Discussion
The CORRECT order of basicity for the following compounds is
Answer: C
The order of basicity is II \(\gt\) I \(\gt\) III.
II is 2-imidazoline, a cyclic amidine. Protonating its C=N nitrogen gives a cation whose positive charge is shared equally by both nitrogens through resonance, so it is a strong base, with a conjugate acid pKa around 11, far above the other two.
I is imidazole. Its pyridine-like nitrogen is protonated to give the symmetric imidazolium ion, which is also stabilised by resonance between the two nitrogens, so imidazole (conjugate acid pKa about 7) is a stronger base than pyridine.
III is pyridine. Its lone pair sits in an sp2 orbital and its conjugate acid gains no such resonance sharing, so it is the weakest base of the three (pKa about 5.2). Orders that put pyridine above imidazole or the imidazoline anywhere but first are wrong.
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