In the reaction of an aldehyde or ketone with semicarbazide (H2N-NH-CO-NH2), why does condensation occur exclusively at the hydrazine amino group (-NH-NH2) rather than the amide amino group (-CO-NH2)?

Answer and explanation

Correct answer: B. Its amide lone pair is delocalized into the carbonyl group by resonance and has reduced nucleophilicity

  1. Step 1: Semicarbazide has the molecular formula , containing two distinct groups.
  2. Step 2: The group directly attached to the carbonyl group (amide nitrogen) participates in strong resonance: .
  3. Step 3: This resonance delocalization strongly reduces the nucleophilic electron density on the amide nitrogen.
  4. Step 4: In contrast, the terminal hydrazine nitrogen is isolated from the carbonyl group by an intervening spacer; its lone pair is localized and acts as a potent nucleophile, forming the semicarbazone: .
Answer
The lone pair on the amide -NH2 group is delocalized into the carbonyl π-system by resonance, whereas the hydrazine -NH2 lone pair remains fully available for nucleophilic attack
Why not the others: 'The hydrazine -NH2 carries a permanent positive charge' is false; neutral amines carry no permanent formal charge; 'The amide -NH2 is too acidic...' misattributes the lack of reactivity to acidity rather than reduced nucleophilicity; 'The amide group undergoes rapid tautomerization into an enol...' does not represent semicarbazide behavior.

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