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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. part 1. Reactions of Arenes; Electrophilic Aromatic Substitution. Electrophilic Aromatic Substitution

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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 part 1

  2. Reactions of Arenes; Electrophilic Aromatic Substitution • Electrophilic Aromatic Substitution • Halogenation, nitration, sulfonation, Friedel Crafts alkylation, Fridel Crafts Acylation • Second Substitution • Placement of 2nd substituent (o,p or m) • Rate of substitution • Third Substitution • Electrophilic Aromatic Substitution of Napthalene and Heterocycles

  3. Aromatic Compounds React Differently Than Alkenes Addition Substitution

  4. H E + – + + E Y H Y O -C-R Electrophilic aromatic substitutions include: Nitration (-NO2) Sulfonation (-SO3H) Halogenation (-Br or –Cl) Friedel-Crafts Alkylation (-R) Friedel-Crafts Acylation

  5. H H H SO2OH NO2 Br FeBr3 H2SO4 heat + + + HONO2 Br2 HOSO2OH + + + H2O HBr H2O

  6. O O H H C(CH3)3 CCH2CH3 AlCl3 AlCl3 + + (CH3)3CCl CH3CH2CCl + + HCl HCl

  7. Basic Electrophilic Aromatic Substitution Mechanism • Form Strong Electrophile, E+ • E+ Attacks P electrons of Aromatic Ring • Restoration of Aromatic Ring / Catalyst

  8. E+ H H E H H H H H + H H H H H Electrophilic Aromatic Substitution Mechanism Step 1: attack of electrophileon -electron system of aromatic ring highly endothermic carbocation is allylic, but not aromatic

  9. H H E H H H H E + H H H H H H+ Electrophilic Aromatic Substitution Mechanism Step 2: loss of a proton from the carbocationintermediate highly exothermic this step restores aromaticity of ring

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

  11. Step 1a; Formation of Strong Electrophile

  12. NO2+ H H H H NO2 NO2 H H H H H H + + H H H H H H H H Step 1b: attack of nitronium cationon -electron system of aromatic ring Step 2: Water molecule will abstract a proton from the carbocation intermediate H H H NO2 H H H+

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

  14. + – •• •• •• •• • • • + Br Br Br Br FeBr3 FeBr3 H Br • • • FeBr3 •• •• •• •• + Br2 + HBr Lewis base Lewis acid Complex Halogenation of Benzene NET REACTION Electrophile is a Lewis acid-Lewis basecomplex between FeBr3 and Br2.

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

  16. + – •• • 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 ••

  17. Limitations to Friedel Crafts Alkylation • Linear alkyl groups can not be put on ring this way due to carbocation rearrangement.

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

  19. O O H CCH2CH3 AlCl3 + CH3CH2CCl + HCl Electrophile is an acyl cation + + •• • • CH3CH2C O CH3CH2C O • • Friedel-Crafts Acylation of Benzene Note; Since carbocation is stabalized by resonance; rearrangement is unlikely (this is different than alkylation)

  20. O O H CR Zn(Hg), HCl CH2R Putting a Linear Alkyl Group on Ring;Acylation-Reduction H2NNH2, KOH,triethylene glycol,heat RCCl OR AlCl3

  21. (CH3)2CHCH2Cl CH2CH(CH3)3 C(CH3)3 AlCl3 Example: Prepare isobutylbenzene isobutylbenzene No! Friedel-Crafts alkylation of benzene using isobutyl chloride fails because of carbocation rearrangement. Actual Product

  22. O (CH3)2CHCCl CH2CH(CH3)3 Zn(Hg)HCl O CCH(CH3)2 Use Acylation-Reduction Instead + AlCl3

  23. 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.

  24. H H H SO2OH NO2 Br FeBr3 H2SO4 heat + + + HONO2 Br2 HOSO2OH + + + H2O HBr H2O

  25. O O H H C(CH3)3 CCH2CH3 AlCl3 AlCl3 + + (CH3)3CCl CH3CH2CCl + + HCl HCl

  26. Effect on Rate Activating substituents increase the rate of EAS compared to that of benzene. (-OH, -CH3) Deactivating substituents decrease the rate of EAS compared to benzene. (-Cl, -NO2)

  27. Rate of Nitration Depends on What is Already on the Ring Ring Deactivators Ring Activators

  28. Effect on Regioselectivity Ortho-para directors direct an incoming electrophile to positions ortho and/or para to themselves. Meta directors direct an incoming electrophile to positions meta to themselves.

  29. Placement of 2nd Substituent Depends on What the 1st Substituent is FeBr3 NO2 is a meta director; while Br is an ortho /para director

  30. CH3 CH3 CH3 CH3 NO2 HNO3 acid NO2 NO2 Nitration of Toluene o- and p-nitrotoluene together comprise 97% of the product a methyl group is an ortho-para director + + 63% 3% 34%

  31. E+ H H E H H H H H + H H H H H Mechanism • Form Strong Electrophile • Electrophile Adds to Benzene Ring to Form the Most Stable Carbocation • Restoration of Aromaticity

  32. CH3 CH3 CH3 NO2 H H H H H + H + + H H H H H H NO2 NO2 H H H more stable less stable Three Possible Carbocations May Be Formed Carbocation Stability Controls Regioselectivity gives ortho gives para gives meta

  33. CH3 CH3 NO2 NO2 H H + H H + + H H H H H H ortho Nitration of Toluene CH3 this resonance form is a tertiary carbocation NO2 H H H H H

  34. CH3 CH3 CH3 H H H H H H + + + H H H H H H NO2 NO2 H H NO2 H para Nitration of Toluene this resonance form is a tertiary carbocation

  35. CH3 CH3 CH3 H H H H H H + + + H H H H H H NO2 NO2 NO2 H H H meta Nitration of Toluene all the resonance forms of the rate-determining intermediate in the meta nitration of toluene have their positive charge on a secondary carbon

  36. Nitration of Toluene: Interpretation • The rate-determining intermediates for ortho and para nitration each have a resonance form that is a tertiary carbocation. All of the resonance forms for the rate-determining intermediate in meta nitration are secondary carbocations. • Tertiary carbocations, being more stable, are formed faster than secondary ones. Therefore, the intermediates for attack at the ortho and para positions are formed faster than the intermediate for attack at the meta position. This explains why the major products are o- and p-nitrotoluene.

  37. Ortho/Para DirectorsA. Lone Pair on Atom Bond to Benzene Ring -X ; -F, -Cl, -Br, -I B. Alkyl Groups Bond to Benzene Ring -CH3 -CH2CH3

  38. NET REACTION

  39. Ortho, Para Attack

  40. + + ERG ERG H H H H X H H H H X H H Lone Pair Stabilizes Intermediates forortho and para Substitution comparable stabilization not possible for intermediate leading to meta substitution

  41. Meta Attack

  42. Ortho/Para DirectorsA. Lone Pair on Atom Bond to Benzene Ring -X ; -F, -Cl, -Br, -I B. Alkyl Groups Bond to Benzene Ring -CH3 -CH2CH3

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