Acid anhydride


 * This article is about an organic functional group: for the anhydrides of inorganic compounds, see acidic oxide.

An acid anhydride is an organic compound that has two acyl groups bound to the same oxygen atom. When the two acyl groups are directly derived from a carboxylic acid (the most common case), the general formula is RC(O)OC(O)R. Symmetrical acid anhydrides of this type are named by replacing the word acid in the name of the parent carboxylic acid by the word anhydride. Thus, CH3C(O)OC(O)CH3 is called acetic anhydride.

Important acid anhydrides
Acetic anhydride is a major industrial chemical, prepared either by the partial hydration of ketene or by the carbonylation of methyl acetate. The cyclic acid anhydride maleic anhydride is used in organic synthesis as dienophiles in the Diels-Alder reaction. Mixed (or unsymmetrical) acid anhydrides, such as acetic propionic anhydride, CH3C(O)OC(O)CH2CH3, are known. One or both acyl groups of an acid anhydride may also be derived from a sulfonic acid or a phosphonic acid. The mixed anhydride 1,3-Bisphosphoglycerate is an intermediate in the formation of ATP via glycolysis.

Preparation
Acid anhydrides are usually prepared by the dehydration of the corresponding acid(s). The conditions vary from acid to acid, but phosphorus pentoxide is a commonly-used laboratory dehydrating agent.
 * 2 CH3COOH →  CH3C(O)OC(O)CH3  +  H2O

Maleic anhydride is prepared by aerobic oxidation of butane.

Reactions
Acid anhydrides are a source of reactive acyl groups, and their reactions and uses resemble those of acyl halides. In reactions with protic substrates, the reactions afford equal amounts of the acylated product and the carboxylic acid:
 * RC(O)OC(O)R +  HY  →  RC(O)Y  +  RCO2H

for HY = H2O, HOR (alcohols), HNR'2 (ammonia, primary, secondary amines), aromatic ring (see Friedel-Crafts acylation)

Acid anhydrides tend to be less electrophilic than acyl chlorides, and only one acyl group is transferred per molecule of acid anhydride, which leads to a lower atom efficiency. The low cost, however, of acetic anhydride makes it a common choice for acetylation reactions. Furthermore, the avoidance of chlorocarbons is preferred from the environmental perspective.

Sulfur analogues
Sulfur can replace oxygen, either in the carbonyl group or in the bridge. In the former case, the name of the acyl group is enclosed in parentheses to avoid ambiguity in the name, e.g., (thioacetic) anhydride, CH3C(S)OC(S)CH3. When two acyl groups are attached to the same sulfur atom, the resulting compound is called a thioanhydride, e.g., acetic thioanhydride, CH3C(O)SC(O)CH3.