Which of the following methods are used for the conversion of carboxylic acids into acid chlorides
`(RCOOHrarrRCOCl)`?

Options

A. `RCOOH+SOCl_(2)rarr`
B. `RCOOH+PCl_(5)rarr`
C. `RCOOH+Cl_(2)rarr`
D. `RCOOH+P+Cl_(2)rarr`

Answer

To convert carboxylic acids (RCOOH) into acid chlorides (RCOCl), we can use several methods. Let's analyze the methods provided in the question step by step.

### Step-by-Step Solution:

1. **Method 1: Reaction with Thionyl Chloride (SOCl2)**
- When a carboxylic acid (RCOOH) reacts with thionyl chloride (SOCl2), the reaction proceeds as follows:
\[ RCOOH + SOCl2 \rightarrow RCOCl + SO2 + HCl \]
- This reaction is effective and leads to the formation of the desired acid chloride (RCOCl) along with by-products SO2 and HCl.
- **Conclusion**: This method is valid for converting carboxylic acids to acid chlorides.

2. **Method 2: Reaction with Phosphorus Pentachloride (PCl5)**
- In this method, carboxylic acid (RCOOH) reacts with phosphorus pentachloride (PCl5):
\[ RCOOH + PCl5 \rightarrow RCOCl + POCl3 + HCl \]
- Here, the acid chloride (RCOCl) is formed along with phosphorous oxychloride (POCl3) and hydrochloric acid (HCl).
- **Conclusion**: This method is also valid for the conversion of carboxylic acids to acid chlorides.

3. **Method 3: Reaction with Chlorine (Cl2)**
- The reaction of carboxylic acid with chlorine (Cl2) does not lead to the formation of acid chlorides. Chlorine does not replace the hydroxyl group (OH) in carboxylic acids directly.
- **Conclusion**: This method is not valid for converting carboxylic acids to acid chlorides.

4. **Method 4: Hel-Wolhard-Zelinsky Reaction**
- This reaction involves the use of X2 (where X can be Cl or Br) in the presence of red phosphorus and is primarily used for the halogenation of alpha-hydrogens in carboxylic acids. It does not directly convert carboxylic acids to acid chlorides.
- **Conclusion**: This method is not valid for the conversion of carboxylic acids to acid chlorides.

### Final Conclusion:
The valid methods for converting carboxylic acids to acid chlorides are:
- **Method 1**: Reaction with Thionyl Chloride (SOCl2)
- **Method 2**: Reaction with Phosphorus Pentachloride (PCl5)