anhydride [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.

acid anhydride, chemical compound that reacts with water to form an acid (see acids and bases). Anhydrides of inorganic acids are usually oxides of nonmetallic elements. Carbon dioxide, CO₂, is the anhydride of carbonic acid, H₂CO₃ . Nitrogen pentoxide, N₂O₅, is the anhydride of nitric acid, HNO₃ . Phosphorus pentoxide, P₂O₅, is the anhydride of phosphoric acid, H₃PO₄ . Sulfur dioxide, SO₂, is the anhydride of sulfurous acid, H₂SO₃ . Sulfur trioxide, SO₃, is the anhydride of sulfuric acid, H₂SO₄ . Anhydrides of organic acids, like the acids themselves, contain the carbonyl group, CO. Organic anhydrides include acetic anhydride or ethanoic anhydride, (CH₃C--O)₂O, and benzoic anhydride, (C₆H₅C--O)₂O.

Any chemical compound obtained, either in practice or in principle, by eliminating water (H₂O) from another compound. Examples of inorganic anhydrides are sulfur trioxide, SO₃, which is derived from sulfuric acid, H₂SO₄, and calcium oxide, CaO, which is derived from calcium hydroxide Ca(OH)₂. The most important organic anhydride is acetic anhydride, (CH₃CO)₂O, a raw material for making cellulose acetate (used for films, fibres, and plastic goods) and aspirin. It can be thought of as acetic acid minus water. Organic anhydrides are very important starting materials for organic synthesis, as they can give rise to carboxylic acids, esters, or amides under the proper conditions.

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