Acid anhydride An acid anhydride is a chemical compound made by removing a molecule of water from two equivalents of some oxoacid(s), especially carboxylic acids, and connecting them with the remaining oxygen atom. They can also be thought of as the result of replacing an acidic hydrogen atom with an acyl group. There are different kinds of acid anhydrides. They have different chemical formulas. Some anhydrides can be made (as the description suggests), by the dehydration reaction of two equivalents of acid, usually with a strong desiccant like phosphorus pentoxide. Others are too unstable and must be made in other ways. Formic anhydride is the simplest acid anhydride, but it is too unstable to be made as a pure compound.[1] Acetic anhydride is the simplest stable anhydride, and is an important commodity chemical. Types When the two acids in an anhydride are the same, it makes a symmetric anhydride; when they are different, a mixed anhydride. Some polyprotic acids can form a cyclic anhydride, connecting two parts of one molecule. Maleic anhydride and phthalic anhydride are important cyclic anhydrides.