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pyrrole anion vs imidazole anion

I at first thought it was the right one because it has 2 lone pairs so I would assume it would have more affinity towards hydrogen protons. However, the resonance forms of the imidazole anions place a negative charge on both nitrogens, so the resonance hybrid would have a partial negative charge on both nitrogens. While for the pyrrole anion, the negative charge is localized on one nitrogen, making me think that it is less stable than the imidazole anion and more basic. Any thoughts?


1 Answer 1


Yours thoughts seem to be correct. Imidazole pKa is 14.5 and 6.8. Pyrrole pKa is 16.5. You do have a resonance form for the imidazole ring. Additionally, nitrogen is electron withdrawing. The localized negative charge on deprotonated pyrrole will be more basic.

Both nitrogens in imidazole can be protonated, hence 2 pKa values.


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