Acid anhydrides are organic chemicals with an oxygen atom bonded to two acyl groups, with varying properties depending on the acyl groups. They can cause harm if ingested, inhaled, or come into contact with skin. They are commonly produced from carboxylic acids and used in industry, such as acetic anhydride in the production of aspirin and cellulose acetate. Maleic anhydride is used in polyester resin production and toners for printers.
An acid anhydride is any member of a group of organic chemicals with molecules that include an oxygen atom bonded simultaneously to two groups of atoms called acyl groups. There are several types of acid anhydride, with properties varying 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 chemical properties of the molecule. Acyl groups are functional groups containing one carbon atom, one oxygen atom and one 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 bonded to an oxygen atom. There are many possible acyl groups that can join with oxygen in this way, and therefore many different acid anhydrides with varying chemical properties. Most types of acid anhydride are symmetric, with an oxygen atom bonded to identical functional groups, but there are exceptions to this.
Acidic anhydrides vary in their properties. Some are solid at room temperature, usually in the form of pellets or flakes, while others are liquid. They are generally clear if liquid and white if solid and often produce acrid and pungent odors when exposed to air. They can cause irritation or burns if they come in 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 result from chemical reactions involving carboxylic acids, a group of organic acids. The name of the resulting acid anhydride is also 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 one molecule of acetic anhydride ((CH3CO)2O). Some forms of acid anhydride are formed from other types of organic acid, such as phosphonic and sulfonic acids, or from inorganic acids such as phosphoric acid (H3PO4).
Various 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 polyester resins in molding compounds and toners for laser printers and copiers.
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