anhydrid

An anhydride is a chemical compound formed by the removal of a molecule of water from another compound, a dehydration process. In common usage the term often refers to acid anhydrides, which form when water is removed from two carboxylic acids or from an oxoacid, so that hydration returns the acid. Some classifications distinguish organic anhydrides (derived from carboxylic acids) from inorganic or oxide anhydrides (derived from oxoacids).

Organic acid anhydrides are formed by dehydrating two carboxylic acid molecules, or by removing water from a carboxylic acid derivative. They have the structure R-CO-O-CO-R' and are reactive toward water, alcohols, and amines. Common examples include acetic anhydride (from acetic acid) and phthalic anhydride (from o-phthalic acid); maleic anhydride is another widely used example, especially in polymerization.

Inorganic acid anhydrides are oxides that form acids upon hydration. Notable examples are sulfur trioxide SO3, the anhydride of sulfuric acid; phosphorus pentoxide P4O10, the anhydride of phosphoric acid; and dinitrogen pentoxide N2O5, the anhydride of nitric acid. These species are typically highly reactive with moisture and are used in industrial processes such as fertilizer production, esterification catalysts, and various synthesis steps.

Properties and handling: Anhydrides are usually reactive with water and air and are often hygroscopic or corrosive. They are typically dried or stored under inert atmospheres. Reactions of anhydrides with nucleophiles involve transfer of an acyl group, enabling acetylation, esterification, and amide formation in organic synthesis.

Notes: They differ from hydrates and salts of acids; the term reflects their status as the waterless forms of acids.