Anhydride

In chemistry, an anhydride is typically an oxide of a nonmetallic element or an organic radical, capable of forming an acid by uniting with the elements of water. The anhydride is so called because it may be formed from an acid by the removal of water.

Examples of inorganic anhydrides include dinitrogen pentoxide, which is the anhydride of nitric acid, and sulfur trioxide, which is the anhydride of sulfuric acid. Useful organic anhydrides include acetic anhydride (an acid anhydride), formed by the condensation of acetic acid:

2 CH₃COOH → (CH₃CO)₂O + H₂O

Anhydrides are typically 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 do the reaction leaves water behind that 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.