Acid anhydrides are the residues remaining after an oxoacid loses one or more molecules of water. Generally, for inorganic acids, a single molecule of the acid directly loses one molecule of water to form its corresponding anhydride; in this process, the oxidation state of the element responsible for the acid's acidity remains unchanged. Organic acid anhydrides, conversely, are formed through intermolecular dehydration reactions involving two or more molecules of the acid. Only oxoacids possess corresponding anhydrides; oxygen-free acids do not.
Acid anhydrides can generally be viewed as oxides derived from acids through dehydration (though organic acid anhydrides are not classified as oxides). Many anhydrides are capable of reacting with water to regenerate the original acid. Based on the nature of the parent acid, anhydrides can be categorized as follows: Inorganic acid anhydrides are formed via the dehydration and condensation of one or two acid molecules-for example, carbonic anhydride (carbon dioxide, CO₂) and nitric anhydride (dinitrogen pentoxide, N₂O₅). Organic acid anhydrides are compounds formed through the dehydration and condensation of two molecules of a monoacid or one molecule of a diacid; although they are not oxides, they are nonetheless referred to as acid anhydrides.
