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Ch. 15 - 1

Chapter 15 (test 2). Electrophilic Aromatic Substitution General Mechanism Halogention, Nitration, sulfonation Friedel-Crafts Alkylation & Acylation Friedel-Crafts limitations Substituent Affects activation, deactivation, orientation Details of affects ortho-para directing, meta-directing

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Ch. 15 - 1

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  1. Chapter 15 (test 2) Electrophilic Aromatic Substitution General Mechanism Halogention, Nitration, sulfonation Friedel-Crafts Alkylation & Acylation Friedel-Crafts limitations Substituent Affects activation, deactivation, orientation Details of affects ortho-para directing, meta-directing Benzylic chemistries Alkenylbenzene Synthesis blocking & protecting groups Disubstituted benzene Allylic benzylic substitution Birch reduction 27 Modified from sides of William Tam & Phillis Chang Ch. 15 - 1

  2. Electrophilic Aromatic Substitution RXs (EAS) Overall reaction Ch. 15 - 2

  3. General Mechanism for Electrophilic Aromatic Substitution Different chemistry with alkene Ch. 15 - 4

  4. Ch. 15 - 3

  5. Electrophilic Aromatic Substitution Benzene does not undergo electrophilicaddition Ch. 15 - 5

  6. Mechanism Ch. 15 - 6

  7. Halogenation of Benzene Requires a Lewis acid catalyst Reactivity: F2 >> Cl2 > Br2 >> I2 Ch. 15 - 9

  8. Catalyst Ch. 15 - 11

  9. Mechanism(Cont’d) Ch. 15 - 12

  10. F2: too reactive mixture Ch. 15 - 14

  11. I2: very unreactive needs LA-oxidizing agent (e.g. HNO3, Cu2+, H2O2) Ch. 15 - 15

  12. Nitration of Benzene Electrophile = NO2⊕ (nitronium ion) Ch. 15 - 16

  13. Mechanism Ch. 15 - 17

  14. Sulfonation r.d.s repeat next slide Ch. 15 - 20

  15. repeat Ch. 15 - 21

  16. (heat) Sulfonation & Desulfonation-useful! Ch. 15 - 22

  17. Friedel–Crafts Alkylation Electrophile = R⊕(not vinyl or aryl) R = 2o or 3o Ch. 15 - 23

  18. -H(+) RX Mechanism Ch. 15 - 24

  19. Other carbocation Ch. 15 - 27

  20. Ch. 15 - 28

  21. Friedel–Crafts Acylation Acyl group: Electrophile is R–C≡O⊕ (acylium ion) Ch. 15 - 29

  22. RX and Mechanism Ch. 15 - 30

  23. Prep Acid chlorides (or acyl chlorides) Ch. 15 - 33

  24. Limitations of Friedel–Crafts Reactions (not formed) carbocations rearrangement Ch. 15 - 35

  25. Reason 1o cation (not stable) 3o cation Ch. 15 - 36

  26. Questions? Ch. 15 - 3

  27. Problems: Friedel–Crafts alkylations, acylations, etc. with withdrawing groups & amines(basic) generally give poor yields deactivating gps Ch. 15 - 37

  28. Basic amino groups (–NH2,–NHR, & –NR2) form strong electron withdrawing gps with acids Not Friedel-Crafts reactive Ch. 15 - 38

  29. Another problem: polyalkylations can occur More common with activated aromatic rings Ch. 15 - 40

  30. Clemmensen Reduction recall Use Clemmensen reduction to avoiding rearrangements Ch. 15 - 41

  31. How? Ch. 15 - 45

  32. Substituents effect reactivity & regiochemistry of substitution faster or slower than Y = EDG (electron-donating group) or EWG (electron-withdrawing group) Ch. 15 - 46

  33. meta m ortho o para p Substituents effect reactivity & regiochemistry of substitution possibilities Y = EDG (electron-donating group) or EWG (electron-withdrawing group) Ch. 15 - 48

  34. Reactivity towards electrophilic aromatic substitution Ch. 15 - 56

  35. Rate-determining-step: aromatic ring -electrons attacking the E • Regiochemistry: directing effect • General aspects • Eithero-, p- directing orm-directing Ch. 15 - 57

  36. Ch. 15 - 59

  37. Classification of substituents Ch. 15 - 64

  38. Classification of substituents Ch. 15 - 65

  39. arenium ion stabilized t.s. stabilized Effect of Electron-Donating (releasing) and Electron-Withdrawing Groups If G is electron-donating group then reaction is faster than with benzene Ch. 15 - 67

  40. arenium ion destabilized t.s. destabilized If G is an electron-withdrawing then reaction is slower than with benzene Ch. 15 - 68

  41. Ch. 15 - 69

  42. Inductive and Resonance Effects: Orientation Two types of EDG (1) resonance donation of e(-)s into the benzene ring (2) e(-)-inductive donation (through σ bond) Ch. 15 - 70

  43. Two types of EDG Positive resonance effect is stronger than positive inductive effect (if the atom directly attacked to the benzene is in the same row as carbon) Ch. 15 - 71

  44. EWGnegative resonance (mesomeric) • or by negative inductive effect Deactivate the ring by resonance effect Deactivate the ring by negative inductive effect Ch. 15 - 72

  45. Meta-Directing Groups EWG = –COOR, –COR, –CHO, –CF3, –NO2, etc. (EWG ≠ halogen) Ch. 15 - 73

  46. etc. etc. For example “if” ortho or para (highly unstable, negative inductive effect of –CF3) Ch. 15 - 74

  47. meta positive charge never on a carbon adjacent to the EWG Ch. 15 - 76

  48. Ortho–Para-Directing Groups EDG = –NR2, –OR, –OH, etc. Ch. 15 - 77

  49. EDG - para extra resonance structure, positive resonance effect Ch. 15 - 78

  50. EDG - ortho (extra resonance) Ch. 15 - 79

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