The reaction by which Benzaldehyde can not be prepared

Options

A.
B.
C.
D.

Answer

To solve the question of which reaction does not yield benzaldehyde, we will analyze each option given in the question step by step.

### Step 1: Identify the reactions in the options

1. **Rosenmund Reaction**: This reaction involves the conversion of acyl chlorides to aldehydes using hydrogen in the presence of a catalyst (usually palladium on barium sulfate). In this case, if we start with benzoyl chloride (an acyl chloride), we will obtain benzaldehyde.

2. **Gattermann Reaction**: This reaction involves the synthesis of aldehydes from benzene using carbon monoxide and hydrochloric acid in the presence of anhydrous aluminum chloride. This reaction will also yield benzaldehyde when starting from benzene.

3. **Clemmensen Reduction**: This reaction involves the reduction of carbonyl compounds (aldehydes or ketones) to alkanes using zinc amalgam and hydrochloric acid. However, it does not work with carboxylic acids. If we start with benzoic acid, there will be no reaction, and thus, benzaldehyde will not be formed.

4. **Itard Reaction**: This reaction involves the oxidation of toluene to benzaldehyde using chromyl chloride (CrO2Cl2) in a non-polar solvent. This reaction will yield benzaldehyde.

### Step 2: Analyze the results

- **Option 1 (Rosenmund Reaction)**: Benzaldehyde is formed.
- **Option 2 (Gattermann Reaction)**: Benzaldehyde is formed.
- **Option 3 (Clemmensen Reduction)**: No reaction occurs, and benzaldehyde is not formed.
- **Option 4 (Itard Reaction)**: Benzaldehyde is formed.

### Conclusion

The reaction in which benzaldehyde cannot be prepared is the **Clemmensen Reduction** (Option 3).

### Final Answer: Option 3

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