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I am comparing the structures of 2-pyridone and a regular (cyclic) amide-what is the main factor contributing to the increased acidity of 2-pyridone compared to a normal amide. For pyridone, I thought the proton was orthogonal to the pi-system, so no delocalisation of the negative charge could take place, so I am confused how it is more acidic?
The main factor is aromaticity.
Let's look at the main mesomeric forms of DMAc and 2-pyridone.
For DMAc the true structure is somewhere in the middle. We know the C-N bond cannot rotate freely but the barrier is quite low. If we look at structure 2 we can imagine that this structure contributes a lot to the acidity of the amide, since we got a protonated imine like structure, while on the left it's more like an amine.
For 2-pyridone the right structure (4) is aromatic, so it is quite stable. In this case this mesomeric structure will contribute to the real structure much more, which means it is closer to the right structure than in case of DMAc. But in structure 4 we basically got a protonated pyridine. Which is very acidic. It also tautomerizes to 2-Hydroxypyridine, in that case we have a phenol like structure, which is quite acidic too.
