Acetaldehyde and Benzaldehyde can be best distinguished by

Fehling's test

The correct answer is option 4. Fehling's test.

Let us explore each test in more detail to understand how they differentiate between acetaldehyde and benzaldehyde:

1. 2,4-DNP Test

2,4-Dinitrophenylhydrazine (2,4-DNP) reacts with aldehydes and ketones to form a 2,4-dinitrophenylhydrazone derivative. This reaction produces a yellow or orange precipitate, indicating the presence of a carbonyl group.

Reaction:

\(\text{R-CHO} + \text{2,4-DNP} \rightarrow R-CH=N-NH-C_6H_3(NO_2)_2 + \text{H}_2\text{O} \)

Acetaldehyde and Benzaldehyde: Both acetaldehyde (CH₃CHO) and benzaldehyde (C₆H₅CHO) will react with 2,4-DNP to form yellow or orange precipitates. Therefore, while this test confirms the presence of aldehydes, it does not distinguish between acetaldehyde and benzaldehyde.

2. Tollen's Test

Tollen’s reagent is a solution of silver nitrate in ammonia. It reacts with aldehydes to produce metallic silver as a silver mirror or black precipitate. This reaction is due to the oxidation of aldehydes to carboxylic acids while reducing silver ions to metallic silver.

Reaction:

\(\text{R-CHO} + 2\text{[Ag(NH}_3)_2]^+ + 2\text{OH}^- \rightarrow \text{R-COO}^- + 2\text{Ag} + 2\text{NH}_3 + \text{H}_2\text{O} \)

Acetaldehyde and Benzaldehyde: Both acetaldehyde and benzaldehyde give a positive Tollen’s test, producing a silver mirror or black precipitate. Since both aldehydes give similar results in this test, Tollen’s test cannot differentiate between acetaldehyde and benzaldehyde.

3. Sodium Carbonate Test

Sodium carbonate reacts with carboxylic acids to produce carbon dioxide gas, which is observed as effervescence (bubbles). Aldehydes and ketones do not react with sodium carbonate, so this test is useful for identifying carboxylic acids.

Reaction:

\(\text{R-COOH} + \text{Na}_2\text{CO}_3 \rightarrow \text{R-COONa} + \text{H}_2\text{O} + \text{CO}_2 \uparrow \)

Acetaldehyde and Benzaldehyde: Neither acetaldehyde nor benzaldehyde reacts with sodium carbonate, as neither is a carboxylic acid. Therefore, this test is not suitable for distinguishing between them.

4. Fehling's Test

Fehling’s solution is a mixture of Fehling’s A (copper(II) sulfate) and Fehling’s B (alkaline tartrate solution). Aldehydes reduce copper(II) ions to copper(I) oxide, forming a red precipitate. Ketones do not react with Fehling’s solution under normal conditions.

Reaction:

\(\text{R-CHO} + 2\text{Cu}^{2+} + 4\text{OH}^- \rightarrow \text{R-COOH} + \text{Cu}_2\text{O} \text{(red precipitate)} + 2\text{H}_2\text{O}\)

Acetaldehyde and Benzaldehyde:

Acetaldehyde (CH₃CHO): Reacts vigorously with Fehling’s solution to form a red precipitate of copper(I) oxide.

Benzaldehyde (C₆H₅CHO): Reacts only weakly with Fehling’s solution and generally does not produce a red precipitate. This is because the aromatic ring in benzaldehyde makes it less reactive with Fehling’s reagent compared to aliphatic aldehydes like acetaldehyde.

Conclusion

Among the tests mentioned:

2,4-DNP test and Tollen’s test both confirm the presence of aldehydes but do not differentiate between acetaldehyde and benzaldehyde.

Sodium carbonate test is not useful for distinguishing between aldehydes.

Fehling’s test can distinguish between acetaldehyde and benzaldehyde because acetaldehyde reacts more vigorously, producing a red precipitate, whereas benzaldehyde reacts weakly or not at all.

Therefore, the test best used to distinguish between acetaldehyde and benzaldehyde is: 4. Fehling's Test.