A biochemist wants to recover free amino acids from a peptide by hydrolysis at room temperature in pH 7 aqueous buffer. After 24 hours, no free amino acids are detected. What best explains this result?
APeptide bonds require a nucleophile to be attacked, and water is too weak a nucleophile at neutral pH
BAmides are the least reactive carboxylic acid derivatives because resonance ties up nitrogen's lone pair, making them resistant to hydrolysis under mild conditions
CThe peptide dissolved completely, so no solid material was left to hydrolyze
DAmide hydrolysis requires an acid chloride intermediate, which is not available at pH 7
Amides resist hydrolysis under mild conditions precisely because nitrogen's lone pair is delocalized into the carbonyl through resonance, reducing the electrophilicity of the carbonyl carbon and making the nitrogen a poor leaving group. This is why proteins can survive in aqueous environments — amide bonds are the least reactive carboxylic acid derivatives. Complete acid or base hydrolysis of peptides requires concentrated acid or base and elevated temperatures.
Question 2 Multiple Choice
Why is the C–N bond in an amide shorter and rotationally restricted compared to a typical C–N single bond in an amine?
AThe carbonyl oxygen withdraws electrons from nitrogen through induction, stiffening the C–N bond
BNitrogen's lone pair donates into the carbonyl π-system through resonance, giving the C–N bond roughly 40% double-bond character
CThe large carbonyl group creates steric hindrance that physically prevents rotation around C–N
DHydrogen bonding between amide groups in proteins locks the conformation in place
The restricted rotation is an electronic effect, not a steric one. Resonance donation of nitrogen's lone pair into the adjacent carbonyl creates a second resonance structure with C=N double-bond character and negative charge on oxygen. The C–N bond in the resonance hybrid is shorter (between single and double bond) and has a rotational barrier of ~75 kJ/mol. This planarity is the structural origin of protein backbone rigidity and the regular geometry of secondary structures.
Question 3 True / False
The nitrogen in an amide is a stronger base than a typical amine nitrogen because the adjacent carbonyl group stabilizes the positive charge on nitrogen after protonation.
TTrue
FFalse
Answer: False
This is exactly backwards. Resonance donation of nitrogen's lone pair into the carbonyl makes amide nitrogen a *weaker* base (pKa of conjugate acid ≈ −1) compared to a typical amine (pKa ≈ 10–11). Because the lone pair is tied up in resonance with the carbonyl, it is less available to accept a proton. The carbonyl does not stabilize the protonated form — it competes with protonation for the nitrogen's lone pair.
Question 4 True / False
The planarity of the peptide (amide) bond, enforced by restricted C–N rotation, is essential for the formation of protein secondary structures like α-helices and β-sheets.
TTrue
FFalse
Answer: True
True. Because the amide bond locks the six atoms of the amide unit into a plane, the overall fold of a protein is determined by rotations around only the bonds flanking each rigid amide unit (the φ and ψ backbone angles). This restricted geometry is what makes regular secondary structures like α-helices and β-sheets geometrically possible — they arise from repeating patterns of these constrained rotational angles. If amide bonds rotated freely, proteins would have far more conformational flexibility and stable secondary structures would be much harder to maintain.
Question 5 Short Answer
Why are amides simultaneously excellent hydrogen bond donors and acceptors, yet poor nucleophiles and weak bases? Explain using resonance.
Think about your answer, then reveal below.
Model answer: Resonance delocalization of nitrogen's lone pair into the carbonyl creates partial negative charge on oxygen and partial positive charge on nitrogen. The carbonyl oxygen (partial negative charge) accepts hydrogen bonds readily, and the N–H (in primary/secondary amides) donates hydrogen bonds. But this same resonance means nitrogen's lone pair is not freely available — it is tied up in the π-system — making nitrogen a poor nucleophile and a very weak base (pKa of conjugate acid ≈ −1). The two properties are two sides of the same resonance coin.
This is the central paradox of amide chemistry: the resonance that creates excellent hydrogen bonding (partial charges on both O and N, N-H donor) also destroys nucleophilicity and basicity by delocalization of the nitrogen lone pair. Nature exploits both consequences simultaneously — amide hydrogen bonds hold protein secondary structures together, while amide bond resistance to hydrolysis (from low reactivity) keeps proteins intact in aqueous environments.