|Jmol-3D images||Image 1
|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)|
|Solubility in water||2.6 g/100 mL, see text|
|Refractive index (nD)||1.3901|
|Legal status||Poison (S6)(AU)
|EU classification||Corrosive (C)|
|R-phrases||R10, R20/22, R34|
|S-phrases||(S1/2), S26, S36/37/39, S45|
|Flash point||49 °C (120 °F; 322 K)|
|Related acid anhydrides||Propionic anhydride|
|Related compounds||Acetic acid
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
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.
Structure and properties
Acetic anhydride, like most acid anhydrides, are flexible molecules with nonplanar structures. 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 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
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. 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
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
A former industrial route to vinyl acetate involved the intermediate ethylidene diacetate. This geminal diacetate was obtained by reaction of acetaldehyde and acetic anhydride in the presence of a ferric chloride catalyst:
- 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. 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 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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