Question

Why does the protonation of an amide occur at O rather than N?

Answer 1

Consider the possible resonance structures of an amide linkage, `R-C(=O)NHR'`

Explanation:

`R-C(=O)NHR'<=>R-C(-O^-)=N^+HR'`.

The lone pair on the nitrogen is conceived to be delocalized over 3 centres, and one of the resonance structures places a formal negative charge on the oxygen centre. Since the lone pair is no longer localized on nitrogen, the basicity of this centre should be reduced, and apparently it is.

Upon protonation, evidently `R-C(-OH)=N^+HR'`, with a quaternized nitrogen centre, is a significant resonance isomer. Of course, I don't know offhand what the `pK_a` values for an amide are. I suggest you get an advanced organic chemistry text in order to put some numbers into your argument.