Identify the reagents A and B respectively in the following reactions
`CH_3COOH overset(A)(rarr)CH_3COCl overset(B)(rarr)CH_3CHO`

Options

A. `SOCl_2, H_2//pd - BaSO_4`
B. `H_2//pd - BaSO_4.SOCl_2`
C. `SOCl_2, H_2O_2`
D. `SOCl_2, OsO_4`

Answer

To identify the reagents A and B in the given reactions:

1. **Starting Reaction**: The reaction begins with acetic acid (CH₃COOH) converting to acetyl chloride (CH₃COCl).

2. **Reagent A**: To convert acetic acid to acetyl chloride, we typically use thionyl chloride (SOCl₂). This reagent reacts with the hydroxyl group (-OH) of the carboxylic acid, replacing it with a chlorine atom and releasing sulfur dioxide (SO₂) and hydrogen chloride (HCl) as byproducts.

**Reaction**:
\[ CH₃COOH + SOCl₂ \rightarrow CH₃COCl + SO₂ + HCl \]

3. **Reagent B**: The next step is the conversion of acetyl chloride (CH₃COCl) to acetaldehyde (CH₃CHO). This transformation is typically achieved through a reduction process. The Rosenmund reduction is a common method for this conversion, which involves the use of palladium on barium sulfate (Pd/BaSO₄) as a catalyst in the presence of hydrogen gas (H₂).

**Reaction**:
\[ CH₃COCl + H_2 \xrightarrow{Pd/BaSO₄} CH₃CHO + HCl \]

4. **Final Identification**:
- Reagent A is **Thionyl Chloride (SOCl₂)**.
- Reagent B is **Palladium on Barium Sulfate (Pd/BaSO₄)**.

### Summary of the Solution:
- **Reagent A**: Thionyl Chloride (SOCl₂)
- **Reagent B**: Palladium on Barium Sulfate (Pd/BaSO₄)