Hydrogen exchange, base-catalyzed pyridazines

In pyridazine, base-catalyzed hydrogen-deuterium exchange takes place at positions 4 and 5 more easily than at positions 3 or 6. Deuteration of pyridazine 1-oxide in NaOD/DiO... 

Base-catalyzed hydrogen exchange in pyridazine (74) occurs in NaOD-DgO and MeONa-MeOD, ° the positional reactivity being 4(5) > 3(6) in both cases. Once more the decreased reactivity of a center ortho to a nitrogen atom relative to a more removed center is evident like pyridine, pyridazine does not have the regular geometry of benzene. 

In pyridazines, base-catalyzed hydrogendeuterium exchange takes place at positions 4 and 5 more easily than at positions 3 and 6. Pyridazine 1-oxide reacts first at positions 5 and 6 and then at C(3) and C(4). Pyrimidine exchanges most readily at the S-position, next at the 4-position, and least readily at the 2-position. In pyrimidine 1-oxide, the reactivity order is 2>6>4>>5. 1,2,4-Triazines easily undergo base-catalyzed hydrogen exchange at the 2-position. 

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