1. AROMATIC COMPOUNDS By PUAN AZDUWIN BINTI KHASRI

2. Criteria for Aromaticity 1. A compound must have an uninterrupted cyclic cloud of electrons above and below the plane of the molecule 2. The  cloud must contain an odd number of pairs of  electrons. BENZENE Benzene is an aromatic compound because it is cyclic and planar, every carbon in the ring has a p orbital, and the cloud contains three pairs of electrons.

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

4. Monocyclic hydrocarbons with alternating single and double bonds are called annulenes: Cyclobutadiene and cyclooctatetraene are NOT AROMATIC, because they have an even number of  electron pairs

5. 5 Cyclopentadiene does not have an uninterrupted ring of p orbital-bearing atoms not aromatic not aromatic not aromatic not aromatic Cyclopentadienyl cation has an even number of  electron pairs Cyclopentadienyl anion has an uninterrupted ring of  orbital-bearing atoms and an odd number of  electron pairs

6. The resonance hybrid shows that all the carbons in the cyclopentadienyl anion are equivalent

7. 7 These compounds consist of fused benzene rings and are aromatic: Any compound consisting of fused benzene rings is aromatic

8. Aromatic Heterocyclic Compounds A HETEROCYCLIC compound has ring atoms other than carbon A compound does not have to be a hydrocarbon to be aromatic. Example:Heterocyclic Compounds

9. Antiaromaticity Antiaromatic compounds are highly unstable, but the nonplanar versions are stable A compound is classified as being antiaromatic if it fulfills the first criterion for aromaticity but does not fulfill the second criterion. A compound is antiaromatic if it is a planar, cyclic, continuous loop of p orbitals with an even number of pairs of  electrons

10. EXAMPLE: Antiaromaticity

11. 11 Nomenclature of Monosubstituted Benzenes Some are named by attaching “benzene” after the name of the substituent:

12. Some have to be memorized:

13. 13 A benzene substituent is called phenyl. A benzene substituent with a methylene group is called benzyl.

14. Electrophilic Aromatic Substitution Reactions 1. Halogenation 2. Nitration 3. Sulfonation 4.Friedel–Crafts acylation 5.Friedel–Crafts alkylation

15. General Mechanism for Electrophilic Aromatic Substitution of Benzene Carbocation intermediate

16. 16 1. Halogenation of Benzene LEWIS ACID CATALYST

17. 17 Lewis acid weakens the Br–Br (or Cl–Cl) bond, which makes the halogen a better electrophile:

18. Mechanism for bromination The catalyst is regenerated: B: Bromide or Benzene

19. 19

20. 20 2. Nitration of Benzene Nitration of benzene with nitric acid requires sulfuric acid as a catalyst.

21. 21 Nitronium ion formation: Mechanism for Nitration;

22. 3.Sulfonation of Benzene Fuming sulfuric acid (a solution of in sulfuric acid) or concentrated sulfuric acid is used to sulfonate aromatic rings

23. Mechanism for sulfonation Mechanism for desulfonation Sulfonation of benzene is a reversible reaction.

24. sulfonation of benzene Desulfonation of benzenesulfonic acid Reaction coordinate diagram for the sulfonation the desulfonation A A B B C C SULFONATION A-B RATE DETERMINING STEP Has a smaller rate constant (Higher energy hill, thus slower reaction) than B-C SULFONATION A-B RATE DETERMINING STEP Has a smaller rate constant (Higher energy hill, thus slower reaction) than B-C DESULFONATION B-A RATE DETERMINING STEP C-B has a smaller rate constant than B-A (because once B is formed, its easier for B to get to C and proceed to A) DESULFONATION B-A RATE DETERMINING STEP C-B has a smaller rate constant than B-A (because once B is formed, its easier for B to get to C and proceed to A)

25. Friedel–Crafts Acylation vs Friedel– Crafts Alkylation

26. 4.Friedel–Crafts Acylation Reactions Either an acyl halide or an acid anhydride can be used for Friedel–Crafts acylation.

27. 27 Mechanism for Friedel–Crafts acylation: Must be carried out with more than one equivalent of AlCl 3 :

28. 28 5.Friedel–Crafts Alkylation of Benzene The Friedel–Crafts alkylation reaction substitutes an alkyl group for a hydrogen.

29. 29 Mechanism for Friedel–Crafts alkylation:

30. The carbocation will rearrange to a more stable species:

31. 31 However, 100% of the 2-methyl-2-phenylbutane product can be obtained if a bulky alkyl halide is used:

32. 32 Friedel–Crafts alkylation will not produce a good yield of an alkylbenzene containing a straight-chain group, because the carbocation will rearrange: Acylium ions, however, do not rearrange:

33. 33 Methodologies Used for the Reduction Step There are more general methods available to reduce a ketone carbonyl group to a methylene group

34. Using Coupling Reactions to Alkylate Benzene The Gilman reagent: The Stille reaction: The Suzuki reaction:

35. The resulting halide product can undergo a nucleophilic substitution reaction:

36. 36 Oxidation of an alkyl group bonded to a benzene ring Provided that a hydrogen is bonded to the benzylic carbon,

37. 37 The same reagent that oxidizes alkyl substituents will oxidize benzylic alcohols:

38. 38 However, aldehydes or ketones can be generated if a milder oxidizing agent is used:

39. 39 Reducing a Nitro Substituent

40. 40 It is possible to selectively reduce just one of the two nitro groups:

41. 41 Summary of Electrophilic Aromatic Substitution Reactions

42. 42 Summary of Friedel–Crafts Reactions

43. THE END