Aromaticity. Reactions of Benzene Chapter 15

1. Aromaticity.Reactions of Benzene Chapter 15 Chapter 15

2. Contents of Chapter 15 • Aromaticity • Heterocyclic Compounds • Chemical Consequences of Aromaticity • Molecular Orbital Description of Aromaticity • Reactivity Considerations • Mechanism for Electrophilic Substitution • Halogenation/Nitration/Sulfonation of Benzene • Friedel–Crafts Reactions Chapter 15

3. Aromaticity • Benzene is a cyclic compound which has a planar structure with a delocalized cloud of p electrons above and below the plane of the ring Chapter 15

4. Criteria for Aromaticity • There must be an uninterrupted ring of p orbital-bearing atoms leading to a delocalized  cloud • For the p cloud to be cyclic, the molecule must be cyclic • For the p cloud to be uninterrupted, every ring atom must have a p orbital • For the p cloud to form, each p orbital must be able to overlap the p orbital on either side Chapter 15

5. Criteria for Aromaticity • The  cloud must have an odd number of pairs of  electrons, or (2n+1)•2 = 4n+2  electrons • Hückel’s rule Chapter 15

6. Aromaticity • cyclooctatetraene is nonaromatic • It is not planar Chapter 15

7. Aromaticity resonance broken 2  electrons 4  electrons nonaromatic aromatic antiaromatic Chapter 15

8. Aromaticity Chapter 15

9. Aromaticity • The criteria for aromaticity also can be applied to polycyclic hydrocarbons • Naphthalene (5 pairs of p electrons), phenanthrene (7 pairs of p electrons), and chrysene (9 pairs of p electrons) all are aromatic Chapter 15

10. Heterocyclic Compounds • Lone pair can’t be in p orbital because p orbital used to build  bond with adjacent carbon(s) • The lone pair on pyridine’s nitrogen is in an sp2 hybrid, not part of the 3-pair aromatic  system Chapter 15

11. Heterocyclic Compounds • In pyrrole the lone pair could be put into either an sp3 hybrid or a p orbital with bonds in sp2 hybrid • Pyrrole puts the lone pair in a p orbital, making 3 pairs of  electrons (aromatic is more stable) Chapter 15

12. Antiaromaticity • A compound is classified an antiaromatic if it • Has an uninterrupted planar cyclic  system • Has an even number of electron pairs (4n  electrons) Chapter 15

13. Antiaromaticity • Both cyclobutadiene and cyclopentadienyl cation are planar p systems and the number of p electron pairs is even Chapter 15

14. Antiaromaticity • An aromatic compound is more stable than an analogous cyclic compound with localized electrons • An antiaromatic compound is less stable than the analogous cyclic compound with localized electrons Chapter 15

15. Heterocyclic Compounds • In above structures the N lone pairs could be put into either sp3 hybrids or p orbitals with bonds to N in sp2 hybrids • Both the lone pairs and bonds to N put into into sp3 hybrids, minimizing antiaromaticity • First structure is forced to be planar so it’s still antiaromatic. • Second structure can become nonplanar but not enough to put pi bonds perpendicular to each other so it still slightly antiaromatic. Chapter 15

16. Heterocyclic Compounds • In furan and thiophene there are 2 pairs of unshared electrons - one is an sp2hybrid orbital and one pair is in a p orbital, like pyrrole (3 pairs of  electrons, aromatic) Chapter 15

17. Heterocyclic Compounds Chapter 15

18. Heterocyclic Compounds • Quinoline, indole, imidazole, purine, and pyrimidine also are aromatic heterocyclic compounds Chapter 15

19. Chemical Consequences of Aromaticity Chapter 15

20. Chemical Consequences of Aromaticity • Cyclopentadiene has such a low pKa because of the stability of the anion formed when the hydrogen ionizes - the anion is aromatic Chapter 15

21. Chemical Consequences of Aromaticity • Cycloheptatrienyl bromide is ionic because of the stability of the aromatic cycloheptatrienyl cation Chapter 15

22. Reactivity Considerations • The benzene ring consists of a ring with p electrons above and below • Electrophiles are attracted to a benzene ring and form a nonaromatic carbocation intermediate (a cyclohexadienyl cation) carbocation intermediate Chapter 15

23. Electrophilic Substitution • Electrophilic addition doesn’t occur (would destroy aromaticity) Chapter 15

24. Reactivity Considerations Chapter 15

25. Mechanism for Electrophilic Substitution Reactions Chapter 15

26. Halogenation of Benzene Chapter 15

27. Halogenation of Benzene Chapter 15

28. Halogenation of Benzene Chapter 15

29. Nitration of Benzene Chapter 15

30. Sulfonation of Benzene Chapter 15

31. Sulfonation of Benzene Chapter 15

32. Friedel–Crafts Acylation Chapter 15

33. Friedel–Crafts Alkylation Chapter 15

34. Friedel-Crafts Alkylation • Two problems with Friedel–Crafts alkylation • Reaction proceeds through a carbocation which is subject to rearrangement Chapter 15

35. Friedel-Crafts Alkylation • The reaction product is more reactive toward Friedel–Crafts alkylation than the original reactant, leading to multiple substitutions • A large excess benzene must be used to minimize multiple substitutions Chapter 15

36. Friedel-Crafts Alkylation • Even with primary alkyl halides rearrangements occur via incipient primary carbocations • The carbocation never really forms, but the incipient carbocation remains complexed with the catalyst and behaves like a primary cation Chapter 15

37. Alkylation via Acylation Followed by Reduction • Problems associated with Friedel–Crafts alkylation can be avoided by conducting an acylation followed by a reduction of the carbonyl group to a methylene group (CH2) Chapter 15

38. Alkylation via Acylation Followed by Reduction • Two methods of reduction available • Clemmensen reduction in acid solution • Wolff-Kishner reduction in basic solution Chapter 15

39. Alkylation via Acylation Followed by Reduction • The method of reduction depends on other groups on the molecule Chapter 15