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Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution

H. E.  +.  –. +. +. E. Y. H. Y. Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution. H. E.  +.  –. +. +. E. Y. H. Y. 12.1 Representative Electrophilic Aromatic Substitution Reactions of Benzene. H. E.  +.  –. +. +. E. Y. H. Y.

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Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution

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  1. H E + – + + E Y H Y Chapter 12Reactions of Arenes:Electrophilic Aromatic Substitution

  2. H E + – + + E Y H Y 12.1Representative Electrophilic Aromatic Substitution Reactions of Benzene

  3. H E + – + + E Y H Y Electrophilic aromatic substitutions include: Nitration Sulfonation Halogenation Friedel-Crafts Alkylation Friedel-Crafts Acylation

  4. H NO2 Nitration of Benzene H2SO4 + HONO2 + H2O Nitrobenzene(95%)

  5. H SO2OH Sulfonation of Benzene heat + HOSO2OH + H2O Benzenesulfonic acid(100%)

  6. H Br Halogenation of Benzene FeBr3 + Br2 + HBr Bromobenzene(65-75%)

  7. H C(CH3)3 Friedel-Crafts Alkylation of Benzene AlCl3 + (CH3)3CCl + HCl tert-Butylbenzene(60%)

  8. O O H CCH2CH3 CH3CH2CCl Friedel-Crafts Acylation of Benzene AlCl3 + + HCl 1-Phenyl-1-propanone(88%)

  9. 12.2Mechanistic PrinciplesofElectrophilic Aromatic Substitution

  10. E+ H H E H H H H H + H H H H H Step 1: Attack of Electrophileby -Electron System of Aromatic Ring Highly endothermic. Carbocation is allylic, but not aromatic.

  11. H H H E H H + BH+ Step 2: Loss of a Proton from the CarbocationIntermediate Highly exothermic. This step restores aromaticity of ring. H H E H + H H H • B • • = base B •

  12. H H E H + H H H H H H H + E+ + H+ H H H E H H H H

  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.

  14. 12.3Nitration of Benzene

  15. H NO2 + Electrophile isnitronium ion. • • O N O • • •• •• Nitration of Benzene H2SO4 + HONO2 + H2O

  16. NO2+ H H NO2 H H H H H + H H H H H Step 1: Attack of Nitronium Cationby -Electron System of Aromatic Ring

  17. H H H NO2 H H + BH+ Step 2: Loss of a Proton from the Carbocation Intermediate H H NO2 H + H H H • B •

  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

  19. 12.4Sulfonation of Benzene

  20. H SO2OH – •• •• • • O O • + • •• Several electrophiles present: a major one is sulfur trioxide. S • O • •• Sulfonation of Benzene heat + HOSO2OH + H2O

  21. SO3 H H SO3– H H H H H + H H H H H Step 1: Attack of Sulfur Trioxideby -Electron System of Aromatic Ring

  22. H H H SO3– H H + BH+ Step 2: Loss of a Proton from the Carbocation Intermediate H H SO3– H + H H H • B •

  23. H H H H H SO3– H SO3H H H H H Step 3: Protonation of Benzenesulfonate Ion H2SO4

  24. 12.5Halogenation of Benzene

  25. H Br Halogenation of Benzene FeBr3 + Br2 + HBr Electrophile is a Lewis acid-Lewis basecomplex between FeBr3 and Br2.

  26. + – •• •• •• •• • • • Br Br Br Br FeBr3 • • • •• •• •• •• Complex The Br2-FeBr3 Complex The Br2-FeBr3 complex is more electrophilic than Br2 alone. + FeBr3 Lewis base Lewis acid

  27. H H Br H + H H H + FeBr4– Step 1: Attack of Br2-FeBr3 Complex by -Electron System of Aromatic Ring + – Br Br FeBr3 H H H H H H

  28. H H H Br H H + BH+ Step 2: Loss of a Proton from the CarbocationIntermediate H H Br H + H H H • B •

  29. 12.6Friedel-Crafts Alkylation of Benzene

  30. H C(CH3)3 H3C + Electrophile is tert-butyl cation. CH3 C H3C Friedel-Crafts Alkylation of Benzene AlCl3 + (CH3)3CCl + HCl

  31. + – •• • AlCl3 (CH3)3C Cl • •• + – •• • + AlCl3 (CH3)3C Cl • •• Role of AlCl3 Acts as a Lewis acid to promote ionizationof the alkyl halide. •• + (CH3)3C Cl AlCl3 ••

  32. + C(CH3)3 H H C(CH3)3 H + H H H Step 1: Attack of tert-Butyl Cationby -Electron System of Aromatic Ring H H H H H H

  33. H H H C(CH3)3 H H BH+ Step 2: Loss of a Proton from the CarbocationIntermediate H H C(CH3)3 H + H H H • B •

  34. H C(CH3)3 AlCl3 + (CH3)2CHCH2Cl Isobutyl chloride tert-Butylbenzene(66%) Rearrangements in Friedel-Crafts Alkylation Carbocations are intermediates. Therefore, rearrangements can occur.

  35. 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%)

  36. H •• AlCl3 H3C Cl C CH2 •• CH3 – •• • AlCl3 Cl • •• Rearrangements in Friedel-Crafts Alkylation + – H + + H3C C CH2 CH3

  37. H Reactions Related to Friedel-Crafts Alkylation Cyclohexene is protonated by sulfuric acid, giving cyclohexyl cation which is attacked by the benzene ring. H2SO4 + Cyclohexylbenzene(65-68%)

  38. 12.7Friedel-Crafts Acylation of Benzene

  39. H Electrophile is an acyl cation. + + •• • • CH3CH2C O CH3CH2C O • • Friedel-Crafts Acylation of Benzene O O CCH2CH3 AlCl3 + CH3CH2CCl + HCl

  40. O H H CCH2CH3 H + H H H Step 1: Attack of the Acyl Cationby -Electron System of Aromatic Ring + O CCH2CH3 H H H H H H

  41. O O H H CCH2CH3 H H H CCH2CH3 H + H H H H H + BH+ Step 2: Loss of a Proton from the CarbocationIntermediate • B •

  42. O O O H CCH3 CH3COCCH3 O + CH3COH Acid Anhydrides Can be used instead of acyl chlorides. AlCl3 + Acetophenone(76-83%)

  43. 12.8Acylation-Reduction

  44. O O H CR Zn(Hg), HCl CH2R 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

  45. 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 H2NNH2and KOH is called theWolff-Kishner reduction. RCCl H2NNH2, KOH,triethylene glycol,heat AlCl3 CH2R

  46. Example: Prepare Isobutylbenzene No! Friedel-Crafts alkylation of benzene using isobutyl chloride fails because of rearrangement. (CH3)2CHCH2Cl CH2CH(CH3)3 AlCl3

  47. C(CH3)3 AlCl3 Recall + (CH3)2CHCH2Cl Isobutyl chloride tert-Butylbenzene(66%)

  48. O (CH3)2CHCCl CH2CH(CH3)2 Zn(Hg)HCl O CCH(CH3)2 Use Acylation-Reduction Instead + AlCl3

  49. 12.9Rate and Regioselectivity in Electrophilic Aromatic Substitution A substituent already present on the ring can affect both the rate and regioselectivityof electrophilic aromatic substitution.

  50. Effect on Rate Activating substituents increase the rate of EAS compared to that of benzene. Deactivating substituents decrease the rate of EAS compared to benzene.

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