Acid anhydrides have the general formula (RCO)2O, and appear to be the dehydration product of two carboxylic acid molecules. They are anhydrides, as they form acids when reacted with water. In practice, carboxylic acids do not readily dehydrate to form acid anhydrides, so they must be produced by other means, such as a substitution reaction between an acid chloride and a carboxylate ion .
The reactions of acid anhydrides are similar to those of acid chlorides. Notably, acetic anhydride can form esters with phenols (acetic acid cannot), making it useful in the production of drugs such as Aspirin.