The hydroxyl group ($\text{-OH}$) of an alcohol or a carboxylic acid can be readily replaced by a chlorine atom ($\text{-Cl}$) upon reaction with phosphorus halides such as phosphorus pentachloride ($\text{PCl}_5$), phosphorus trichloride ($\text{PCl}_3$), or thionyl chloride ($\text{SOCl}_2$) .

For example, the reaction with alcohols is given by: $\text{R-OH} + \text{PCl}_5 \rightarrow \text{R-Cl} + \text{POCl}_3 + \text{HCl}$ .

Similarly, carboxylic acids react with $\text{PCl}_5$ to form acyl chlorides ($\text{R-COCl}$). While concentrated hydrochloric acid ($\text{HCl}$) can replace the $\text{-OH}$ group of primary and secondary alcohols in the presence of a $\text{ZnCl}_2$ catalyst , it cannot replace the $\text{-OH}$ group of carboxylic acids. Therefore, phosphorous pentachloride is the correct general reagent for both.