Definitions

acid anhydride

acid anhydride

acid anhydride, chemical compound that reacts with water to form an acid (see acids and bases). Anhydrides of inorganic acids are usually oxides of nonmetallic elements. Carbon dioxide, CO2, is the anhydride of carbonic acid, H2CO3 . Nitrogen pentoxide, N2O5, is the anhydride of nitric acid, HNO3 . Phosphorus pentoxide, P2O5, is the anhydride of phosphoric acid, H3PO4 . Sulfur dioxide, SO2, is the anhydride of sulfurous acid, H2SO3 . Sulfur trioxide, SO3, is the anhydride of sulfuric acid, H2SO4 . Anhydrides of organic acids, like the acids themselves, contain the carbonyl group, CO. Organic anhydrides include acetic anhydride or ethanoic anhydride, (CH3C--O)2O, and benzoic anhydride, (C6H5C--O)2O.

An acid anhydride is an organic compound that has two acyl groups bound to the same oxygen atom. Most commonly, the acyl groups are derived from the same carboxylic acid, the formula of the anhydride being (RC(O))2O. 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, (CH3CO)2O is called acetic anhydride. Mixed (or unsymmetrical) acid anhydrides, such as acetic formic anhydride (see below), are known.

Important acid anhydrides

Acetic anhydride is a major industrial chemical widely used for preparing acetate esters. The cyclic acid anhydride maleic anhydride is used in organic synthesis as dienophile in the Diels-Alder reaction. 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 prepared in industry by diverse means. Acetic anhydride is mainly produced by the carbonylation of methyl acetate. Maleic anhydride is produced by the oxidation of butane. For pedogical and practical considerations, laboratory routes emphasize the dehydration of the corresponding acids. The conditions vary from acid to acid, but phosphorus pentoxide is a common dehydrating agent:
2 CH3COOH + P4O10CH3C(O)OC(O)CH3 + "(HO)2P4O9"
Acid chlorides are also effective precursors:
CH3C(O)Cl + HCO2Na → HCO2COCH3 + NaCl

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))2S).

References

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