To convert a carboxylic acid directly into its corresponding acid chloride derivative, we need to use a reagent that can replace the -OH group of the carboxylic acid with a -Cl group. The most commonly used reagent for this transformation is thionyl chloride (SOCl2), which is represented by the formula (COCl)2.



The correct answer is option 'D' - (COCl)2. Here's why:

1. Thionyl Chloride (SOCl2):
- Thionyl chloride is a versatile reagent commonly used for converting carboxylic acids to acid chlorides.
- It reacts with carboxylic acids to form acid chlorides by replacing the -OH group with a -Cl group.
- The reaction proceeds through an intermediate where the -OH group is converted into a good leaving group (-OSO2Cl) before being replaced by -Cl.
- The byproducts of this reaction are sulfur dioxide (SO2) and hydrogen chloride (HCl).

2. Other Options:
- Option 'A' - HCl: Hydrochloric acid (HCl) is not suitable for this conversion because it does not possess the ability to replace the -OH group of the carboxylic acid with a -Cl group.
- Option 'B' - NaOCl: Sodium hypochlorite (NaOCl), also known as bleach, is not a suitable reagent for converting carboxylic acids to acid chlorides.
- Option 'C' - CH3COCl: Acetyl chloride (CH3COCl) is not a suitable reagent for converting carboxylic acids to acid chlorides. It is mainly used to convert carboxylic acids into acyl chlorides with the formation of acetic acid as a byproduct.



The correct reagent for converting a carboxylic acid directly into its corresponding acid chloride derivative is thionyl chloride (SOCl2), represented by the formula (COCl)2. This reagent replaces the -OH group of the carboxylic acid with a -Cl group, forming the desired acid chloride.