1. WWU-Chemistry Aromatic Substitution Electrophilic

2. WWU-Chemistry Sect. 22.1 Nomenclature

3. WWU-Chemistry Nomenclature-- examples

4. WWU-Chemistry 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)

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

6. WWU-Chemistry Sect. 22.5: Nitration conc. HNO 3 and H 2 SO 4 react to make electrophile, NO 2 + nitro aromatics are important intermediates reduction of nitro groups give anilines

7. WWU-Chemistry Mechanism of Aromatic Nitration Step 1: Where does the electrophile come from? Nitronium ion (NO 2 + ) is the electrophile that reacts with the benzene ring.

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

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

10. WWU-Chemistry Sect. 22.6: Halogenation active electrophile is a bromonium or chloronium ion need Lewis acid catalyst ( FeX 3 ) to activate X 2

11. WWU-Chemistry Sect. 22.7: Friedel-Crafts Alkylation alkyl halide + AlCl 3 -->carbocation + AlCl 3 X - watch out for carbocation rearrangements! more than one alkylation can occur --> mixtures!

12. WWU-Chemistry Friedel-Crafts Acylation acid chloride + AlCl 3 --> acylium ion + AlCl 4 - cation rearrangements are NOT observed! acylation will only occur ONCE... reaction VERY sensitive to substituents-- an acyl group prevents further reaction

13. WWU-Chemistry

14. Aromatic substitution on Benzene Sect. 22.8:Sulfonation (skip, fall 06) Sect. 22.9:Summary

15. WWU-Chemistry 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

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

17. WWU-Chemistry 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.

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

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

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

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

22. WWU-Chemistry ortho meta para BAD!

23. WWU-Chemistry meta substitution preferred because the + charge is never next to the CO 2 R group

24. WWU-Chemistry 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.

25. WWU-Chemistry Deactivation during substitution All meta directors are strongly deactivating relative to benzene.

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

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

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

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

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

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

32. WWU-Chemistry Some more good stuff!

33. WWU-Chemistry An explosive!

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

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

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

37. WWU-Chemistry

38. Other n = 1 aromatics ::: :: 6 electrons 6 electrons, one pair not involved! 6 electrons, one pair not involved! All are aromatic!

39. WWU-Chemistry

40. n = 0 aromatic: 2 pi electrons

41. WWU-Chemistry

42. Some Antiaromatic compounds : Not aromatic!

43. WWU-Chemistry 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