A-Level Amines, amides and amino acids: worked solution

Question

Pyrrole and imidazole, like pyridine, are heterocyclic compounds. Pyrrole is more prone to electrophilic substitution than benzene. Letting 'E+' represent the electrophile, draw the mechanism for electrophilic substitution on pyrrole in a way that accounts for its greater reactivity relative to benzene.

Step 1: The electrophile E+ adds to a ring carbon of pyrrole (position 2). Draw a curly arrow from the ring/nitrogen system to E+, giving a positively charged intermediate. Crucially, the nitrogen lone pair can be donated into the ring so the positive charge is delocalised onto N (an extra resonance form not available to benzene). Step 2: A curly arrow from the C-H bond expels H+, restoring the aromatic ring and giving the 2-substituted pyrrole. Greater reactivity than benzene: the N lone pair makes the ring more electron-rich AND it stabilises the cationic (Wheland) intermediate more strongly, lowering the activation energy relative to benzene.