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Chemistry

How Aromaticity Dictates the Acidity of Pyrrole versus Pyridine

Quick fact

Pyrrole is about 10^16 times more acidic than ammonia, while pyridine is a weak base. The key is that in pyrrole, the nitrogen lone pair is part of the aromatic sextet, so protonation would destroy aromaticity, making the N–H proton readily lost. In pyridine, the lone pair is not part of the aromatic system, so it can accept a proton without losing aromaticity.

Why this is interesting

You might expect pyrrole, with an NH group, to be a base like ammonia. Yet pyrrole is actually more acidic than many alcohols. How can a nitrogen-containing compound be acidic?