Question

Compare the basicity of pyridine,pyrrole,pyrazolle and imidaxole

Answer 1

Answer:

In pyrrole, the lone pair electrons of the nitrogen atom is actively involved with the two carbon-carbon double bonds in the 5-member ring to form a conjugated system of pi electrons, leading to greater stability of the molecule.Pyridine, on the other hand, already has a stable conjugated system of 3 double bonds in the aromatic hexagonal ring, like benzene. Hence the lone pair electrons on the N atom in pyridine can be easily donated to a H+ ion or a Lewis acid.

Therefore, pyridine is a stronger base than pyrrole.

As we all know that nitrogen containing compounds basicity depends on its ability to donate the lone pair of nitrogen. Pyrazole form a dimer so it's lone pairs are busy in intramolecular hydrogen bonding. This type of dimer structure is not possible in imidazole . So it's lone pairs are available compare to pyrazole.

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Answer 2

Explanation:

pyridine (pkb- 8.77): Weak base but stronger base than pyrrole

pyrrole (pkb-13.6) : Non basic (or can refer as a very weakly basic in nature).

pyrazole (pkb-11.5) : The inductive effect is larger in pyrazole because the two N atoms are closer Hence, the pyrazole is a weaker base compare to imidazole.

imidazole (pkb-7.0) : Moderately strong base

So, basicity order ===> imidazole > pyridine> pyrazole > pyrrole

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