Question

Molecules like acetamide ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{CONH}}}_{\mathbf{\text{2}}}\mathbf{\text{)}}$ can be protonated on either their O or N atoms when treated with a strong acid like HCl. Which site is more readily protonated and why?

Short answer

The oxygen (O) atom is quickly protonated with strong HCl because of electron delocalization.

Step-by-step solution

Nucleophilicity

The nucleophilicity term comes when an atom has more number of free lone pairs of electrons that attract the electron-deficient group.

The more the electron-donating group around the nucleophilic atom, the greater is the nucleophilicity.

Determination of nucleophilic atom

In the given compound acetamide, there are two atoms, nitrogen, and oxygen which have lone pair of electrons. The nitrogen atom delocalizes its lone pair of electrons towards the carbonyl carbon (electrophile).

Therefore, the oxygen atom’s nucleophilicity increases due to the donation of electrons towards the carbonyl group.

Hence, the oxygen atom is more readily protonated with the treatment of HCl.

Representation of enhanced nucleophilicity of oxygen atom