In order to show the general method for the preparation of acid chloranhydrides, we will take that of acetic acid, CH_{3}·COOH, as an example. Phosphorus pentachloride is placed in a glass retort, and acetic acid poured over it; hydrochloric acid is then evolved, and the substance distilling over directly after is a very volatile liquid, boiling at 50°, and having all the properties of the chloranhydrides. With water it forms hydrochloric and acetic acids. The reaction here taking place may be explained thus: the substitution of the oxygen taken from the acetic acid (from its carboxyl) by two atoms of chlorine from the PCl_{5} should be as follows: CH_{3}·COOH + PCl_{5} = CH_{3}·COHCl_{2} + POCl_{3}. But the compound CH_{3}·COHCl_{2} does not exist in a free state (because it would indicate the possibility of the formation of compounds of the type CX_{6}, and carbon only gives those of the type CX_{4}); it therefore splits up into HCl and the chloranhydride CH_{3}·COCl. The general scheme for the reaction of phosphorus pentachloride with hydrates ROH is exactly the same as with water; namely, ROH with PCl_{5}, gives POCl_{3} + HCl + RCl--that is a chloranhydride.[28 bis] [28 bis] This reaction indeed proceeds very easily and completely with a number of hydroxides, if they do not react on hydrochloric acid and phosphorus oxychloride, which is the case when they have alkaline properties. When the hydroxide is bibasic and is present in excess, it not unfrequently happens that the elements of water are taken up: R(OH)_{2} + PCl_{5} = RO + 2HCl + POCl_{3}. The anhydride RO may then be converted into chloranhydride, RO + PCl_{5} = RCl_{2} + POCl_{3}--that is, phosphorus pentachloride brings about the substitution of O by Cl_{2}. Thus carbonyl chloride, COCl_{2}, boron chloride, 2BCl_{3}, and succinic chloride, C_{4}H_{4}O_{2}Cl_{2}, &c., are respectively obtained by the action of phosphoric chloride on carbonic, boric, and succinic anhydrides. Phosphorus pentachloride reacts in a similar manner on the aldehydes, RCHO, forming RCHCl_{2}, and on the chloranhydrides themselves--for example, with acetic chloride, CH_{3}.COCl (when heated in a closed tube), it forms a substance having the composition CH_{3}CCl_{3}.