anhydride

SO₃+H₂O → H₂SO₄

anhydride (ănhī'drīd, -drĭd) [Gr.,=without water], chemical compound formed by removing water, H₂O, from another compound; the anhydride can also react with water to form the original compound. An acid anhydride reacts with water to form an acid; e.g., sulfur trioxide, SO₃, reacts with water to form sulfuric acid, H₂SO₄. A basic anhydride reacts with water to form a base; e.g., calcium oxide, CaO, reacts with water to form calcium hydroxide, Ca(OH)₂. Anhydrides of organic acids have many uses. They react with alcohols to form esters; e.g., acetic anhydride, (CH₃CO)₂O, reacts with ethanol, C₂H₅OH, to form ethyl acetate, CH₃COOC₂H₅, a useful solvent. They also react with ammonia and primary or secondary amines to form amides. Other important acid anhydrides include maleic anhydride and phthalic anhydride.