• 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 in the rate determining step of the mechanism.
There are basically five such reactions given in the following table.

Alkylation and Acylation which make use of anhydrous AlCl₃ a Lewis acid as a catalyst are known as Friedel-Craft reactions.

Exercise:

1. Write chemical equations for these five reactions.
2. What is nitration?
3. Give a method of preparing an aromatic ketone.
4. Write the electrophiles involved in these reactions.
5. What is a Lewis acid?. Give two examples of Lewis acids other than AlCl₃.