In organic chemistry, the compounds most commonly involved are carboxylic acids.
Compounds involved are often acids or bases; in such cases, the anhydrides can be called acid anhydrides or base anhydrides. This does not imply that the anhydrides are not themselves acids or bases; they are. But the more hydrated forms may be more familiar or convenient.
Anhydride is sometimes used as a synonym for acid anhydride.
Following are two reactions, one with a typical acid and base, one with their anhydrides. The first one is more practical.
- 2 NaOH (aq) + H2SO4 (aq) → Na2SO4 + 2H2O (l)
- Na2O (s) + SO3 (g) → Na2SO4 (aq)
In general, anhydrides are more reactive than their corresponding acids, as they can react with water to form their corresponding acid. They often are good dehydrating agents. Acetic anhydride is useful in the acetylation of salicylic acid, as using acetic acid to perform the reaction leaves water behind, which can destroy the product, acetylsalicylic acid, or aspirin.
In biology, most of the high-energy phosphate compounds are formed from the condensation of the phosphate ion with a phosphorylated sugar. The resulting pyrophosphate bond is a classic anhydride bond.