Reaction by which benzaldehyde cannot be prepared is :

Options

A.
B.
C.
D.

Explanation

To determine which reaction does not yield benzaldehyde, we will analyze each provided reaction step by step.

### Step-by-Step Solution:

1. **Reaction with Toluene and Chromyl Chloride (CrO2Cl2)**
- **Reactants:** Toluene (C6H5CH3) + CrO2Cl2 (in carbon disulfide)
- **Process:** Toluene reacts with chromyl chloride to form a brown addition product.
- **Hydrolysis:** Upon hydrolysis, the brown product breaks down to yield benzaldehyde (C6H5CHO).
- **Conclusion:** This reaction successfully generates benzaldehyde.

2. **Rosenmund Reaction with Benzoyl Chloride**
- **Reactants:** Benzoyl chloride (C6H5C(O)Cl) + Pd/BaSO4 (in boiling xylene)
- **Process:** This reaction is known as the Rosenmund reaction. Benzoyl chloride is reduced to benzaldehyde.
- **Conclusion:** This reaction also successfully generates benzaldehyde.

3. **Gattermann-Koch Synthesis**
- **Reactants:** Benzene (C6H6) + CO + HCl (in presence of anhydrous AlCl3)
- **Process:** This is the Gattermann-Koch aldehyde synthesis, which introduces an aldehyde group to the benzene ring.
- **Conclusion:** This reaction generates benzaldehyde as well.

4. **Reaction of Benzoic Acid with Zinc and Mercury**
- **Reactants:** Benzoic acid (C6H5COOH) + Zn + Hg (in concentrated HCl)
- **Process:** This reaction does not yield benzaldehyde but rather a modified product (likely benzoyl chloride).
- **Conclusion:** This reaction does NOT generate benzaldehyde.

### Final Answer:
The reaction by which benzaldehyde cannot be prepared is the fourth option, which involves the reaction of benzoic acid with zinc and mercury in concentrated HCl.