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AROMATIC COMPOUNDS

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  1. AROMATIC COMPOUNDS By PUAN AZDUWIN BINTI KHASRI

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

  3. 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

  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 p electron pairs

  5. not aromatic not aromatic aromatic Cyclopentadienedoes not have an uninterrupted ring of p orbital-bearing atoms Cyclopentadienylcationhas an even number of p electron pairs Cyclopentadienyl anion has an uninterrupted ring of p orbital-bearing atoms and an odd number of p electron pairs

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

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

  9. Antiaromaticity A compound is antiaromatic if it is a planar, cyclic, continuous loop of porbitals with an even number of pairs of p electrons 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.

  10. EXAMPLE: Antiaromaticity

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

  12. Some have to be memorized:

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

  14. Electrophilic Aromatic Substitution Reactions • Halogenation • Nitration • Sulfonation • Friedel–Crafts acylation • Friedel–Crafts alkylation

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

  16. 1. Halogenationof Benzene LEWIS ACID CATALYST LEWIS ACID CATALYST

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

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

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

  20. Nitronium ion formation: Mechanism for Nitration;

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

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

  23. Reaction coordinate diagram for the sulfonation the desulfonation SULFONATION A-B RATE DETERMINING STEP Has a smaller rate constant (Higher energy hill, thus slower reaction) than B-C sulfonation of benzene B 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) C A Desulfonation of benzenesulfonic acid

  24. Friedel–Crafts Acylation vs Friedel–Crafts Alkylation

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

  26. Mechanism for Friedel–Crafts acylation: Must be carried out with more than one equivalent of AlCl3:

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

  28. Mechanism for Friedel–Crafts alkylation:

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

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

  31. 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:

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

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

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

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

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

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

  38. Reducing a Nitro Substituent

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

  40. Summary of Electrophilic Aromatic Substitution Reactions

  41. Summary of Friedel–Crafts Reactions

  42. THE END