Reaction with PCl5

Carboxylic acids react with phosphorus(V) chloride, PCl₅, to form acyl chlorides. The OH group of the acid is replaced by a chlorine atom.RCOOH + PCl₅ → RCOCl + POCl₃ + HClConditions: solid PCl₅ is added to the acid at room temperature. Everything must be dry, because both PCl₅ and the acyl chloride react with water.Observations: the reaction is vigorous and gives off steamy white fumes of hydrogen chloride. The HCl turns damp blue litmus paper red and forms white smoke with ammonia.The other product, POCl₃, is phosphorus oxychloride. The acyl chloride is separated from it by fractional distillation, as they have different boiling temperatures.Test for an OH group: steamy fumes of HCl are also produced when PCl₅ reacts with alcohols, so this reaction is used as a general test for the OH group. It shows that OH is present but does not tell an alcohol from a carboxylic acid; the sodium hydrogencarbonate test does that.Worked example: ethanoic acid reacts with PCl₅:
CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl
The organic product is ethanoyl chloride. One mole of acid gives one mole of HCl, so 0.0500 mol of ethanoic acid gives 0.0500 mol of HCl gas.