Question

Propose a mechanism for the following reaction:

Short answer

The mechanism followed here is an electrophilic substitution reaction.

Step-by-step solution

Electrophile

An electrophile is an electron-deficient species, so an electron-rich species attack it.

In pyrrole, the electrophilic substitution occurs at position 2 due to more resonance structures and, hence, more stability.

Mechanism of reaction

The reaction is an electrophilic substitution reaction and will occur at position 2 of the pyrrole ring. The mechanism is as follows:

• The oxygen in CH2O attacks the proton of HCl and forms an electrophile having a positive charge on oxygen.
• This electrophile attacks the pyrrole at position 2 and delocalization of electron to nitrogen of the pyrrole ring.
• The oxygen of the OH group on the pyrrole ring attacks the proton of HCl and forms an alkene intermediate after water removal.
• This intermediate electrophile attacks another molecule of pyrrole and forms are given molecule.

Mechanism