Electriphilic substitution in Pyrrole

Pyrrole and furan react under very mild conditions

• α- Substitution favoured over β-substitution, because of more resonance forms for intermediate and so the charge is less localized (also applies to the transition state)
• Some β-substitution usually observed - depends on X and substituents

Nitration of Furans

Pyrrole undergoes electrophilic substitution reactions at position C-2

i. Halogenation: Pyrrole reacts with halogens  [ X2 ( X2 = Cl2, Br2 and I2 ) ] to give tetrahalopyrrole. For example, the Reaction of bromine with pyrrole gives tetrabromopyrrole. 

ii. Nitration: Nitration of pyrrole is achieved by reacting it with HNO3 in acetic anhydride. The reaction of HNO3 and anhydride resulted in acetyl nitrate in which -NO2 acts as an electrophile.

iii. Sulphonation: Sulphonation of pyrrole is achieved by reacting it with sulfur trioxide ( SO3) -

pyridine mixture in ethylene chloride.

iv. Friedel-Crafts Acylation: Reaction of pyrrole with acetic anhydride under heating condition give 2-acetylpyrrole.

v. Diazotization: Pyrrole reacts with benzene diazonium chloride in an acidic medium to give 2-phenylazopyrrole.

vi. Reimer-Tiemann Reaction: Pyrrole reacts with Chloroform in presence of KOH to give 2-Formylpyrrole. This reaction is known as the Reimer-Tiemann reaction. It also takes place through an electrophilic substitution reaction mechanism.

Thank you