SolutionsAcid-Base Properties
1. Where does it protonate?
Why is an amide protonated on oxygen and not on nitrogen?
The nitrogen lone pair is delocalized towards the carbonyl; protonation on oxygen spreads the positive charge over O, C and N (three resonance forms), whereas protonation on N would destroy that delocalization.
2. Amide or amine
Acetamide (pKa of the conjugate acid ≈ −0.5) and ethylamine (≈ 10.6): which is more basic, and by roughly how much?
Ethylamine, by about 1011 times: the pKa difference is about 11 units.
In the amine the nitrogen lone pair is localized and available; in the amide it is tied up in resonance with the C=O.