|Jmol-3D images||Image 1
|Molar mass||102.09 g mol−1|
|Density||1.082 g cm−3, liquid|
−73.1 °C, 200 K, -100 °F
139.8 °C, 413 K, 284 °F
|Solubility in water||2.6 g/100 mL, see text|
|Refractive index (nD)||1.3901|
|EU classification||Corrosive (C)|
|R-phrases||R10, R20/22, R34|
|S-phrases||(S1/2), S26, S36/37/39, S45|
|Flash point||49 °C|
|Related acid anhydrides||Propionic anhydride|
|Related compounds||Acetic acid
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. 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.
Structure and properties
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.
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).
- CH3CO2CH3 + CO → (CH3CO)2O
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.
- H2C=C=O + CH3COOH → (CH3CO)2O (ΔH = −63 kJ/mol)
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.
- CH3COOH H2C=C=O + H2O (ΔH = +147 kJ/mol)
- CH3COCH3 → H2C=C=O + CH4
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. In these conversions, acetic anhydride is viewed as a source of CH3CO+. Alcohols and amines are readily acetylated. For example, the reaction of acetic anhydride with ethanol yields ethyl acetate:
- (CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3COOH
- (CH3CO)2O + C6H6 → CH3COC6H5 + CH3CO2H
- Cp2Fe + (CH3CO)2O → CpFe(C5H4COCH3)
Acetic anhydride dissolves in water to approximately 2.6% by weight. Aqueous solutions have limited stability because, like most acid anhydrides, acetic anhydride hydrolyses to give carboxylic acids. In this case, acetic acid is formed:
- (CH3CO)2O + H2O → 2 CH3CO2H
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. 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)
Acetic anhydride is an irritant and flammable. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression. The vapour of acetic anhydride is harmful.
- Acetic anhydride was first synthesized in 1852 by French chemist Charles Frédéric Gerhardt (1816-1856). See: Charles Gerhardt (1852) “Recherches sur les acides organiques anhydres,” Comptes Rendus, vol. 34, pages 755-758.
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