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Aromatic Chemistry
Most dyes, pesticides, pharmaceutical drugs, polymers of explosives are aromatic compounds and
contain a benzene ring
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In a
conjugated system, the electron charge from the pi bond is spread over more of the moleculeelectrons have become delocalised
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As transitions energies decrease the
absorptions move towards visible spectrum and give colour
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Kehule proposed that benzene existed in
two states and shifted back and forth between the two
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Each C has four bonding
(valence) electrons; one is used to bond to H and two are used to bond to
two C’s
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This explains problems with Kehules theory and benzenes nonpolarity
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Evidence to Support the Benzene Structure;
Isomers;
Kehule predicted that there would be four isomers of dibromobenzene, but only three exist: 1,2dibromobenzene, 1,3- dibromobenzene, 1,4- dibromobenzene
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Addition Reactions
Benzene would be a very reactive molecule if it contained 3 C=C bouble bonds as it does in Kehules
proposed structure but it is not
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C-C Bond Length
Bond length of C=C in an alkene= 0
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154 nm
Bond length of C-C in benzene= 0
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Therefore this
suggests that for Kehules structure, complete hydrogenation is -119kJ mol-1 x 3 =357kJ mol-1
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Summary
1
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Does not react as it should if it had double bonds like ethane
3
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Delocalisation of electrons accounts for observed intermediate C-C bond length

Reactions of Benzene
Benzene undergoes reduction reactions as one of the six hydrogen atoms is subsitituted by the
reacting functional group
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Electrophilic Substitution
1
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Electrophile bonds to the benzene ring via two of the six delocalised electrons, leaving a
partially delocalised system containing four delocalised electrons
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3
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Electrons from this bond re- form the stable delocalised
six electron system
Mechanism

The reaction conditions are generally severe;
 Heat
 Concentrated reagents and catalysts to make these reactions take place
This is because of the high stability of the delocalised benzene structure
Electrophiles generated easily are; NO2+, Nitro group and CH3C+O, Acyl group

Nitration of Benzene via Electrophilic Subsitution
Conditions/ Reagents; Concentrated sulphuric acid, H2SO4 and concentrated nitric acid, HNO3 at 50°c
The nitric acid provides the nitro group
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C6H6 + HNO3  C6H5NO2 + H2O
Step one- Formation of the electrophile +NO2
Overall;
2H2SO4 + HNO3  +NO2 + H3O+ + 2HSO4By

H2SO4 + HNO3  HSO4- + H2NO3
H2NO3  +NO2 + H2O
H2O + H2SO4  H3O+ + HSO4-

Nitrobenzenes can be used as explosives e
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TNT (Trinitrotoluene)

Nitrobenzene is a very important chemical as it can be subsequently converted to phenylamine (An
aromatic amine) by reduction of the nitro group, +NO2 to NH2
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Diazonium salts allow benzene ring to react with nuclophiles which cannot be done directly
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Friedel- Crafts Acylation Reactions
Friedel- Crafts Acylation Reactions are electrophilic substitution reactions where an acyl (or
an alkyl group) is attached to the ring
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AlCL3 and FeCl3 are Lewis Acids so accept a lone pair
of electrons from the chloride of the acyl chloride
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Reaction Conditions;
 80°c
 Ensure reaction mixture totally free of water- AlCl3 and FeCl3 react vigorously with water
Step 1; Generation of nucleophile

Step 2; reaction of nucleophile with benzene

Catalyst reforms; AlCl4- + H+  AlCl3 + HCl
Can then reduce product using nickel catalyst and hydrogen gas

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