An acid anhydride is any member of a group of organic chemicals with molecules that include an oxygen atom simultaneously bound to two groups of atoms called acyl groups. A number of different types of acid anhydride exist, with properties that vary according to their different acyl groups. Many are produced and used in modern industry.
Organic molecules contain groups of atoms called functional groups, which determine the molecule's chemical properties. Acyl groups are functional groups containing a carbon atom, an oxygen atom, and an alkyl group, which refers to a set of functional groups composed entirely of carbon and hydrogen atoms joined by single bonds. Both acyl groups are in turn single-bonded to an oxygen atom. There are many possible acyl groups that can join with oxygen in this way, and thus many different acid anhydrides with varying chemical properties. Most types of acid anhydride are symmetrical, with an oxygen atom bonded to identical functional groups, but exceptions to this exist.
Acid anhydrides vary in their properties. Some are solid at room temperature, usually in the form of pellets or flakes, while others are liquids. They are generally clear if liquid and white if solid and often produce acrid, pungent odors when exposed to the air. They can cause irritation or burns if they come into contact with the eyes or skin, irritation or damage to the lungs if their fumes are inhaled, and burn to the gastrointestinal tract if ingested. Excessive exposure without proper medical attention can cause serious injury or death.
Most commonly, acid anhydrides are derived from chemical reactions involving carboxylic acids, a group of organic acids. The name of the resulting acid anhydride is likewise commonly derived from the name of the original acid. For example, the condensation reaction involving two molecules of a type of carboxylic acid called acetic acid (CH3COOH) results in a single molecule of water (H2O) and a molecule of acetic anhydride ((CH3CO)2O). Some forms of acid anhydride are formed from other types of organic acid, such as the phosphonic and sulfonic acids, or from inorganic acids such as phosphoric acid (H3PO4).
Several acid anhydrides are used in modern industry. Acetic anhydride is commonly used as a reagent for reactions involving certain organic compounds and is involved in the production of aspirin and cellulose acetate, which is used as a synthetic fiber and as a substrate, or base, for the photosensitive chemicals in photographic film. Another example is maleic anhydride (C2H2(CO)2O), which is used in the production of the polyester resins in molding compounds and toner for laser printers and photocopiers.