| Acetic anhydride | |
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acetic anhydride |
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ethanoic anhydride |
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Other names
Ethanoyl ethanoate |
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| Identifiers | |
| CAS number | 108-24-7 |
| PubChem | 7918 |
| ChemSpider | 7630 |
| UNII | 2E48G1QI9Q |
| EC number | 203-564-8 |
| ChEBI | CHEBI:36610 |
| RTECS number | AK1925000 |
| Jmol-3D images | Image 1 Image 2 |
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| Properties | |
| Molecular formula | C4H6O3 |
| Molar mass | 102.09 g mol−1 |
| Appearance | clear liquid |
| Density | 1.082 g cm−3, liquid |
| Melting point |
−73.1 °C, 200 K, -100 °F |
| Boiling point |
139.8 °C, 413 K, 284 °F |
| Solubility in water | 2.6 g/100 mL, see text |
| Refractive index (nD) | 1.3901 |
| Hazards | |
| MSDS | ICSC 0209 |
| EU Index | 607-008-00-9 |
| EU classification | Corrosive (C) |
| R-phrases | R10, R20/22, R34 |
| S-phrases | (S1/2), S26, S36/37/39, S45 |
| NFPA 704 |
2
2
1
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| Flash point | 49 °C |
| Autoignition temperature |
316 °C |
| Explosive limits | 2.7–10.3% |
| Related compounds | |
| Related acid anhydrides | Propionic anhydride |
| Related compounds | Acetic acid Acetyl chloride |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O.[1] Commonly abbreviated Ac2O, it is the simplest isolatable acid anhydride and is a widely used reagent in organic synthesis. It is a colorless liquid that smells strongly of acetic acid, formed by its reaction with the moisture in the air.
Formic anhydride is an even simpler acid anhydride, but it spontaneously decomposes, especially once removed from solution.
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Contrary to what its Lewis structure seems to predict, acetic anhydride, like many other acid anhydrides that are free to rotate, has experimentally been found to be aplanar. The pi system linkage through the central oxygen offers very weak resonance stabilization compared to the dipole-dipole repulsion between the two carbonyl oxygens. However, the energy barriers to bond rotation between each of the optimal aplanar conformations are quite low.[2]
Like most acid anhydrides, the carbonyl carbon of acetic anhydride is a potent electrophile as the leaving group for each carbonyl carbon (a carboxylate) is a good electron-withdrawing leaving group. The internal asymmetry may contribute to acetic anhydride's potent electrophilicity as the asymmetric geometry makes one side of a carbonyl carbon more reactive than the other, and in doing so tends to consolidate the electropositivity of a carbonyl carbon to one side (see electron density diagram).
Acetic anhydride is produced by carbonylation of methyl acetate:[3]
This process involves the conversion of methyl acetate to methyl iodide and an acetate salt. Carbonylation of the methyl iodide in turn affords acetyl iodide, which reacts with acetate salts or acetic acid to give the product. Rhodium iodide and lithium iodide are employed as catalysts. Because acetic anhydride is not stable in water, the conversion is conducted under anhydrous conditions. In contrast, the Monsanto acetic acid process, which also involves a rhodium catalyzed carbonylation of methyl iodide, is at least partially aqueous.
To a decreasing extent, acetic anhydride is also prepared by the reaction of ethenone (ketene) with acetic acid at 45–55 °C and low pressure (0.05–0.2 bar).[4]
Ketene is generated by dehydrating acetic acid at 700–750 °C in the presence of triethyl phosphate as a catalyst or (in Switzerland and the CIS) by the thermolysis of acetone at 600–700 °C in the presence of carbon disulfide as a catalyst.[4]
The route from acetic acid to acetic anhydride via ketene was developed by Wacker Chemie in 1922,[5] when the demand for acetic anhydride increased due to the production of cellulose acetate.
Due to its low cost, acetic anhydride is purchased, not prepared, for use in research laboratories.
Acetic anhydride is a versatile reagent for acetylations, the introduction of acetyl groups to organic substrates.[6] In these conversions, acetic anhydride is viewed as a source of CH3CO+. Alcohols and amines are readily acetylated.[7] For example, the reaction of acetic anhydride with ethanol yields ethyl acetate:
Often a base such as pyridine is added to function as catalyst. In specialized applications, Lewis acidic scandium salts have also proven effective catalysts.[8]
Aromatic rings are acetylated, usually in the presence of an acid catalyst. Illustrative is the conversion of benzene to acetophenone:
Ferrocene can be acetylated as well:[9]
Acetic anhydride dissolves in water to approximately 2.6% by weight.[10] Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give acetic acid:[11]
As indicated by its organic chemistry, Ac2O is mainly used for acetylations leading to commercially significant materials. Its largest application is for the conversion of cellulose to cellulose acetate, which is a component of photographic film and other coated materials. Similarly it is used in the production of aspirin (acetylsalicylic acid), which is prepared by the acetylation of salicylic acid.[12] It is also used as a wood preservative via autoclave impregnation to make a longer lasting timber.
In starch industry, acetic anydride is a common acetylation compound, used for the production of modified starches (E1414, E1420, E1422)
Because of its use for the synthesis of heroin by the diacetylation of morphine, acetic anhydride is listed as a U.S. DEA List II precursor, and restricted in many other countries.[13]
Acetic anhydride is an irritant and flammable. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression.[14] The vapour of acetic anhydride is harmful.[15]
When mixed with hydrogen peroxide, an excess of acetic anhydride reacts with one of the reaction products peracetic acid and forms highly shock sensitive and explosive diacetyl peroxide.