Electrophilic substitution reactions of furan

 Electrophilic substitution reactions of furan involve the replacement of one or more hydrogen atoms in the furan ring with electrophiles. Furan, being an aromatic heterocycle with a five-membered ring containing four carbon atoms and one oxygen atom, undergoes these reactions differently compared to benzene due to the presence of the oxygen atom.

1. Nitration: Furan can be nitrated using a mixture of nitric acid and sulfuric acid. However, nitration is less straightforward compared to benzene because the reaction is highly sensitive and typically results in a mixture of products.

   • Reaction: ${\text{C}}_4{\text{H}}_4\text{O}+{\text{HNO}}_3\to {\text{NO}}_2\text{-furan}+\text{by-products}$

2. Sulfonation: Furan reacts with sulfuric acid to form furan-2-sulfonic acid. This reaction also tends to produce a mixture of products due to the instability of the intermediates.

   • Reaction: ${\text{C}}_4{\text{H}}_4\text{O}+{\text{H}}_2{\text{SO}}_4\to {\text{C}}_4{\text{H}}_3\text{O}{\text{SO}}_3\text{H}$

3. Friedel-Crafts Alkylation: Furan can undergo Friedel-Crafts alkylation reactions, but this process can sometimes lead to polysubstitution due to the ring's reactivity.

   • Reaction: ${\text{C}}_4{\text{H}}_4\text{O}+\text{R-X}\to \text{R-furan}+\text{HX}$

4. Friedel-Crafts Acylation: Similarly, furan can undergo Friedel-Crafts acylation to form acyl derivatives. The reaction generally involves acyl chlorides and aluminum chloride.

   • Reaction: ${\text{C}}_4{\text{H}}_4\text{O}+\text{R-CO-Cl}\to \text{R-CO-furan}+\text{HCl}$

5. Halogenation: Furan reacts with halogens (like chlorine or bromine) in the presence of a catalyst, but it often leads to a complex mixture of products, including di- and tri-substituted derivatives.

   • Reaction: ${\text{C}}_4{\text{H}}_4\text{O}+{\text{X}}_2\to \text{X-furan}+\text{other products}$

Due to the electron-donating effect of the oxygen atom, furan is more reactive in electrophilic substitution reactions compared to benzene, but this also makes controlling the reaction conditions crucial to obtaining the desired products.