Formation of acyl chloride

Carboxylic acids, heated with any of three different reagents -- PCl3, PCl5, or SOCl2 (thionyl chloride) -- are converted to the corresponding ACYL CHLORIDE, the -OH of the original -COOH group being replaced, in each case, by -Cl. THIONYL CHLORIDE is specifically the PREFERRED reagent of the three, for a genuinely practical reason: BOTH of its reaction byproducts, sulfur dioxide (SO2) and hydrogen chloride (HCl), are gases at reaction temperature, and so both simply escape from the reaction flask on their own, leaving a clean acyl-chloride product behind with no messy byproduct to separate out afterwards: R-COOH + SOCl2, on heating, gives R-COCl + SO2(up-arrow) + HCl(up-arrow). With phosphorus trichloride instead: 3 R-COOH + PCl3, on heating, gives 3 R-COCl + P(OH)3 …