1. Electrophilic Substitution Reactions of Benzene
Aim:
To describe the electrophilic substitution of arenes with concentrated nitric acid in the presence of concentrated sulphuric acid.
Outline the mechanism of the above reaction.

2. The Reactions of Benzene
The reactions of benzene usually involve the replacement of 1or more hydrogen atoms on the ring being replaced by a functional group in a SUBSTITUTION reaction.
Substitution allows the ring to retain its delocalised Π system of electrons and its stability.
Stability means it is far less reactive than its electron cloud would suggest. In fact it needs a catalyst for reactions to take place.

3. Why are aromatic compounds so unreactive?
Although there is a delocalised Π cloud above and below the plane of the ring electrophiles are far less attracted to the cloud than they are in alkenes.
In ethene, for example there are 2 Π electrons per single covalent bond which form a region of localised high electron density which attracts electrophiles readily.
In benzene there are 6 electrons delocalised over the whole 6 bonds in the ring, leading to a much lower (1/2?) electron density.

4. Mechanism of electrophilic substitution
The electrophile must carry a full positive charge.
Catalysts are usually required.
Benzene is very important in the chemical industry.
Used to make:
Explosives, dyes, pharmaceuticals, insecticides, detergents, polymers etc.
Explosives and dyes are derived from NITROBENZENE.

5. Neither nitric acid nor sulphuric acid will react with benzene.
Nitration of Benzene
Neither nitric acid nor sulphuric acid will react with benzene.
However NITRATING MIXTURE does, readily and exothermically.
At 50oC use equal volumes conc HNO3 and conc H2SO4
C6H6 +HNO3 → C6H5NO3 + H2O
Nitrobenzene
Yellow oil bp 211oC

6. production of the electrophile
the benzene ring is a centre of high electron density which makes it susceptible to electrophilic attack
the nitryl cation, NO2+, is the electrophile in this reaction
it is produced by the reaction of concentrated nitric acid with concentrated sulphuric acid
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7. H2 NO3+ + H2SO4 → H3O+ + NO2+ + H3O+
the initial reaction
HNO H2SO4
HSO4- + H2 NO3+
H2 NO3+ + H2SO4 → H3O+ + NO2+ + H3O+
i.e. HNO3 + 2H2SO4 → NO2+ + 2HSO4- +H3O+
the sulphuric acid acts as a catalyst in this reaction
it is regenerated later in the reaction mechanism
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8. mechanism
the benzene molecule has 6 delocalised electrons in a molecular orbital
the NO2+ electrophile is attracted to this centre of high electron density situated above and below the ring
an electron pair move from the delocalised ring system to the NO2+ forming a new covalent bond
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9. electrons move from the benzene ring towards the electrophile
electrophilic attack
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10. the intermediate formed is very unstable
Note that the positive charge is delocalised over 5 carbon atoms
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11. mechanism
the stable delocalised ring system is reformed as the unstable intermediate breaks down
the covalent bond between the hydrogen and the carbon atom in the benzene ring breaks heterolytically
the delocalised benzene ring system is reformed
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12. the hydrogen - carbon bond breaks
the delocalised molecular orbital reforms
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13. the H+ reacts with HSO4- to regenerate sulphuric acid
the products
the H+ reacts with HSO4- to regenerate sulphuric acid
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14. mechanism the product of this reaction is nitrobenzene
this is an electrophilic substitution reaction
the halogens e.g.chlorine will undergo a similar reaction with benzene in the presence of a catalyst such as aluminium chloride producing chlorobenzene
CARE!!benzene and chlorine undergo an addition reaction in the presence of UV light the product is 1,2,3,4,5,6-hexachlorocyclohexane
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15. + home Electrophilic Substitution 1 – Nitration of benzene
H2SO HNO3 NO HSO H2O
NO2
+NO2 +
NO2 H H+
home
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16. + home Electrophilic Substitution 2 – Bromination of benzene
Br AlCl3 Br AlBrCl3- Br
+Br + Br H
HBr
home
AlCl3
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17. + home Electrophilic Substitution 3 – Alkylation of benzene
CH3Cl AlCl3 CH AlCl4-
CH3
+CH3 +
CH3 H
HCl
home
AlCl3
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18. + home Electrophilic Substitution 4 – Acylation of benzene
CH3COCl AlCl3 CH3CO AlCl4-
CH3 +C O +
CH3 C O H
CH3 C O
HCl
home
AlCl3
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19. + home Bromination of methylbenzene – ring substitution
Br AlBr3 Br AlBr4-
CH3
CH3 + H Br
CH3 Br
Br+
HBr
home
AlBr3
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