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Aromatic Substitution

Aromatic Substitution. Chapter 22. Sections to skip!!. Skip sections 22.1 (most), 22.3, 22.4, and 2.13 through 22.15 Keep 22.16 (review from c. 351, for some of you) Keep 22.17. Problems. In text: 4 - 6 8 - 13 20 - 29 End of Chapter: 1 - 5 9 - 13 . Sect. 22.1 Nomenclature.

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Aromatic Substitution

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  1. Aromatic Substitution Chapter 22 WWU-Chemistry

  2. Sections to skip!! Skip sections 22.1 (most), 22.3, 22.4, and 2.13 through 22.15 Keep 22.16 (review from c. 351, for some of you) Keep 22.17 WWU-Chemistry

  3. Problems • In text: 4 - 6 8 - 13 20 - 29 • End of Chapter: 1 - 5 9 - 13 WWU-Chemistry

  4. Sect. 22.1 Nomenclature WWU-Chemistry

  5. Catechols WWU-Chemistry

  6. Nomenclature-- examples WWU-Chemistry

  7. Chapter 22: Aromatic Substitutions, Monosubstitution reactions on benzene Sect. 22.2 Electrophilic aromatic substitution mechanism Sect. 22.5 Nitration Sect. 22.6. Halogenation Sect. 22.7 Friedel-Crafts Reactions Sect. 22.8 Sulfonation Reactions (skip fall 2006) WWU-Chemistry

  8. Sect. 22.2 Electrophilic aromatic substitution. WWU-Chemistry

  9. Sect. 22.5: Nitration • conc. HNO3 and H2SO4 react to make electrophile, NO2+ • nitro aromatics are important intermediates • reduction of nitro groups give anilines WWU-Chemistry

  10. Mechanism of Aromatic NitrationStep 1: Where does the electrophile come from? Nitronium ion (NO2+) is the electrophile that reacts with the benzene ring. WWU-Chemistry

  11. Mechanism of Aromatic Nitration (Step 2) WWU-Chemistry

  12. Mechanism of Aromatic Nitration (Step 3) WWU-Chemistry

  13. Sect. 22.6: Halogenation • active electrophile is a bromonium or chloronium ion • need Lewis acid catalyst ( FeX3 ) to activate X2 WWU-Chemistry

  14. Sect. 22.7: Friedel-Crafts Alkylation • alkyl halide + AlCl3 -->carbocation + AlCl3X- • watch out for carbocation rearrangements! • more than one alkylation can occur --> mixtures! WWU-Chemistry

  15. Friedel-Crafts Acylation • acid chloride + AlCl3 --> acylium ion + AlCl4- • cation rearrangements are NOT observed! • acylation will only occur ONCE... • reaction VERY sensitive to substituents-- an acyl group prevents further reaction WWU-Chemistry

  16. WWU-Chemistry

  17. Aromatic substitution on Benzene • Sect. 22.8: Sulfonation (skip, fall 06) • Sect. 22.9: Summary WWU-Chemistry

  18. Sect. 22.10 and 22.11: Directing effects • methoxy group releases electrons by resonance effect: ortho and para director • nitro group withdraws electrons by inductive and resonance effect: meta director WWU-Chemistry

  19. These are ortho and para directors! All are electron releasing!! WWU-Chemistry

  20. All ortho/para directing groups have pairs of electrons next to the benzene ring! The only exception are alkyl groups. They are also ortho/para directors. WWU-Chemistry

  21. Why do ortho/para groups direct as they do? Resonance!! WWU-Chemistry

  22. These are meta directors! All are electron withdrawing!! WWU-Chemistry

  23. Now let’s look at a meta directing group This is an example of Electrophilic Aromatic Substitution (EAS). WWU-Chemistry

  24. Why does the nitration reaction take place preferentially at the meta position? Let’s ask a “what if” question. WWU-Chemistry

  25. ortho BAD! meta para BAD! WWU-Chemistry

  26. meta substitution preferred because the + charge is never next to the CO2R group WWU-Chemistry

  27. Activation during substitution All ortho and para directing groups are activating relative to benzene, except halogen substituents. Halogens are weakly deactivating but are still o, p- directors. WWU-Chemistry

  28. Deactivation during substitution • All meta directors arestrongly deactivating relative to benzene. WWU-Chemistry

  29. Sect. 22.12 and 22.17: Some synthetic examples involving aromatic substitution WWU-Chemistry

  30. ortho/para directors can work together with meta directors. They reinforce each other. WWU-Chemistry

  31. Strong o/ p directors win over weak o, p and meta directors. WWU-Chemistry

  32. Substitution RARELY occurs in-between two substituents--too hindered! WWU-Chemistry

  33. Some groups can be modified to change their directing effects. WWU-Chemistry

  34. Good stuff! Order of reaction is critical! WWU-Chemistry

  35. Some more good stuff! WWU-Chemistry

  36. An explosive! WWU-Chemistry

  37. 2,4,6-Trinitrotoluene = TNT WWU-Chemistry

  38. Some miscellaneous examples • Nitration of 3-nitrobenzoic acid • Acylation of 1,3-dimethylbenzene • Acylation of 1,4-dimethylbenzene • Make 2-methyl-1-phenylpropane WWU-Chemistry

  39. Sect. 22.16 Aromaticity and Huckel’s Rule Aromatic compounds 4n + 2 pi electrons n = 1 6 pi electrons systems : WWU-Chemistry

  40. WWU-Chemistry

  41. Other n = 1 aromatics : : : : : 6 electrons, one pair not involved! 6 electrons, one pair not involved! 6 electrons All are aromatic! WWU-Chemistry

  42. WWU-Chemistry

  43. n = 0 aromatic: 2 pi electrons WWU-Chemistry

  44. WWU-Chemistry

  45. Some Antiaromatic compounds : Not aromatic! WWU-Chemistry

  46. Diazonium ions, Azo Dyes and the Sandmeyer Reaction- from Chapter 23 (not covered 06) Sect 23.16: Diazonium ion formation Sect 23.17: Sandmeyer reaction Sect 23.19: Azo dyes WWU-Chemistry

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