Carboxylic acid on treating with which reagent does not give acid chloride ?

Options

A. `PCl_5`
B. `PCl_3`
C. `SOCl_2`
D. `Cl_2`

Answer

To solve the question of which reagent does not convert carboxylic acid to acid chloride, we can analyze the reactions of carboxylic acids with various reagents.

### Step-by-Step Solution:

1. **Understanding Carboxylic Acids**:
- Carboxylic acids have the general formula RCOOH, where R is a hydrocarbon group. The functional group is -COOH.

2. **Reagents that Convert Carboxylic Acids to Acid Chlorides**:
- **PCl5 (Phosphorus Pentachloride)**:
- When carboxylic acid reacts with PCl5, it replaces the -OH group with a -Cl, forming an acid chloride (RCOCl) and releasing POCl3 and HCl.
- **PCl3 (Phosphorus Trichloride)**:
- Similar to PCl5, PCl3 also converts carboxylic acids to acid chlorides, producing phosphoric acid as a byproduct.
- **SOCl2 (Thionyl Chloride)**:
- SOCl2 reacts with carboxylic acids to produce acid chlorides, along with sulfur dioxide (SO2) and HCl as byproducts.

3. **Reagent that Does Not Convert Carboxylic Acids to Acid Chlorides**:
- **Cl2 (Chlorine)** in the presence of red phosphorus:
- This reaction leads to the formation of alpha-halo carboxylic acids instead of acid chlorides. This reaction is known as the Hal-Wolhard-Zelinsky reaction.

4. **Conclusion**:
- Among the reagents discussed, Cl2 in the presence of red phosphorus does not yield acid chlorides from carboxylic acids. Therefore, the answer to the question is that carboxylic acid does not give acid chloride when treated with Cl2.

### Final Answer:
Carboxylic acid does not give acid chloride when treated with **Cl2 (Chlorine)** in the presence of red phosphorus.

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