An acid anhydride or, simply, anhydride is the product of formal condensation of two oxoacid molecules with the release of a water molecule. The most common anhydrides in organic chemistry are those derived from carboxylic acids. (In organic chemistry, the term “anhydride” is customarily used to mean “carboxylic acid anhydride”.) Carboxylic acid anhydrides have the general structural formula 1, in which R1 and R2 could be hydrogen atoms, alkyl groups, aryl groups, or any combination thereof.
The O=C—O—C=O group in a carboxylic acid anhydride is called the anhydride group.