Which of the following does not give red-brown precipitate with Fehling reagent?

Question

Accepted Answer

Option 1, $C_{6}H_{5}CHO$ (benzaldehyde), is the correct answer because it does not give a red-brown precipitate with Fehling's reagent. Fehling's test is used to detect the presence of reducing sugars and aldehydes. It involves the reduction of copper(II) ions to copper(I) oxide, which forms a red-brown precipitate.

1. **Explanation for Option 1 ($C_{6}H_{5}CHO$):**
   - Benzaldehyde is an aromatic aldehyde. Aromatic aldehydes generally do not react with Fehling's reagent because they lack the necessary structure to reduce the copper(II) ions in the reagent. The presence of the benzene ring in benzaldehyde stabilizes the carbonyl group, making it less reactive towards oxidation by Fehling's reagent. Therefore, benzaldehyde does not produce a red-brown precipitate.

2. **Explanation for Option 2 (HCHO):**
   - Formaldehyde (HCHO) is a simple aliphatic aldehyde. It readily reacts with Fehling's reagent, reducing the copper(II) ions to copper(I) oxide, which forms a red-brown precipitate. This is because formaldehyde is a strong reducing agent.

3. **Explanation for Option 3 ($CH_{3}CHO$):**
   - Acetaldehyde ($CH_{3}CHO$) is another aliphatic aldehyde that reacts with Fehling's reagent. It reduces the copper(II) ions to copper(I) oxide, resulting in the formation of a red-brown precipitate. Acetaldehyde is a typical reducing aldehyde that easily undergoes oxidation.

4. **Explanation for Option 4 ($CH_{3}-CH_{2}-CH_{2}-CH_{2}-CHO$):**
   - Butyraldehyde ($CH_{3}-CH_{2}-CH_{2}-CH_{2}-CHO$) is also an aliphatic aldehyde. Like other aliphatic aldehydes, it reacts with Fehling's reagent, reducing the copper(II) ions to copper(I) oxide and forming a red-brown precipitate. Its structure allows it to act as a reducing agent.

In summary, Option 1 ($C_{6}H_{5}CHO$) is the correct answer because benzaldehyde, being an aromatic aldehyde, does not react with Fehling's reagent to give a red-brown precipitate, unlike the other aliphatic aldehydes listed in the options.