 201 & 202 12-35

  36. H C(CH3)3 AlCl3 Rearrangements in Friedel-Crafts Alkylation Isobutyl chloride is the alkyl halide. But tert-butyl cation is the electrophile. + (CH3)2CHCH2Cl Isobutyl chloride tert-Butylbenzene(66%) Dr. Wolf's CHM 201 & 202 12-36

  37. H •• AlCl3 H3C Cl C CH2 •• CH3 – •• • AlCl3 Cl • •• Rearrangements in Friedel-Crafts Alkylation + – H + + H3C C CH2 CH3 Dr. Wolf's CHM 201 & 202 12-37

  38. H Reactions Related to Friedel-Crafts Alkylation Cyclohexene is protonated by sulfuric acid, giving cyclohexyl cation which attacks the benzene ring H2SO4 + Cyclohexylbenzene(65-68%) Dr. Wolf's CHM 201 & 202 12-38

  39. Friedel-Crafts Acylation of Benzene Dr. Wolf's CHM 201 & 202 12-39

  40. H + + •• • • CH3CH2C O CH3CH2C O • • Friedel-Crafts Acylation of Benzene O O CCH2CH3 AlCl3 + CH3CH2CCl + HCl Electrophile is an acyl cation Dr. Wolf's CHM 201 & 202 12-40

  41. O O CCH2CH3 + CCH2CH3 Step 1: attack of the acyl cationon p-electron system of aromatic ring H H H H H H H + H H H H H Dr. Wolf's CHM 201 & 202 12-41

  42. O O CCH2CH3 CCH2CH3 Step 2: loss of a proton from the carbocationintermediate H H H H H H + H H H H H H+ Dr. Wolf's CHM 201 & 202 12-42

  43. O O O H CCH3 CH3COCCH3 O + CH3COH Acid Anhydrides can be used instead of acyl chlorides AlCl3 + Acetophenone(76-83%) Dr. Wolf's CHM 201 & 202 12-43

  44. Acylation-Reduction Dr. Wolf's CHM 201 & 202 12-44

  45. O O H CR Acylation-Reduction permits primary alkyl groups to be attachedto an aromatic ring Reduction of aldehyde and ketonecarbonyl groups using Zn(Hg) and HCl is called the Clemmensen reduction. RCCl AlCl3 Zn(Hg), HCl CH2R Dr. Wolf's CHM 201 & 202 12-45

  46. O O H CR Acylation-Reduction permits primary alkyl groups to be attachedto an aromatic ring Reduction of aldehyde and ketonecarbonyl groups by heating with H2NNH2 and KOH is called theWolff-Kishner reduction. RCCl H2NNH2, KOH,triethylene glycol,heat AlCl3 CH2R Dr. Wolf's CHM 201 & 202 12-46

  47. Example: Prepare isobutylbenzene No! Friedel-Crafts alkylation of benzene using isobutyl chloride fails because of rearrangement. (CH3)2CHCH2Cl CH2CH(CH3)2 AlCl3 Dr. Wolf's CHM 201 & 202 12-47

  48. C(CH3)3 AlCl3 Recall + (CH3)2CHCH2Cl Isobutyl chloride tert-Butylbenzene(66%) Dr. Wolf's CHM 201 & 202 12-48

  49. O (CH3)2CHCCl CH2CH(CH3)2 O CCH(CH3)2 Use Acylation-Reduction Instead + AlCl3 Zn(Hg)HCl Dr. Wolf's CHM 201 & 202 12-49

  50. Rate and Regioselectivity in Electrophilic Aromatic Substitution A substituent already present on the ring can affect both the rate and regioselectivityof electrophilic aromatic substitution. Dr. Wolf's CHM 201 & 202 12-50

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