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 tetroxide, which is the anhydride of nitric acid, and sulfur trioxide, which is the anhydride of sulfuric acid. Useful organic anhydrides include acetic anhydride, formed by the condensation of acetic acid:
- 2 CH3COOH → (CH3CO)2O + H2O
In biology, most of the high energy phosphate compounds are formed from the condensation of the phosphate ion with a phosporylated sugar. The resulting pyrophosphate bond is a classic anhydride bond.