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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For example:sodium oxide is an anhydride of sodium hydroxide, and sulfur trioxide is an anhydride of sulfuric acid.
In organic chemistry, the compounds most commonly involved are carboxylic acids.
Compounds involved are often acids or bases; in such cases, the anhydrides can be called acid anhydrides or base anhydrides. This does not imply that the anhydrides are not themselves acids or bases; they are. But the more hydrated forms may be more familiar or convenient.
Anhydride is sometimes used as a synonym for acid anhydride.
Following are two reactions, one with a typical acid and base, one with their anhydrides. The first one is more practical.
In general, anhydrides are more reactive than their corresponding acids, as they can react with water to form their corresponding acid. They often are good dehydrating agents. Acetic anhydride is useful in the acetylation of salicylic acid, as using acetic acid to perform the reaction leaves water behind, which can destroy the product, acetylsalicylic acid, or aspirin.
In biology, most of the high-energy phosphate compounds are formed from the condensation of the phosphate ion with a phosphorylated sugar. The resulting pyrophosphate bond is a classic anhydride bond.