The correct answer is Option (4) → 2,4-DNP test
Formic acid (HCOOH) behaves differently in these tests due to its unique structure, containing both a carboxylic acid group and an aldehydic-like hydrogen.
- Fehling's test: Formic acid acts as a reducing agent and gives a positive test (brick-red precipitate of Cu₂O), similar to aldehydes.
- Tollen's test: Formic acid gives a positive test (silver mirror), as it reduces ammoniacal silver nitrate.
- Sodium bicarbonate test: As a carboxylic acid, formic acid reacts vigorously with NaHCO₃, evolving CO₂ gas (brisk effervescence): HCOOH + NaHCO₃ → HCOONa + CO₂ + H₂O.
- 2,4-DNP test: Formic acid does not respond, as it fails to form the characteristic orange/red precipitate of a hydrazone. This is because the carbonyl in carboxylic acids is less electrophilic (due to resonance stabilization in the carboxylate form), preventing nucleophilic addition by 2,4-dinitrophenylhydrazine. This test is specific for free aldehyde/ketone groups, which formic acid lacks in typical carbonyl reactivity.
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