Acetic anhydride

Acetic anhydride is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is 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:
 * 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 and lithium iodides 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 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.

Due to its low cost, acetic anhydride is purchased, not prepared, for use in research laboratories.

Uses
Ac2O is mainly used for the acetylation of cellulose to cellulose acetate for photographic film and other applications.

In general alcohols and amines are acetylated. For example, the reaction of acetic anhydride with ethanol is:
 * (CH3CO)2O +  CH3CH2OH → CH3CO2CH2CH3  +  CH3CO2H

Often a base such as pyridine is added to function as catalyst. Lewis acidic scandium salts are also effective catalysts.

Aspirin, acetyl salicylic acid, is prepared by the acetylation of salicylic acid using acetic anhydride. 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 DEA List II Precursor, and restricted in many other countries.

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:
 * (CH3CO)2O +  H2O  →  2 CH3CO2H

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.