acid anhydride

(chemistry) An acid with one or more molecules of water removed; for example, SO₃ is the acid anhydride of H₂SO₄, sulfuric acid.

One of an important class of reactive organic compounds derived from acids via formal intermolecular dehydration.

Anhydrides of straight-chain acids containing from 2 to 12 carbon atoms are liquids with boiling points higher than those of the parent acids. They are relatively insoluble in cold water and are soluble in alcohol, ether, and other common organic solvents. The lower members are pungent, corrosive, and weakly lacrimatory. Anhydrides from acids with more than 12 carbon atoms and cyclic anhydrides from dicarboxylic acids are crystalline solids.

Because the direct intermolecular removal of water from organic acids is not practicable, anhydrides must be prepared by means of indirect processes. A general method involves interaction of an acid salt with an acid chloride.

Acetic anhydride, the most important aliphatic anhydride, is manufactured by air oxidation of acetaldehyde, using as catalysts the acetates of copper and cobalt, shown in the reaction below.

Cyclic anhydrides are obtained by warming succinic or glutaric acids, either alone, with acetic anhydride, or with acetyl chloride. Under these conditions, adipic acid first forms linear, polymeric anhydride mixtures, from which the monomer is obtained by slow, high-vacuum distillation. Cyclic anhydrides are also formed by simple heat treatment of cis-unsaturated dicarboxylic acids, for example, maleic and glutaconic acids; and of aromatic 1,2-dicarboxylic acids, for example, phthalic acid. Commercially, however, both phthalic (structure 1) and maleic (2)

Anhydrides are used in the preparation of esters. Ethyl acetate and butyl acetate (from butyl alcohol and acetic anhydride) are excellent solvents for cellulose nitrate lacquers. Acetates of high-molecular-weight alcohols are used as plasticizers for plastics and resins. Cellulose and acetic anhydride give cellulose acetate, used in acetate rayon and photographic film. The reaction of anhydrides with sodium peroxide forms peroxides (acetyl peroxide is violently explosive), used as catalysts for polymerization reactions and for addition of alkyl halides to alkenes. In Friedel-Crafts reactions, anhydrides react with aromatic compounds, forming ketones such as acetophenone.

Maleic anhydride reacts with many dienes to give hydroaromatics of various complexities (Diels-Alder reaction). Maleic anhydride is used commercially in the manufacture of alkyd resins from polyhydric alcohols. Soil conditioners are produced by basic hydrolysis of the copolymer of maleic anhydride with vinyl acetate.

Phthalic anhydride and alcohols form esters (phthalates) used as plasticizers for plastics and resins. Condensed with phenols and sulfuric acid, phthalic anhydride yields phthaleins, such as phenolphthalein; with m-dihydroxybenzenes under the same conditions, xanthene dyes form, for example, fluorescein. Phthalic anhydride is used in manufacturing glyptal resins (from the anhydride and glycerol) and in manufacturing anthraquinone. Heating phthalic anhydride with ammonia gives phthalimide, used in Gabriel's synthesis of primary amines, amino acids, and anthranilic acid (o-aminobenzoic acid). With alkaline hydrogen peroxide, phthalic anhydride yields monoperoxyphthalic acid, used along with benzoyl peroxide as polymerization catalysts, and as bleaching agents for oils, fats, and other edibles.

Anhydrides react with water to form the parent acid, with alcohols to give esters, and with ammonia to yield amides; and with primary or secondary amines, they furnish N-substituted and N,N-disubstituted amides, respectively. See also Acid halide; Acylation; Carboxylic acid; Diels-Alder reaction; Ester; Friedel-Crafts reaction.

The noun has one meaning:

Meaning #1: organic compounds that react with water to form an acid
  Synonym: acyl anhydrides

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

General structure of an acid anhydride derived from carboxylic acids

An acid anhydride is an organic compound which 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."

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.

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