4. aromatic compounds-students_copy


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4. aromatic compounds-students_copy

  1. 1. 4. Aromatic Compounds Prepared by Joyce Tiong For UEMK1013 October 2012
  2. 2. Benzene• 1825, Michael Faraday found a compound with C:H ratio of 1:1.• 1834, Eilhard Mitscherlich produced the same compound and found the molecular formula of C6H6.• It was then named benzene.• Other compounds with low C:H ratios were then classified as aromatic compounds. 2
  3. 3. Benzene• Benzene occurs in two Kekulé structures. Kekulé structures: resonance structure of the benzene ring with alternate double and single bonds 3
  4. 4. Properties of Benzene• Benzene ring is planar.• All C-C bond lengths are the same (1.397 Å) and all bond angles are 120o.• Delocalisation of the electrons gives benzene great stability. 4
  5. 5. Reactions of Benzene• Reacts with Br2 in the presence of FeCl3 catalyst to form bromobenzene + HBr 5
  6. 6. Annulenes• Annulenes: cyclic hydrocarbons with alternating single and double bonds.• Eg.: Benzene = 6-annulene.• Criteria for annulenes: MUST have – Cycle with conjugated double bonds – Planar to allow overlapping of π-orbitals 6
  7. 7. Exceptions of Aromatic Compounds• Cyclobutadiene and Cyclooctatetraene are NOT aromatic compounds because they don’t form Kekulé structures.• Cyclobutadiene (C4H4) is very reactive, it dimerizes (forms C8H8) very quickly.• Cyclooctatetraene adds Br2 readily. 7
  8. 8. Hückel’s Rule• For a cyclic compound with alternating double and single bonds, if: – (4N+2) = aromatic – (4N) = antiaromatic – N = 0, 1, 2, … – Example: • Benzene (C6H6): • 6 = 4N+2, N = 1 • ∴ aromatic • Cyclobutadiene (C4H4): • 4 = 4N, N = • ∴ anti-aromatic 8
  9. 9. Aromatic, anti-aromatic, non- anti- non- aromatic Cyclic Planar Every C-atom Fulfill with π orbital Hückel’s Rule Aromatic Yes Yes Yes 4N+2Anti-aromatic Yes Yes Yes 4NNon-aromatic No Maybe No No 9
  10. 10. Reactions of Aromatic Compounds• Halogenation of Benzene• I) Bromination – Br2 donates a pair of electron to a strong Lewis acid catalyst, FeBr3 to form a stronger electrophile. 10
  11. 11. Bromination of Benzene– Benzene attacts to form sigma complex.– Bromine ion from FeBr4- acts as a weak base to remove a proton to form bromobenzene and HBr. 11
  12. 12. Chlorination of Benzene• Chlorination is similar to bromination.• AlCl3 is used as a Lewis acid catalyst together with Cl2 to form chlorobenzene. 12
  13. 13. Iodination of Benzene• Nitric acid (HNO3) oxidizes iodine to an iodonium ion. 13
  14. 14. Nitration of Benzene• Use sulfuric acid (H2SO4) with nitric acid (HNO3) to form nitronium ion electrophile.• Sulfuric acid acts as a catalyst. 14
  15. 15. Other reactions…• Sulfonation• Desulfonation 15
  16. 16. Other reactions…• Nitration of Toluene• Bromination of aniline 16
  17. 17. Summary of activators• Functional group reactivity Increasing Reactivity 17
  18. 18. Nitration of Nitrobenzene• Nitrobenzene is 100,000 less reactive than benzene.• Nitration is performed in concentrated HNO3 and H2SO4 >100oC.• Forms majority meta- isomers. 18
  19. 19. Friedel- Friedel-Crafts Alkylation• Synthesis of alkylbenzenes from alkyl halides and a Lewis acid (usually AlCl3 or FeCl3). 19
  20. 20. Friedel- Friedel-Crafts Acylation• Acyl chloride (RCO-Cl) reacts with benzene to form phenyl ketone (an acylbenzene).• Reaction is analogous to alkylation, but the final product is a phenyl ketone. 20
  21. 21. Example of Friedel-Crafts Acylation Friedel- 21
  22. 22. Other Reactions… (I)• Clemmensen Reduction  acylbenzene  alkylbenzene upon treatment with HCl and amalgamated zinc Amalgamated: to mix a metal with mercury. 22
  23. 23. Other Reactions… (II)• Chlorination of Benzene – Occurs with high heat and pressure (or light) – Results in benzene hexachloride, used as an insecticide. Benzene Benzene hexachloride 23
  24. 24. Other Reactions… (III)• Catalytic Hydrogenation of benzene – With catalysts: Pt, Pd, Ni, Ru or Rh 24
  25. 25. Side Chain Oxidation (I)• Alkylbenzenes can be oxidized to benzoic acid by hot KMnO4 or Na2Cr2O7 in H2SO4.• Produces carboxylate salt of benzoic acid. 25
  26. 26. Side Chain Halogenation (I)• Occurs in the presence of benzylic position. 26
  27. 27. Side Chain Halogenation (II)• Chlorination• Bromination 27