This document discusses aromatic compounds and benzene reactions. It covers criteria for aromaticity including Hückel's rule. Electrophilic aromatic substitution reactions on benzene such as halogenation, nitration, sulfonation, Friedel-Crafts acylation and alkylation are explained. The mechanisms and products of these reactions are described. Methods to modify substituents on benzene rings through oxidation, reduction and coupling reactions are also summarized.

1. Chapter 14
Aromaticity
• Reactions of
Benzene
Organic Chemistry
5th Edition
Paula Yurkanis Bruice
Irene Lee
Case Western Reserve University
Cleveland, OH
©2007, Prentice Hall

2. Aromatic Compounds Are Unusually Stable
Benzene is an aromatic compound

3. Benzene is unusually stable because of electron
delocalization
Compounds with unusually large resonance energies,
like benzene, are called aromatic compounds

4. Criteria for Aromaticity
1. A compound must have an uninterrupted cyclic cloud
of p electrons above and below the plane of the
molecule
2. The p cloud must contain an odd number of pairs
of p electrons

5. Hückel’s Rule
For a planar, cyclic compound to be aromatic, its
uninterrupted p cloud must contain (4n + 2) p electrons,
where n is any whole number

6. Monocyclic hydrocarbons with alternating single and
double bonds are called annulenes
Cyclobutadiene and cyclooctatetraene are not aromatic
because they have an even number of p electron pairs
Furthermore, cyclooctatetraene is nonplanar

7. Cyclopentadiene does not have an uninterrupted ring of
p orbital-bearing atoms
not
aromatic
not
aromatic
aromatic
Cyclopentadienyl cation has an even number of p
electron pairs
Cyclopentadienyl anion has an uninterrupted ring of p
orbital-bearing atoms and an odd number p electron pairs

8. The resonance hybrid shows that all the carbons in the
cyclopentadienyl anion are equivalent
Each carbon has exactly one-fifth of the negative charge
associated with the anion

9. These compounds are aromatic

10. Aromatic Heterocyclic Compounds
A heterocyclic compound is a cyclic compound in which
one or more of the ring atoms is an atom other than
carbon

11. Pyridine Is Aromatic

12. Pyrrole Is Aromatic
The lone-pair electrons on the nitrogen atom of pyrrole
are p electrons

13. Furan and thiophene are aromatic compounds like pyrrole

14. Examples of Heterocyclic
Aromatic Compounds

15. The Effect of Aromaticity on the pKa
Values of Some Compounds

16. Why is the pKa of cyclopentadiene so much lower than
that of ethane?
The conjugate base is aromatic

17. Aromaticity influences chemical reactivity
The cycloheptatrienyl cation is aromatic

18. A compound is antiaromatic if it is a planar, cyclic
compound with an uninterrupted ring of p orbital-bearing
atoms, and the p cloud must contain an even number of
pairs of p electrons
Antiaromaticity
Antiaromatic compounds are highly unstable

19. A Molecular Orbital Description of
Aromaticity and Antiaromaticity
Aromatic compounds are stable because they have filled
bonding p molecular orbitals

20. Nomenclature of Monosubstituted Benzenes
Some are named by attaching “benzene” after the name
of the substituent

21. Some have to be memorized

22. When a benzene ring is a substituent, it is called a
phenyl group
A benzene ring with a methylene group is called a benzyl
group

23. With the exception of toluene, benzene rings with an alkyl
substituent are named as alkyl-substituted benzenes or
as phenyl-substituted alkanes
Aryl group (Ar) is the general term for either a phenyl
group or a substituted phenyl group

24. Aromatic compounds such as benzene undergo
electrophilic aromatic substitution reactions

25. Recall in Section 3.6…

26. Benzene is a nucleophile that reacts with an electrophile
An electrophilic substitution yields an aromatic product
which is significantly more stable than the addition
reaction

27. Reaction Coordinate Diagrams for the
Two Benzene Reactions

28. There are five common electrophilic aromatic substitution
reactions
1. Halogenation
2. Nitration
3. Sulfonation
4.Friedel-Crafts acylation
5.Friedel-Crafts alkylation

29. General Mechanism for Electrophilic
Aromatic Substitution of Benzene

30. Halogenation of Benzene

31. Lewis acid weakens the Br-Br (or Cl-Cl) bond, which
makes the halogen a better electrophile

32. Mechanism for bromination
The catalyst is regenerated

35. Nitration of Benzene

37. Sulfonation of Benzene

38. Sulfonic acid is a strong acid

39. Sulfonation is reversible

41. Friedel–Crafts Acylation Reactions
The electrophile is an acylium ion

42. Mechanism for Friedel-Crafts acylation
Must be carried out with more than one equivalent of
AlCl3

43. Friedel-Crafts Alkylation of Benzene

44. Mechanism for Friedel-Crafts alkylation

45. The carbocation will rearrange to a more stable species

46. However, a 100% of the 2-methyl-2-phenylbutane
product can be obtained if a bulky alkyl halide is used

48. It is not possible to obtain a good yield of an
alkylbenzene containing a straight-chain group via
Friedel–Crafts alkylation due to carbocation
rearrangement
Acylium ions, however, do not rearrange

49. Methodologies Used for
the Reduction Step

50. Using Coupling Reactions to Alkylate Benzene
The Gilman reagent
The Stille reaction

51. The Suzuki reaction

52. One needs to consider an alternative if there is another
functional group present in the compound

53. Chemical Modification of Substituents of Benzene
Reactions of alkyl substituents

54. The resulting halide product can undergo a nucleophilic
substitution reaction

55. Remember that halo-substituted alkyl groups can also
undergo E2 and E1 reactions (Section 9.8)

56. Substitutions with double and triple bonds can undergo
catalytic hydrogenation (Section 4.11 and 6.9)

58. Oxidation of an alkyl group bonded to a benzene ring…
Provided that a hydrogen is bonded to the benzylic
carbon,

59. The same reagent that oxidizes alkyl substituents will
oxidize benzylic alcohols

60. However, aldehydes or ketones can be generated if a
milder oxidizing agent is used

61. Reducing a Nitro Substituent

62. It is possible to selectively reduce just one of the two nitro
groups