See image — Aromatic Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Pyrrole is an electron-rich aromatic heterocycle. Electrophilic aromatic substitution (EAS) on pyrrole preferentially occurs at the C-2 position (alpha position, adjacent to nitrogen) rather than the C-3 position (beta position). This is because the transition state/intermediate (sigma complex) formed by attack at C-2 is more stable — the positive charge can be delocalized onto nitrogen at the alpha position more effectively than at the beta position. Reasoning: In pyrrole, the nitrogen lone pair is part of the aromatic system and donates electron density most effectively to the C-2 and C-5 (alpha) positions. When an electrophile attacks at C-2, the resulting arenium ion has three resonance structures, one of which places the positive charge directly on nitrogen (which is stabilized by the nitrogen lone pair). When attack occurs at C-3 (beta), only two resonance structures are available and the positive charge cannot be placed directly on nitrogen, making this intermediate less stable. For sulphonation (electrophilic substitution with SO3 or H2SO4), the electrophile preferentially attacks at the most electron-rich and kinetically/thermodynamically favored position, which is C-2 (alpha position) of pyrrole. In the numbering shown in the question, the NH is at position 1, C-2 is the alpha carbon adjacent to NH (which corresponds to answer option b = position 2), C-3 is the beta position (marked with arrow 3), and C-4 is the other beta position (marked with arrow 4). Why other options fail: - Option (a) C-1: This is the nitrogen atom bearing NH, not a carbon; substitution does not occur here under normal EAS conditions. - Option (c) C-3: Beta position, less favored due to less effective lone pair delocalization. - Option (d) C-4: Also a beta position, less favored for the same reason as C-3. Therefore, sulphonation is most favourable at C-2 (alpha position adjacent to NH). Therefore, the correct answer is B.