Organic Chemistry 2 · Amine basicity and electronic structure
Why an amide nitrogen is not basic
Conditions: Acetamide, CH3CONH2, compared with ethylamine, CH3CH2NH2.
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The question
Ethylamine is readily protonated on nitrogen by dilute acid, but acetamide is not. Why is the amide nitrogen not basic?
Reveal the answer and explanation
Answer and explanation
Correct answer The nitrogen lone pair is delocalised into the C=O group, so protonating N would destroy amide resonance
Correct: the lone pair is part of the amide π system, and strong acids protonate the oxygen instead.
Not this one Amide nitrogens have no lone pair
The lone pair is there; it is shared with the carbonyl rather than free.
Not this one The acetyl group is too bulky for a proton to reach nitrogen
A proton is tiny; the effect is electronic, not steric.
Not this one The nitrogen is sp³ and holds its lone pair too tightly
Amide nitrogen is close to sp² and planar; delocalisation, not hybridization, is the main reason.
Worked solution
The nitrogen lone pair of an amide is delocalised into the C=O π system (amide resonance), giving the C–N bond partial double-bond character. Protonating nitrogen would destroy that resonance, so amides are very weak bases (pKaH about −0.5) and, when strong acid does protonate them, it protonates the carbonyl oxygen.
What you'll practice
Explain why the nitrogen of an amide is not a basic site, unlike the nitrogen of an alkyl amine.
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