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Thursday, January 23, 2020

Wednesday, January 22, 2020

What is correct basicity order of pyridine, pyridazine, pyrimidine and pyrazine ?


Pyridine is more basic as compared to pyridazine, pyrimidine and pyrazine because pyridazine, pyrimidine and pyrazine containing two nitrogen atoms which are exerted inductive effect to each other so availability of lone pair for donation is less.

But pyridazineis  more basic as expected, it may be due to electrostatic repulsion of the lone pairs of the vicinal nitrogens. This repulsion destabilizes the non-protonated structure and therefore makes protonation easier.

Why pyridine is more basic than Pyrrole?

In case of Pyrrole  the lone pair electrons of the nitrogen atom is involved in conjugated system of pi electrons of five membered ring leading to greater stability of the molecule due to acquiring aromatic character. Hence less available for donation.
While in case pyridine already has a stable conjugated system of three double bonds in an aromatic ring, like benzene. Hence the lone pair electrons on the N atom in pyridine is localized and  more available for donation and  easily donated to a H+ ions. Therefore, pyridine is a stronger base than Pyrrole.