Explain why amides have significantly higher boiling points than amines of similar molar mass, yet amides are far less basic than amines.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Amides can form strong intermolecular hydrogen bonds (N–H···O=C) that are more extensive than the H‑bonding of amines, so their boiling points are higher.
The carbonyl group in an amide delocalises the lone pair on nitrogen into the C=O π system; the lone pair becomes part of a resonance structure and is less available.
Because this lone pair is delocalised, it cannot be donated to a proton; the conjugate acid of an amide has a pKa≈0, making amides very weak bases.
In contrast, the lone pair on an amine is localised and freely available to accept a proton, giving a conjugate acid pKa≈10 and higher basicity.
The carbonyl group in an amide delocalises the lone pair on nitrogen into the C=O π system; the lone pair becomes part of a resonance structure and is less available.
Because this lone pair is delocalised, it cannot be donated to a proton; the conjugate acid of an amide has a pKa≈0, making amides very weak bases.
In contrast, the lone pair on an amine is localised and freely available to accept a proton, giving a conjugate acid pKa≈10 and higher basicity.
Examiner tips
- Mention H‑bonding for boiling point and resonance for basicity; link the two points; use correct pKa values to show basicity difference.
- Use the exact terminology: ‘intermolecular hydrogen bonds’, ‘delocalisation’, ‘lone pair’, ‘conjugate acid’.
Common mistakes
- Confusing the role of the lone pair in hydrogen bonding with its role in basicity; writing that the lone pair is used for H‑bonding donation.
- Giving the wrong pKa values or not specifying the difference between amide and amine basicity.
Mark scheme (4 marks)
- Amides can form intermolecular hydrogen bonds (N–H···O=C) that are stronger/more extensive than those in amines, leading to higher boiling points.
- The carbonyl group in amides delocalises/withdraws the lone pair on nitrogen through resonance (conjugation), reducing its availability for hydrogen bonding donation — accept: the lone pair is part of a π system / delocalised into the C=O.
- Because the lone pair on nitrogen in an amide is delocalised/less available, it cannot be readily donated to a proton (H⁺), so amides are much weaker bases (pKa of conjugate acid ≈ 0) compared with amines (pKa ≈ 10).
- In amines, the nitrogen lone pair is localised and freely available to accept a proton, explaining their relatively high basicity, whereas in amides the same delocalisation that strengthens intermolecular interactions simultaneously reduces basicity.
Key terms in this question
amide · amine · boiling point
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