Which reagent is used to convert a carboxylic acid, $\text{RCOOH}$, into an acid chloride, $\text{RCOCl}$?

Solution

1. Carboxylic acids react with ${\text{SOCl}}_2$ to produce acid chlorides.
2. The reaction is $\text{RCOOH}+{\text{SOCl}}_2\to \text{RCOCl}+{\text{SO}}_2+\text{HCl}$.
3. The gaseous byproducts ${\text{SO}}_2$ and $\text{HCl}$ drive the reaction to completion.
4. Hence the answer is (A).