anidride

Anhydride, in chemistry often rendered as anhydride or anidride in some languages, is a compound that forms when water is removed from another substance, typically an acid. Anhydrides react with water to re-form the corresponding acid (or base, in the case of basic anhydrides). The term covers both organic and inorganic species.

Organic anhydrides, also called acyl anhydrides, are typically produced by the condensation of two carboxylic acid molecules with loss of water. Common examples include acetic anhydride (two acetate groups linked by an anhydride oxygen), phthalic anhydride, and maleic anhydride. These compounds are important reagents in organic synthesis, used for acetylation, ring opening, and as intermediates in polymer and polymerizable monomer production. They are usually reactive toward nucleophiles and can be hydrolyzed by water to yield the parent carboxylic acids.

Inorganic anhydrides include acid anhydrides, formed by nonmetal oxides that, on contact with water, yield acids. Examples are carbon dioxide (CO2) forming carbonic acid H2CO3, sulfur trioxide (SO3) forming sulfuric acid H2SO4, chlorine heptoxide (Cl2O7) forming perchloric acid HClO4, and phosphorus pentoxide (P2O5 or P4O10) forming phosphoric acid H3PO4. There are also basic anhydrides, typically metal oxides such as calcium oxide (CaO) and sodium oxide (Na2O), which form hydroxides when dissolved in water (Ca(OH)2, NaOH).

Applications of anhydrides span drying and dehydration roles, acetylation and functionalization in organic synthesis, and the production of acids and resins in inorganic and polymer chemistry. They are typically handled under controlled moisture-free conditions due to their propensity to hydrolyze.