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.^[1] 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.^[2] Thus, CH₃C(O)OC(O)CH₃ is called acetic anhydride.

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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.^[3]

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,^[2] 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,^[2] e.g., acetic thioanhydride, CH₃C(O)SC(O)CH₃.

References

1. ^ International Union of Pure and Applied Chemistry. "acid anhydrides". Compendium of Chemical Terminology Internet edition.
2. ^ ^{a b c} International Union of Pure and Applied Chemistry (1993). "Recommendation R-5.7.7." A Guide to IUPAC Nomenclature of Organic Compounds. Oxford: Blackwell Science. ISBN 0-632-03488-2. pp. 123–25.
3. ^ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.