anhydride

An anhydride is a chemical compound formed by the removal of water from one or more acid molecules. The term is used in two related senses: organic chemistry, for compounds derived from carboxylic acids by dehydration, and inorganic chemistry, for oxide derivatives that are the anhydrides of mineral or polyprotic acids.

In organic chemistry, acid anhydrides are typically formed by the condensation of two carboxyl groups, yielding a general structure of R-CO-O-CO-R' (symmetric cases give (RCO)2O). They are generally reactive toward water, hydrolyzing to the corresponding carboxylic acids, and they undergo nucleophilic acyl substitution with alcohols and amines to give esters and amides. Common examples include acetic anhydride (CH3CO)2O, phthalic anhydride C6H4(CO)2O, maleic anhydride, and succinic anhydride. Uses include acetylation of alcohols and amines and serving as precursors in polymer and resin synthesis, notably phthalic anhydride in polyesters.

Inorganic anhydrides are binary compounds formed by elements with oxygen that correspond to the parent acids after water removal. Examples include sulfur trioxide (SO3), the anhydride of sulfuric acid; phosphorus pentoxide (P4O10), the anhydride of phosphoric acid; carbon dioxide (CO2), the anhydride of carbonic acid; and dinitrogen pentoxide (N2O5), the anhydride of nitric acid. These species often hydrolyze readily in water, forming the respective acids, and many are highly reactive or corrosive. Anhydrides are important both as intermediates in synthesis and as reagents that enable selective introduction of acyl or oxide functions.