Aromaticity. Reactions of Benzene Chapter 8

1. Aromaticity.Reactions of Benzene Chapter 8 Chapter 8

2. Contents of Chapter 15 • Aromaticity • Heterocyclic Compounds • Chemical Consequences of Aromaticity • Nomenclature • Reactivity Considerations • Mechanism for Electrophilic Substitution • Halogenation/Nitration/Sulfonation of Benzene • Friedel–Crafts Reactions • Substituent Effects • Retrosynthetic Analysis Chapter 8

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 8

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 8

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 8

6. Aromaticity • cyclooctatetraene is nonaromatic • It is not planar Chapter 8

7. Aromaticity resonance broken 2  electrons 4  electrons nonaromatic aromatic antiaromatic Chapter 8

8. Aromaticity Chapter 8

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 8

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 8

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 8

12. 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 • Because the lone pairs give these rings an EVEN number of pi electrons these rings are not aromatic • In these cases the lone pairs are put into sp3 hybrid orbitals because nature doesn’t like even numbers of pairs of pi electrons in cyclic pi systems Chapter 8

13. 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 8

14. Heterocyclic Compounds Chapter 8

15. Heterocyclic Compounds • Quinoline, indole, imidazole, purine, and pyrimidine also are aromatic heterocyclic compounds Chapter 8

16. Chemical Consequences of Aromaticity Chapter 8

17. 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 8

18. Chemical Consequences of Aromaticity • Cycloheptatrienyl bromide is ionic because of the stability of the aromatic cycloheptatrienyl cation Chapter 8

19. chlorobenzene nitrobenzene ethylbenzene bromobenzene Naming Monosubstituted Benzenes Chapter 8

20. phenol aniline toluene anisole benzonitrile styrene benzaldehyde benzoic acid Naming Common Monosubstituted Benzenes Chapter 8

21. 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 8

22. Electrophilic Substitution • Electrophilic addition doesn’t occur (would destroy aromaticity) Chapter 8

23. Reactivity Considerations Chapter 8

24. Mechanism for Electrophilic Substitution Reactions Chapter 8

25. Halogenation of Benzene Chapter 8

26. Nitration of Benzene Chapter 8

27. Anilines From Nitrobenzenes • Anilines (aminobenzenes) are always made from nitrobenzenes. • Anilines decompose and make black tar when exposed to electrophilic aromatic substitution (EAS) reaction conditions • For this reason nitrobenzenes are converted to anilines in the very LAST step AFTER all other groups are added by EAS reactions • There are several ways to convert nitrobenzenes to anilines but this course teaches H2 and Pd/C Chapter 8

28. Sulfonation of Benzene Chapter 8

29. Friedel–Crafts Acylation Chapter 8

30. Friedel–Crafts Alkylation Chapter 8

31. Electron-donating Substituents Resonance contributors increase electron density in ortho and para positions. Overall electron density is bigger. Chapter 8

32. Electron-donating Substituents • A pi system can be considered to be polarized in an alternating fashion by substituents for product analysis purposes. • Electrophiles (+ groups) add to – positions • Alkyl groups and atoms with lone pairs polarize the ring carbon thhey are attached to + Chapter 8

33. Electron-donating Substituents Chapter 8

34. Electron-withdrawing Substituents Atoms with + charge polarize the attached ring carbon negative (–) Chapter 8

35. Electron-withdrawing Substituents Chapter 8

36. Naming Disubstituted Benzenes Chapter 8

37. Naming Disubstituted Benzenes Chapter 8

38. Retrosynthetic Analysis • Work backwards from an aromatic compound to figure out how to make it • First if amino group (NH2) is present work it backwards to nitro (NO2) • Next remove one substituent and polarize the ring according to positions of other substituents • If the substituent you removed came from a – carbon remove another substituent, polarize the ring again, and repeat • If remaining substituents don’t agree on ring polarization or latest removed substituent doesn’t remove from a – carbon replace removed substituent and try to remove another one. • Continue until all substituents have been removed • Reverse the retrosynthetic analysis to figure out what reagents to add to benzene in what order Chapter 8

39. Retrosynthetic Analysis Work out synthesis of 4-bromo-3-chloroaniline using retrosynthetic analysis Chapter 8