Which reagent can be employed to distinguish Benzaldehyde and Propanal?

Fehling's Reagent

The correct answer is Option (2) → Fehling's Reagent

Benzaldehyde and propanal are both aldehydes, but they differ in their structure and reactivity. 

Propanal is an aliphatic aldehyde, while benzaldehyde is an aromatic aldehyde. This structural difference affects their behavior toward certain oxidation reagents. 

Fehling's reagent is commonly used to distinguish aliphatic aldehydes from aromatic aldehydes. Fehling's reagent contains a complex of Cu²⁺ ions in alkaline solution. Aliphatic aldehydes are easily oxidized to carboxylate ions, reducing Cu²⁺ to Cu₂O, which appears as a brick-red precipitate.

Aliphatic aldehyde + Cu²⁺ (Fehling's solution) → Carboxylate + Cu₂O (red precipitate)

Propanal reacts with Fehling's reagent and produces a brick-red precipitate of Cu₂O.

Benzaldehyde, being an aromatic aldehyde, does not undergo this oxidation under these conditions due to stabilization of the carbonyl group by the benzene ring. Therefore, no red precipitate forms.

Thus, Fehling's reagent clearly distinguishes the two compounds.

Option 1 — 2,4-DNP reagent -- Both are aldehydes, so they react with 2,4-DNP to give an orange precipitate.  (cannot distinguish)

Option 2 — Fehling's Reagent -- Fehling’s reagent reacts only with aliphatic aldehydes (like Propanal) but not with aromatic aldehydes (like Benzaldehyde). 

Option 3 — Tollen's Reagent -- Tollen’s reagent reacts with both aromatic and aliphatic aldehydes, forming a silver mirror. 

Option 4 — Baeyer's Reagent -- Baeyer’s reagent (KMnO₄) oxidizes both aldehydes and some alcohols, not selective.