1. Aromaticity: Reactions of Benzene and Substituted Benzenes
Essential Organic Chemistry
Paula Yurkanis Bruice
Chapter 7
Aromaticity: Reactions of Benzene and Substituted Benzenes

2. 7.1 Criteria for Aromaticity
Uninterrupted arrangement of p cloud
Cyclic molecule
Every atom in the ring must have a p orbital
Molecule has to be planar
Total number of p-electron pairs is odd (1,3,5…)
Exceptionally stable molecules

3. 7.2 Aromatic Hydrocarbons
cyclobutadiene
benzene
cyclooctatetraene
Cyclobutadiene: planar, cyclic, all atoms have p orbitals,
but 2 pairs of p electrons
Benzene: planar, cyclic, all atoms have p orbitals,
3 pairs of p electrons.
Cyclooctatetraene: nonplanar, cyclic, all atoms have p orbitals, but 4 pairs of p electrons

4. Benzene
Benzene: planar, ring, all atoms have p orbitals, 3 pairs of p electrons.

5. Cyclopentadiene 4 p electrons 4 p electrons 6 p electrons
cyclopentadienyl cation
cyclopentadienyl anion
sp3
4 p electrons 4 p electrons 6 p electrons
planar nonplanar planar
aromatic

6. Cyclopentadienyl anion

8. Cyclopentadienyl anion
Resonance contributors.
Overall every C atom has the same amount of negative charge
charge equally
distributed

10. Cycloheptadienyl cation
6 p electrons 6 p electrons 8 p electrons
planar nonplanar planar
aromatic sp3-atom

11. Cycloheptadienyl cation
Resonance contributors.
Overall every C atom has the same amount of positive charge
charge equally
distributed

12. 7.3 Aromatic Heterocyclic Compounds

13. Pyridine The lone pair is not part of the aromatic ring system.
Electrons are located
in sp2-hybrid orbital.

14. Pyrrole
Lone pair is part of p-system, N is sp2 hybridized

15. Pyrrole
Resonance contributors
pos. charge

16. Furan
Second lone pair is located in sp2-hybridized
Orbital.

17. Furan
Resonance contributors

18. Bicyclic Heterocyclic Aromatics
quinoline
indole
purine

20. 7.4 Nomenclature of Monosubstituted Benzenes
State the name of substituent and add the word “benzene”

21. Nomenclature
Many trivial names in use

22. Nomenclature

24. Electrophilic Aromatic Substitution
7.5 How Benzene Reacts
Electrophilic Aromatic Substitution
Aromatic ring systems react with electrophiles differently than alkenes.
We obtain a substitution rather than an addition product.

25. Mechanism First, we get a carbocation intermediate.
At the carbon that adds the electrophile,
rehybridization (sp2  sp3) takes place.
Molecule loses aromatic character.

26. Mechanism
Carbocation is resonance stabilized.

27. Mechanism
The electron-rich part of the reagent can act either as nucleophile in an addition reaction or as a base to remove a proton.

28. Mechanism Figure: 07-01 Title:
Mechanism for the Reaction of Benzene with an Electrophile
Caption:
When benzene reacts with an electrophile a carbocation intermediate is formed. Because an aromatic product is more stable, the reaction proceeds as a) an electrophilic substitution reaction, rather than b) an electrophilic addition reaction.
Notes:
The carbocation loses a proton at the site of the electrophilic attack and the aromaticity is restored to the benzene ring.

29. Energy Profile

30. 7.6 General Mechanism for Electrophilic Aromatic Substitution Reactions

31. 7.7 Halogenation of Benzene

32. Halogenation of Benzene
1st step is generating an electrophile:
FeBr3 is a Lewis acid that coordinates with Br2 and
polarizes it, generating an electrophilic Br+

33. Halogenation of Benzene

34. 7.8 Nitration of Benzene
Nitration:

35. Nitration of Benzene
Nitration:

36. 7.9 Sulfonation of Benzene

37. Sulfonation of Benzene

38. 7.10 Friedel-Crafts Acylation of Benzene

39. Friedel-Crafts Acylation of Benzene

41. 7.11 Friedel-Crafts Alkylation of Benzene

42. Friedel-Crafts Alkylation of Benzene

44. 7.12 Nomenclature of Disubstituted Benzenes

47. 7.13 The Effect of Substituents on Reactivity
Monosubstituted benzenes undergo electrophilic aromatic substitution.
Substituents influence reactivity.
Substituents determine position of second electrophilic substitution.
Substituents that increase electron density increase reactivity.
Substituents that decrease electron density decrease reactivity.

48. The Effect of Substituents on Reactivity

49. Donating and Withdrawing Electrons by Resonance

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

51. Donating and Withdrawing Electrons by Resonance

52. Electron-withdrawing Substituents
Resonance contributors decrease electron density in ortho and para positions. Overall electron density is lower.

53. Substituent Effects
Both types of substituents, electron Donating and electron Withdrawing, act on the same positions, namely ortho and para, however, in opposite directions.

55. Relative Reactivity of Substituted Benzenes
Activating
Deactivating

56. Figure: UN.T1
Title:
Table The Effects of Substituents on the Reactivity of a Benzene Ring Toward Electrophilic Substitution
Caption:
Those at the top of the table are the most activating substituents when compared to hydrogen. The bottom of the table contains the deactivating groups.
Notes:
Ortho/para directors are generally activating substituents (except the halogens) while meta directors are generally deactivating substituents.

57. Classification of Substituents
Activating Substituents
(ortho/para directing):
prim. Amines
sec. Amines
tertiary amines
phenolic OH
phenol ethers
amides
phenolic esters
alkyl substituents
halides
Strongly
activating
weakly
activating
deactivating

58. Classification of Substituents
Deactivating Substituents (meta directing):
aldehydes
esters
carboxylic acid
nitrile
sulfonic acid
nitro
ammonium
weakly
deactivating
strongly
deactivating

60. 7.14 The Effect of Substituents on Orientation
1. All activating substituents direct an incoming electrophile to the ortho and para positions.

61. The Effect of Substituents on Orientation
2. The weakly deactivating halogens also direct an incoming electrophile to the ortho and para positions.

62. The Effect of Substituents on Orientation
3. All moderately deactiating and strongly deactivating substituents direct an incoming electrophile to the meta position.

63. Figure: 07-04
Title:
Electrophilic Aromatic Substitution
Caption:
The structures of the carbocation intermediates formed from the reaction of an electrophile with toluene at the ortho, meta, and para positions. The electrons in this example are being donated inductively. Any substituent that donates electrons inductively will be an ortho-para director.
Notes:
In this case the ortho and para intermediates are the most stable. Therefore, activating groups are ortho-para directors. The more stable the carbocation, the less energy is required to make the compound and the more rapidly it will be formed.

64. Figure: 07-05
Title:
Electrophilic Aromatic Substitution
Caption:
The structures of the carbocation intermediates formed from the reaction of an electrophile with anisole at the ortho, meta, and para positions. The electrons in this example are being donated by resonance. Any substituent that donates electrons by resonance will be an ortho-para director.
Notes:
In this case the ortho and para intermediates are the most stable. Therefore, activating groups are ortho-para directors. The more stable the carbocation, the less energy is required to make the compound and the more rapidly it will be formed.

65. Figure: 07-06
Title:
Electrophilic Aromatic Substitution
Caption:
The structures of the carbocation intermediates formed from the reaction of an electrophile with protonated aniline at the ortho, meta, and para positions. The electrons in this example are being donated by resonance. Any substituent that withdraws electrons inductively or by resonance will be a meta director.
Notes:
In this case, the ortho-para intermediates are the least stable. Therefore, deactivating groups (except for the halogens) are meta directors.

72. 7.15 The Effect of Substituents on pKa

73. Figure: UN
Title:
Substituent Effects on pKa
Caption:
The pKa of phenol decreases as the substituent’s ability to withdraw electron density increases.
Notes:
Compare para-methylphenol to para-nitrophenol. Nitrophenol is more acidic since the nitro group withdraws electrons from the ring.

74. Figure: UN
Title:
Substituent Effects on pKa
Caption:
The pKa of a benzoic acid and a protonated aniline decreases as the substituent’s ability to withdraw electron density increases.
Notes:
Electron-withdrawing substituents increase acidity whereas electron-donating groups decrease acidity.