anhydride

Anhydride, any chemical compound obtained, either in practice or in principle, by the elimination of water from another compound. Examples of inorganic anhydrides are sulfur trioxide, SO₃, which is derived from sulfuric acid, and calcium oxide, CaO, derived from calcium hydroxide. Sulfur trioxide and other oxides formed by the removal of water from an acid are often called acid anhydrides, whereas those such as calcium oxide that are produced by a base upon the loss of water are designated basic anhydrides.

The most important of the organic anhydrides is acetic anhydride, (CH₃CO)₂O. It is prepared industrially in either of two ways: by atmospheric oxidation of acetaldehyde in the presence of a metal acetate; and from acetic acid by reaction with acetylene or ketene. Other organic anhydrides can be prepared from carboxylic acids by reaction with acetic anhydride, ketene, methoxyacetylene, or isopropenyl acetate. Anhydrides also are produced when acyl halides react with acetic anhydride or with carboxylic acid and pyridine.