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The reaction of a carboxylic acid with phosphorus pentachloride produces an acid chloride. The reaction goes firstly by a cyclic transition state with the removal of HCl. Then via a nucleophilic addition of chloride where the carbonyl is simultaneously protonated. The final step is a cyclic transition state to form the acid chloride, HCl and phosphorus oxychloride. Formation of the strong P=O bond drives the reaction.