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1. 4. Aromatic Compounds
Prepared by
Joyce Tiong
For UEMK1013
October 2012

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.
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3. Benzene
• Benzene occurs in two Kekulé structures.
Kekulé structures: resonance structure of the
benzene ring with alternate double and single bonds
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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.
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5. Reactions of Benzene
• Reacts with Br2 in the presence of FeCl3
catalyst to form bromobenzene + HBr
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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
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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.
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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
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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+2
Anti-aromatic Yes Yes Yes 4N
Non-aromatic No Maybe No No
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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.
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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.
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12. Chlorination of Benzene
• Chlorination is similar to bromination.
• AlCl3 is used as a Lewis acid catalyst together
with Cl2 to form chlorobenzene.
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13. Iodination of Benzene
• Nitric acid (HNO3) oxidizes iodine to an
iodonium ion.
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14. Nitration of Benzene
• Use sulfuric acid (H2SO4) with nitric acid
(HNO3) to form nitronium ion electrophile.
• Sulfuric acid acts as a catalyst.
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15. Other reactions…
• Sulfonation
• Desulfonation
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16. Other reactions…
• Nitration of Toluene
• Bromination of aniline
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17. Summary of activators
• Functional group reactivity
Increasing Reactivity 17

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.
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19. Friedel-
Friedel-Crafts Alkylation
• Synthesis of alkylbenzenes from alkyl halides
and a Lewis acid (usually AlCl3 or FeCl3).
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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.
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21. Example of Friedel-Crafts Acylation
Friedel-
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22. Other Reactions… (I)
• Clemmensen Reduction
 acylbenzene  alkylbenzene upon treatment with
HCl and amalgamated zinc
Amalgamated: to mix a metal with mercury. 22

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
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24. Other Reactions… (III)
• Catalytic Hydrogenation of benzene
– With catalysts: Pt, Pd, Ni, Ru or Rh
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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.
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26. Side Chain Halogenation (I)
• Occurs in the presence of benzylic position.
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27. Side Chain Halogenation (II)
• Chlorination
• Bromination
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