Everything about Acetic Anhydride totally explained
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Acetic anhydride is the
chemical compound with the
formula (CH
3CO)
2O. Commonly abbreviated
Ac2O, it's one of the simplest
acid anhydrides and is a widely used
reagent in
organic synthesis. It is a colorless liquid that smells strongly of
acetic acid, which is formed by its reaction with the moisture in the air.
Production
Acetic anhydride is produced by
carbonylation of
methyl acetate:
» CH
3CO
2CH
3 + CO → (CH
3CO)
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 and lithium iodides are employed as catalysts. Because acetic anhydride isn't stable in water, the conversion is conducted under anhydrous conditions. In contrast, the
Monsanto acetic acid synthesis, 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
ketene with acetic acid. Ketene is generated by dehydrating acetic acid at elevated temperatures. This production method was developed by
Wacker Chemie in 1922, when the demand for acetic anhydride increased due to the production of acetic fibre.
Due to its low cost, acetic anhydride is purchased, not prepared, for use in research laboratories.
Organic chemistry
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 CH
3CO
+.
Alcohols and
amines are readily acetylated. For example, the reaction of acetic anhydride with
ethanol is:
» (CH
3CO)
2O + CH
3CH
2OH →
CH3CO2CH2CH3 + CH
3CO
2H
Often a base such as
pyridine is added to function as catalyst. In specialized applications,
Lewis acidic scandium salts have also proven effective catalysts.
Aromatic rings are acetylated, usually in the presence of an acid catalyst. Illustrative is the conversion of benzene to
acetophenone:
» (CH
3CO)
2O + C
6H
6 → CH
3COC
6H
5 + CH
3CO
2H
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 acetic acid:
» (CH
3CO)
2O + H
2O → 2 CH
3CO
2H
Applications
As indicated by its organic chemistry, Ac
2O 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's used in the production of
aspirin, acetyl salicylic acid, which is prepared by the acetylation of
salicylic acid. Because of its use for the synthesis of
heroin by the diacetylation of
morphine, acetic anhydride (known as 'AA' in
clandestine chemistry circles) is listed as a U.S. DEA List II Precursor, and restricted in many other countries.
Safety
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.
Notes and references
Further Information
Get more info on 'Acetic Anhydride'.
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