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

Pyridineis 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.

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