[bzk]

Hi everyone.  I had a question concerning the basicity of nitrogens in Imidazole ( 2 nitrogens) vs the basicity in the 1 nitrogen in Pyrrolidine.

I understand that the nitrogen not attached to the hydrogen in Imidazole is the most basic of the 3 because of the lone pair that is in the same plane of the aromatic system.  However, I was stuck on the LEAST basic nitrogen of the 3.  I know the lone pair of second nitrogen in Imidazole is in the plane of the aromatic system, but not sure about the lone pair of the nitrogen in Pyrrolidine.  Any help is greatly appreciated. Thanks!

[Dan]

I understand that the nitrogen not attached to the hydrogen in Imidazole is the most basic of the 3 because of the lone pair that is in the same plane of the aromatic system.

What you say is true, but I don't think it explains why it is the most basic N. The N in imidazole (not the NH) is sp² hybridised (and not delocalised) and in pyrrolidine the NH is sp³ hybridised. Typically, sp³ lone pairs (comparing the same atom) are more basic than sp² due to the larger size of the sp³ orbital - this is not observed in this example, so there must be something else going on. That something else is resonance. Protonation of the N (not NH) in imidazole gives a resonance stabilised cation in which the positive charge is shared equally over the two Ns - a cation more stable than the localised protonated pyrrolidine.

For the NH in imidazole vs NH in pyrrolidine, think about the relative availability of the lone  pairs for picking up a proton. Resonance is the key once more.