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 "Carboxylic acid on treating with which reagent does not give acid chloride?", we will analyze the reactions of carboxylic acids with various reagents.

### Step-by-Step Solution:

1. **Understanding Acid Chloride Formation**:
Acid chlorides can be formed from carboxylic acids when they react with certain reagents such as thionyl chloride (SOCl2), phosphorus trichloride (PCl3), or phosphorus pentachloride (PCl5).

2. **Reagent 1: Phosphorus Pentachloride (PCl5)**:
- Reaction:
\[ \text{RCOOH} + \text{PCl5} \rightarrow \text{RCOCl} + \text{POCl3} + \text{HCl} \]
- Conclusion: This reaction produces an acid chloride (RCOCl), so PCl5 is not the answer.

3. **Reagent 2: Phosphorus Trichloride (PCl3)**:
- Reaction:
\[ \text{RCOOH} + \text{PCl3} \rightarrow \text{RCOCl} + \text{H3PO3} \]
- Conclusion: This reaction also produces an acid chloride (RCOCl), so PCl3 is not the answer.

4. **Reagent 3: Thionyl Chloride (SOCl2)**:
- Reaction:
\[ \text{RCOOH} + \text{SOCl2} \rightarrow \text{RCOCl} + \text{SO2} + \text{HCl} \]
- Conclusion: This reaction produces an acid chloride (RCOCl), so SOCl2 is not the answer.

5. **Reagent 4: Chlorine (Cl2)**:
- Reaction:
\[ \text{RCOOH} + \text{Cl2} \rightarrow \text{RCHClCOOH} + \text{HCl} \]
- Conclusion: This reaction does not produce an acid chloride; instead, it produces a chlorinated carboxylic acid. Therefore, Cl2 is the reagent that does not give acid chloride.

6. **Final Answer**:
The reagent that does not give acid chloride when treated with carboxylic acid is **Chlorine (Cl2)**.