Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a colorless liquid that smells strongly of acetic acid, which is formed by its reaction with moisture in the air.
|Preferred IUPAC name
|Systematic IUPAC name
Acetic acid anhydride
3D model (JSmol)
|Molar mass||102.09 g·mol−1|
|Density||1.082 g cm−3, liquid|
|Melting point||−73.1 °C (−99.6 °F; 200.1 K)|
|Boiling point||139.8 °C (283.6 °F; 412.9 K)|
|2.6 g/100 mL, see text|
|Vapor pressure||4 mmHg (20 °C)|
Refractive index (nD)
|Safety data sheet||ICSC 0209|
|R-phrases (outdated)||R10, R20/22, R34|
|S-phrases (outdated)||(S1/2), S26, S36/37/39, S45|
|Flash point||49 °C (120 °F; 322 K)|
|316 °C (601 °F; 589 K)|
|Lethal dose or concentration (LD, LC):|
LC50 (median concentration)
|1000 ppm (rat, 4 hr)|
|US health exposure limits (NIOSH):|
|TWA 5 ppm (20 mg/m3)|
|C 5 ppm (20 mg/m3)|
IDLH (Immediate danger)
Related acid anhydrides
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Structure and propertiesEdit
Acetic anhydride, like most acid anhydrides, is a flexible molecule with a nonplanar structure. The pi system linkage through the central oxygen offers very weak resonance stabilization compared to the dipole-dipole repulsion between the two carbonyl oxygens. 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 has electrophilic character, as the leaving group is carboxylate. 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
The Tennessee Eastman acetic anhydride 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 chloride in the presence of lithium iodide is employed as catalysts. Because acetic anhydride is not stable in water, the conversion is conducted under anhydrous conditions.
- H2C=C=O + CH3COOH → (CH3CO)2O (ΔH = −63 kJ/mol)
Due to its low cost, acetic anhydride is purchased, not prepared, for use in research laboratories.
Acetylation of alcohols and aminesEdit
- (CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3COOH
Acetylation of aromatic ringsEdit
Aromatic rings are acetylated by acetic anhydride. Usually acid catalysts are used to accelerate the reaction. Illustrative are the conversions of benzene to acetophenone and ferrocene to acetylferrocene:
- (C5H5)2Fe + (CH3CO)2O → (C5H5)Fe(C5H4COCH3) + CH3CO2H
Preparation of other acid anhydridesEdit
Precursor to geminal diacetatesEdit
Aldehydes react with acetic anhydride in the presence of an acidic catalyst to give geminal diacetates. A former industrial route to vinyl acetate involved the intermediate ethylidene diacetate, the geminal diacetate obtained from acetaldehyde and acetic anhydride:
- CH3CHO + (CH3CO)2O → (CH3CO2)2CHCH3
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, and is used in the manufacture of cigarette filters. 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 anhydride is a common acetylation compound, used for the production of modified starches (E1414, E1420, E1422)
Acetic anhydride is an irritant and combustible liquid. Because of its reactivity toward water, alcohol foam or carbon dioxide are preferred for fire suppression. The vapour of acetic anhydride is harmful.
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