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Organic Chemistry 2

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Basicity of Pyrrole

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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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A

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

B

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

C

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

D

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