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4. Aromatic Compounds4. Aromatic Compounds
Prepared by
Joyce Tiong For UEMK1013
October 2012
BenzeneBenzene
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• 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.
BenzeneBenzene
• Benzene occurs in two Kekulé structures.
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Kekulé structures: resonance structure of the benzene ring with alternate double and single bonds
Properties of BenzeneProperties 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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Reactions of BenzeneReactions of Benzene
• Reacts with Br2 in the presence of FeCl3 catalyst to form bromobenzene + HBr
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AnnulenesAnnulenes
• 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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Exceptions of Aromatic CompoundsExceptions 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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Hückel’sHückel’s RuleRule
• 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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Aromatic, antiAromatic, anti--aromatic, nonaromatic, non--aromaticaromatic
Cyclic Planar Every C-atom with π orbital
Fulfill 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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Reactions of Aromatic CompoundsReactions 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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BrominationBromination of Benzeneof 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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Chlorination of BenzeneChlorination 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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Iodination of BenzeneIodination of Benzene
• Nitric acid (HNO3) oxidizes iodine to an iodonium ion.
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Nitration of BenzeneNitration 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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Other reactions…Other reactions…
• Sulfonation
• Desulfonation
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Other reactions…Other reactions…
• Nitration of Toluene
• Bromination of aniline
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Summary of activatorsSummary of activators
• Functional group reactivity
17 Increasing Reactivity
Nitration of NitrobenzeneNitration 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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FriedelFriedel--Crafts Crafts AlkylationAlkylation
• Synthesis of alkylbenzenes from alkyl halides and a Lewis acid (usually AlCl3 or FeCl3).
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FriedelFriedel--Crafts Crafts AcylationAcylation
• 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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Example of Example of FriedelFriedel--Crafts Crafts AcylationAcylation
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Other Reactions… (I)Other Reactions… (I)
• Clemmensen Reduction
acylbenzene alkylbenzene upon treatment with HCl and amalgamated zinc
22 Amalgamated: to mix a metal with mercury.
Other Reactions… (II)Other Reactions… (II)
• Chlorination of Benzene
– Occurs with high heat and pressure (or light)
– Results in benzene hexachloride, used as an insecticide.
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Benzene Benzene hexachloride
Other Reactions… (III)Other Reactions… (III)
• Catalytic Hydrogenation of benzene
– With catalysts: Pt, Pd, Ni, Ru or Rh
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Side Chain Oxidation (I)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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Side Chain Side Chain HalogenationHalogenation (I)(I)
• Occurs in the presence of benzylic position.
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Side Chain Side Chain HalogenationHalogenation (II)(II) • Chlorination
• Bromination
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