1 / 92

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

thelma
Télécharger la présentation

Ch. 15 - 1

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related