Aromatic Substitution
Electrophilic Aromatic Substitution
- From the molecular formula Benzene is highly unsaturated, so
it should undergo addition reactions easily.
- Actually it undergoes substitution reactions with the greater
ease (the reason which is termed aromatic nature will be clear
through study of the phenomenon of resonance and the molecular
orbital theory).
- One or more of the hydrogen atoms are replaced by another atom
or group as illustrated.
These aromatic reactions are also termed electrophilic
substitution reactions because an electrophile(electron deficient
species) is involved during the reaction mechanism. There are basically
five such reactions given in the following table.
| Name |
Substituting
group |
Conditions |
Product |
| Nitration |
Nitro:
-NO2 |
Heat with
Conc. H2SO4 and Conc. HNO3 |
Nitrobenzene
C6H5NO2 |
| Sulphonation |
Sulphonic
acid -SO3H |
Heat with
Conc. H2SO4 |
Benzenesulphonic acid C6H5SO3H |
| Halogenation |
Halogen
-X |
Heat, Cl2 ,
FeCl3 |
Chlorobenzene C6H5Cl |
| Alkylation |
Alkyl group
e.g., -CH3 |
Heat, CH3Cl,
anhydrous AlCl3 |
Toluene C6H5CH3 |
| Acylation |
Acyl group
-COCH3 |
Heat, CH3COCl,
anhydrous AlCl3 |
Acetophenone C6H5COCH3 |
Alkylation and Acylation which make use of anhydrous AlCl3 a
Lewis acid as a catalyst are known as Friedel-Craft reactions
Exercise:
-
Write chemical equations for these five reactions
-
What is nitration?
-
Give a method of preparing an aromatic ketone.
-
Write the electrophiles involved in these reactions
-
What is a Lewis acid?.Give two examples other
than AlCl3.
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