In organic chemistry, the term anhydride (from the Greek άνυδρος, ánhydros, without water) denotes an organic compound containing the functional group -CO-O-CO-

In the traditional nomenclature of inorganic chemistry, the term anhydride indicates a compound of oxygen with a non-metallic element which, when combined with water, forms oxygenated acids (oxyacids): thus, for example, sulfuric anhydride SO3 and nitric anhydride N2O5, combined with water, give sulfuric acid and nitric acid, respectively. In the same nomenclature, compounds of metallic elements with oxygen were called oxides: for example, calcium oxide CaO, sodium oxide Na2O, which combined with water give hydrates.

However, since the oxides of many elements have a chemical behavior intermediate between that typical of anhydrides and metal oxides, the most recent nomenclature has abandoned the term of anhydride adopting the term of oxides for compounds of oxygen with non-metallic elements: so, for example, the old name of sulfuric anhydride of the compound SO3 is replaced by that of sulfur trioxide. The name of anhydride is retained in the nomenclature of organic chemistry to indicate compounds which result from the elimination of a molecule of water between the acid groups of two molecules of a monocarboxylic acid or between the two acid groups of the same molecule of a bicarboxylic acid. In this way, for example, acetic acid produces acetic anhydride and succinic acid produces succinic anhydride.

These schemes represent not only a formal derivation, but also the method actually used for the preparation of anhydrides of organic acids: the removal of water from carboxylic acid is brought about by energetic dehydrating agents or even by the action of heat alone. However, dehydration is reversible and the anhydrides react with water at room temperature, transforming back into the carboxylic acids from which they are derived.

Anhydrides are generally more reactive than their corresponding acids, as they can react with water to form them spontaneously. They are often excellent dehydrators.

Acetic anhydride is useful in the acetylation of salicylic acid, as opposed to acetic acid, which, reacting, produces water, able to deteriorate the product, acetylsalicylic acid. Acetylation or generally acylation of hydroxyl functional groups is a common method for the protection of the groups themselves during organic syntheses.

In biology, most high-energy phosphate group bonds are made by condensation of the phosphate anion with phosphorylated sugar. The resulting pyrophosphate bond is a classic phosphorus oxide bond, although in reality the latter does not intervene in the process which is instead conducted by ATP.

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