anhydride ănhī´drīd, –drĭd [key] [Gr.,=without water], chemical compound formed by removing water, H 2O, 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, SO 3, reacts with water to form sulfuric acid, H 2SO 4. 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, (CH 3CO) 2O, reacts with ethanol, C 2H 5OH, to form ethyl acetate, CH 3COOC 2H 5, 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.
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