Organic Chemistry 2

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Basicity of Pyrrole


Pyrrole (1) is an extremely weak base. In fact its conjugate acid (2) has a $\rm {pK}_\rm {a}$ of about 0.4, making it an even stronger acid than most carboxylic acids.

What is the BEST explanation for why pyrrole is such a weak base?

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Protonation of the nitrogen would greatly increase the steric strain because there would be four groups attached to the nitrogen.


Protonation of the nitrogen would form an anti-aromatic ring system, which is too unstable to form.


The nitrogen is part of an aromatic ring, and all nitrogen atoms in an aromatic ring are inert.


Protonation of the nitrogen would disrupt the aromaticity of the ring system.