Question

When pyrrole undergoes electrophilic aromatic substitution, at which position does substitution occur?

Answer 1

Pyrrole is a heterocyclic compound consisting of nitrogen as a hetero atom. Electrophilic aromatic substitution in pyrrole means the hydrogen atom in pyrrole can be replaced by any electrophile. The electrophilic substitution in pyrrole was effective at the ${2^{nd}}$ position due to the formation of resonating structures.

Complete answer:
Heterocyclic compounds are the cyclic organic compounds that have a hetero atoms like sulphur, nitrogen or oxygen as a part in their cyclic structure. Some of the examples of heterocyclic compounds are pyrrole, thiophene, and furan.
Pyrrole is heterocyclic compound consisting of nitrogen as a hetero atom. The molecular formula of pyrrole is ${C_5}{H_5}N$ . It is an aromatic compound with the presence of lone pairs of electrons on nitrogen atoms.
Electrophilic aromatic substitution means the substitution of one electrophile by a stronger electrophile.
When electrophilic substitution takes place at ${2^{nd}}$ position of pyrrole, it will lead to the formation of three resonating structures. When electrophilic substitution takes place at the ${3^{rd}}$ position of pyrrole, it will lead to the formation of two resonating structures only.
More is the resonating structures; more will be the stable compound. Thus, when a pyrrole undergoes electrophilic aromatic substitution, substitution occurs at the ${2^{nd}}$ position of the pyrrole.

Note:
Generally, electrophilic substitution takes place at the ${2^{nd}}$ position of pyrrole. But when the ${2^{nd}}$ position is occupied by any electrophile like halides or any stronger electrophiles then the substitution occurs at the ${3^{rd}}$ position. When there is a more charge distribution, more will be the resonating structures, more will be the stability of the compound.