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Chapter 16

Chapter 16. Reactions of Aromatic Compounds Prepared By : Dr: Ishaq Takruri Philadelphia University Faculty of Pharmacy. Electrophilic Aromatic Substitution Reactions. Overall reaction.

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Chapter 16

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  1. Chapter 16 Reactions of Aromatic Compounds Prepared By : Dr: IshaqTakruri Philadelphia University Faculty of Pharmacy

  2. Electrophilic AromaticSubstitution Reactions • Overall reaction

  3. A General Mechanism for Electro-philic Aromatic Substitution • Different chemistry with alkene

  4. Benzene does not undergo electrophilicaddition, but it undergoes electrophilic aromatic substitution

  5. Mechanism Step 1

  6. MechanismStep 2

  7. Halogenation of Benzene • Benzene does not react with • Br2or Cl2 • unless a Lewis acid is present . • (catalytic amount is usually enough)

  8. Examples • Reactivity: F2 > Cl2 > Br2 > I2

  9. Mechanism 1st.step

  10. Mechanism 2nd. step

  11. Mechanism3rd. step

  12. F2: too reactive, give mixture of mono- di- and highly substituted products

  13. I2: very unreactive even in the presence of Lewis acid, usually need to add an oxidizing agent (e.g. HNO3, Cu2+, H2O2)

  14. Nitration of Benzene • Electrophile in this case is NO2 (nitronium ion)

  15. Mechanism

  16. Mechanism (Cont’d)

  17. Mechanism (Cont’d)

  18. Sulfonation of Benzene • Mechanism • Step 1 • Step 2

  19. Step 3 • Step 4

  20. Sulfonation & Desulfonation

  21. Friedel–Crafts Alkylation • Electrophile in this case is R • R = 2o or 3o • Or (R = 1o)

  22. Mechanism

  23. Mechanism (Cont’d)

  24. Mechanism (Cont’d)

  25. Note: Not necessary to start with alkyl halide, other possible functional groups can be used to generate a reactive carbocation

  26. Friedel–Crafts Acylation • Acyl group: • Electrophile in this case is R–C≡O (acylium ion)

  27. Mechanism

  28. Mechanism (Cont’d)

  29. Mechanism (Cont’d)

  30. Acid chlorides (or acyl chlorides) • Can be prepared by

  31. Limitations of Friedel–CraftsReactions • (1) When the carbocation formed from an alkyl halide, alkene, or alcohol can rearrange to one or more carbocations that are more stable, it usually does so, and the major products obtained from the reaction are usually those from the more stable carbocations

  32. (not formed) (How is this Formed?) For example

  33. Reason 1o cation (not stable) 3o cation (more stable)

  34. (2) Friedel–Crafts reactions usually give poor yields when powerful electron-withdrawing groups are present on the aromatic ring or when the ring bears an –NH2, –NHR, or –NR2 group. This applies to both alkylations and acylations These usually give poor yields in Friedel-Crafts reactions

  35. (3) The amino groups, –NH2, –NHR, and –NR2, are changed into powerful electron-withdrawing groups by the Lewis acids used to catalyze Friedel-Crafts reactions Does not undergo a Friedel-Crafts reaction

  36. (4) Aryl and vinylic halides cannot be used as the halide component because they do not form carbocations readily sp2 sp2

  37. (5) Polyalkylationsoften occur

  38. Synthetic Applications ofFriedel-Crafts Acylations: The Clemmensen Reduction • Clemmensen ketone reduction

  39. Clemmensen ketone reduction • A very useful reaction for making alkyl benzene that cannot be made via Friedel-Crafts alkylations

  40. Clemmensen ketone reduction • Cannot use Friedel-Crafts alkylation

  41. Rearrangements of carbon chain do not occur in Friedel-Crafts acylations (no rearrangement of the R group)

  42. Substituents Can Affect Boththe Reactivity of the Ring and the Orientation of the Incoming Group • Two questions we would like to address here • Reactivity • Regiochemistry

  43. Reactivity faster or slower than Y = EDG (electron-donating group) or EWG (electron-withdrawing group)

  44. Regiochemistry Statistical mixture of o-, m-, p- products or any preference?

  45. d+ d- Electrophilic reagent Arenium ion A substituted benzene

  46. Z donates electrons Y withdraws electrons The ring is more electron rich and reacts faster with an electrophile The ring is electron poor and reacts more slowly with an electrophile

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