- Acid anhydride
An acid anhydride is an
organic compoundthat has two acyl groups bound to the same oxygenatom.GoldBookRef|title=acid anhydrides|url=http://goldbook.iupac.org/A00072.html] 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".BlueBookRef|rec=R-5.7.7|pages=123–25] 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 anhydrideis a major industrial chemical widely used for preparing acetate esters. The cyclic acid anhydride maleic anhydrideis used in organic synthesisas dienophilein the Diels-Alder reaction. One or both acyl groups of an acid anhydride may also be derived from a sulfonic acidor a phosphonic acid. The mixed anhydride 1,3-bisphosphoglycerateis an intermediate in the formation of ATP via glycolysis. [Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISBN 1-57259-153-6.]
Acid anhydrides are prepared in industry by diverse means. Acetic anhydride is mainly produced by the
carbonylationof methyl acetate. [Zoeller, J. R.; Agreda, V. H.; Cook, S. L.; Lafferty, N. L.; Polichnowski, S. W.; Pond, D. M. "Eastman Chemical Company Acetic Anhydride Process" Catalysis Today (1992), volume 13, pp.73-91. doi:10.1016/0920-5861(92)80188-S] Maleic anhydrideis 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 pentoxideis a common dehydrating agent::2 CH3COOH + P4O10 → CH3C(O)OC(O)CH3 + "(HO)2P4O9"Acid chlorides are also effective precursors: [OrgSynth | author = Lewis I. Krimen | title = Acetic Formic Anhydride | collvol = 6 | collvolpages = 8| year = 1988 | prep = CV6P0008] :CH3C(O)Cl + HCO2Na → HCO2COCH3 + NaCl
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 + RCO2Hfor HY = H2O, HOR (alcohols), HNR'2 (ammonia, primary, secondary amines), aromatic ring (see Friedel-Crafts acylation).
Acid anhydrides tend to be less
electrophilicthan 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 anhydridemakes it a common choice for acetylationreactions. Furthermore, the avoidance of chlorocarbons is preferred from the environmental perspective.
Sulfurcan 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).
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