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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