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Choose the reaction which is not possible:

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To understand why the reaction involving Fehling's reagent and aromatic aldehydes is not possible, we need to delve into the chemistry of these compounds and the nature of the reagents involved. 1. Understanding Fehling's Reagent Fehling's reagent consists of two solutions: Fehling's A, which is a blue solution of copper(II) sulfate ($\mathrm{CuSO}_{4}$), and Fehling's B, which is an alkaline solution containing sodium potassium tartrate. When mixed, they form a deep blue solution of complexed copper ions. This reagent is commonly used to test for the presence of aldehydes, particularly in the context of distinguishing between aldehydes and ketones. 2. The Reaction Mechanism Fehling's reagent acts as an oxidizing agent. In the presence of a reducing sugar or an aldehyde, the copper(II) ions ($\mathrm{Cu}^{2+}$) are reduced to copper(I) oxide ($\mathrm{Cu}_{2}O$), which precipitates as a red solid. The general reaction can be summarized as follows: $$ \text{RCHO} + 2 \mathrm{Cu}^{2+} + 5 \mathrm{OH}^{-} \rightarrow \text{RCOO}^{-} + \mathrm{Cu}_{2}\mathrm{O} (s) + 3 \mathrm{H}_2\mathrm{O} $$ Here, RCHO represents the aldehyde. 3. Aromatic Aldehydes and Their Stability Aromatic aldehydes, such as benzaldehyde ($\mathrm{C_6H_5CHO}$), have a stable aromatic ring that makes them less reactive compared to aliphatic aldehydes. The resonance stabilization of the aromatic ring means that the carbonyl carbon is less electrophilic, making it less susceptible to oxidation by mild oxidizing agents like Fehling's reagent. 4. Conclusion: Why the Reaction is Not Possible Due to the stability of the aromatic ring and the nature of the carbonyl group in aromatic aldehydes, they do not undergo oxidation with Fehling's reagent. This is why the reaction involving aromatic aldehydes and Fehling's reagent is not possible. In contrast, aliphatic aldehydes can easily be oxidized, which is why Fehling's test is effective for them but not for aromatic aldehydes.

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