Why does benzaldehyde give a positive Tollen's test (depositing a silver mirror) but fails to reduce Fehling's solution to form a red cuprous oxide precipitate?

Answer and explanation

Correct answer: B. Fehling's solution is a weaker oxidant than Tollen's and cannot oxidize resonance-stabilized aromatic aldehydes

  1. Step 1: Both Tollen's reagent () and Fehling's solution (alkaline tartrate complex) are mild oxidizing agents used to detect aldehydes.
  2. Step 2: Tollen's reagent has a higher standard reduction potential () than Fehling's solution ( in base), making Tollen's reagent a stronger oxidizing agent.
  3. Step 3: In benzaldehyde, resonance donation from the phenyl ring into the carbonyl carbon decreases its electrophilicity and oxidation rate.
  4. Step 4: Consequently, benzaldehyde is readily oxidized by Tollen's reagent (silver mirror), but cannot be oxidized by the weaker Fehling's reagent, a key diagnostic distinction between aliphatic and aromatic aldehydes.
Answer
Fehling's solution is a weaker oxidizing agent than Tollen's reagent and cannot oxidize aromatic aldehydes lacking enolizable α-hydrogens
Why not the others: 'Benzaldehyde is an aliphatic ketone...' is false because benzaldehyde is an aromatic aldehyde; 'Tollen's reagent contains cyanide ions...' is false as it contains ammoniacal silver ions; 'Fehling's solution precipitates benzaldehyde as a chelate...' is chemically inaccurate.

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