Q1(c). Electrophilic substitution of heteroarenes

Answer in brief

Pyrrole is generally much more reactive than thiophene toward electrophilic aromatic substitution; both favour attack at C2.

Key points

• A heteroatom lone pair contributes to the six-π aromatic system, placing excess electron density on ring carbons and making both rings more reactive than benzene.
• Nitrogen 2p orbitals overlap efficiently with carbon 2p orbitals in pyrrole; donation stabilises the σ complex formed during electrophilic attack.
• Sulfur uses larger, more diffuse orbitals; its weaker donation and the cost of disrupting thiophene aromaticity make attack less favourable than in pyrrole.
• C2 attack produces a σ complex whose positive charge is spread over more favourable contributors than after C3 attack, explaining α substitution.