An aldehyde that does not give red precipitate on heating with Fehling's reagent is

Options

A. methanal
B. ethanal
C. benzaldehyde
D. propanal

Explanation

To determine which aldehyde does not give a red precipitate when heated with Fehling's reagent, we need to understand how Fehling's test works and the characteristics of different aldehydes.

### Step-by-Step Solution:

1. **Understanding Fehling's Reagent**:
- Fehling's reagent is a solution of copper(II) sulfate mixed with sodium potassium tartrate. It is used to test for aldehydes. When an aldehyde is present, it reduces the copper(II) ions (Cu²⁺) to copper(I) oxide (Cu₂O), which forms a red precipitate.

2. **Identifying Aldehyde Types**:
- Aldehydes can be classified into two main categories: aliphatic aldehydes (like formaldehyde, acetaldehyde, and propionaldehyde) and aromatic aldehydes (like benzaldehyde).

3. **Analyzing Benzaldehyde**:
- Benzaldehyde (C₆H₅CHO) is an aromatic aldehyde. The carbonyl group (C=O) in benzaldehyde is involved in resonance with the aromatic ring. This resonance stabilizes the carbonyl group, making it less reactive towards oxidation.

4. **Resonance Effect**:
- The resonance structure of benzaldehyde shows that the carbonyl bond has partial double bond character due to the interaction with the aromatic ring. This makes the C=O bond stronger and less susceptible to oxidation by weak oxidizing agents like Cu²⁺.

5. **Conclusion**:
- Since benzaldehyde does not undergo oxidation in the presence of Fehling's reagent, it does not produce the red precipitate. Therefore, the answer to the question is benzaldehyde.

### Final Answer:
The aldehyde that does not give a red precipitate on heating with Fehling's reagent is **benzaldehyde**.