Acetic anhydride, or ethanoic anhydride, is the chemical compound with the chemical formula . Commonly abbreviated , it is one the simplest anhydrides of a carboxylic acid and is widely used in the production of cellulose acetate as well 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.
Structure and properties
Acetic anhydride, like most organic acid anhydrides, is a flexible molecule with a nonplanar structure. The C=O and C-O distances are 1.19 and 1.39 Å.
The
Pi bond through the central oxygen offers very weak resonance stabilization compared to the
dipole repulsion between the two
carbonyl oxygens. The energy barriers to bond rotation between each of the optimal aplanar conformations are quite low.
Production
Acetic anhydride was first synthesized in 1852 by the French chemist Charles Frédéric Gerhardt (1816-1856) by heating potassium acetate with
benzoyl chloride.
[Charles Gerhardt (1852) “Recherches sur les acides organiques anhydres” (Investigations into the anhydrides of organic acids), Comptes rendus … , 34 : 755-758.]
Acetic anhydride is produced by carbonylation of methyl acetate:
The Tennessee Eastman acetic anhydride process involves the conversion of methyl acetate to iodomethane. Carbonylation of the methyl iodide produces acetyl iodide, which reacts with acetate source to give the desired anhydride. Rhodium chloride in the presence of lithium iodide is employed as the catalyst. Because acetic anhydride is not stable in water, the conversion is conducted under anhydrous conditions.
To a decreasing extent, acetic anhydride is also prepared by the reaction of ketene (ethenone) with acetic acid at 45–55 °C and low pressure (0.05–0.2 bar).[.]
- (Δ H = −63 kJ/mol)
The route from acetic acid to acetic anhydride via ketene was developed by Wacker Chemie in 1922,[.] when the demand for acetic anhydride increased due to the production of cellulose acetate.
Due to its low cost, acetic anhydride is usually purchased, not prepared, for use in research laboratories.
Reactions
Acetic anhydride is a versatile reagent for
, the introduction of acetyl groups to organic substrates.
[.] In these conversions, acetic anhydride is viewed as a source of .
Acetylation of alcohols, amines, aromatics
Alcohols and
are readily acetylated.
[.] 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,
scandium salts have also proven effective catalysts.
Aromatic rings are acetylated by acetic anhydride. Usually acid catalysts are used to accelerate the reaction. Illustrative are the conversions of benzene to acetophenone[Roger Adams and C. R. Noller "p-Bromoacetophenone" Org. Synth. 1925, vol. 5, p. 17. ] and ferrocene to acetylferrocene:[.]
Preparation of other acid anhydrides
Dicarboxylic acids are converted to the anhydrides upon treatment with acetic anhydride.
[B. H. Nicolet and J. A. Bender "3-Nitrophthalic Anhydride" Org. Synth. 1927, vol. 7, 74.
] It is also used for the preparation of mixed anhydrides such as that with nitric acid,
acetyl nitrate.
Precursor to geminal diacetates
react with acetic anhydride in the presence of an acidic
catalyst to give geminal diacetates.
[R. T. Bertz "Furfuryl Diacetate" Org. Synth. 1953, 33, 39. ] A former industrial route to
vinyl acetate involved the intermediate ethylidene diacetate, the geminal diacetate obtained from
acetaldehyde and acetic anhydride:
[G. Roscher "Vinyl Esters" in Ullmann's Encyclopedia of Chemical Technology, 2007 John Wiley & Sons: New York. ]
Hydrolysis
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, this reaction product being fully water miscible:
[.]
Enolate formation
Acetic anhydride forms the enolate in the presence of
acetate as base. The enolate can be
chemical trap by condensation with
benzaldehyde. In the 19th century, this chemistry, the
Perkin reaction, was used for the production of
cinnamic acid:
Lewis base properties
The carbonyl groups in acetic anhydride are weakly basic. A number of adducts are known, such as the derivative of titanium tetrachloride, TiCl
4((CH
3CO)
2O).
Applications
As indicated by its organic chemistry, acetic anhydride 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 an active modification agent via
autoclave impregnation and subsequent acetylation to make a durable and long-lasting timber.
Acetic anhydride is commonly used for the production of (E1414, E1420, E1422).
Legal status
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 is restricted in many other countries.
[.]
Safety
Acetic anhydride is an irritant and combustible liquid; it is highly corrosive to skin and any direct contact will result in burns. Because of its reactivity toward water and alcohol, foam or carbon dioxide are preferred for fire suppression.
The vapour of acetic anhydride is harmful.
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