To a lesser extent, acetic anhydride is also prepared by the reaction of ketene (ketene) with acetic acid at 45–55 °C and low pressure (0.05–0.2 bar). [8]

H2C=C=O + CH3COOH → (CH3CO)2O

In 1922, Wacker Chemie [9] developed the route from acetic acid to acetic anhydride via ketene, when the demand for acetic anhydride increased due to the production of cellulose acetate.

Due to its low cost, acetic anhydride is usually purchased, rather than prepared, for use in research laboratories.

Acetic anhydride is a versatile acetylation reagent that introduces acetyl groups into organic substrates. [10] In these transformations, acetic anhydride was considered as the source of CH3CO+.

Acetylation of Alcohols and Amines

Alcohols and amines are readily acetylated. [11] For example, acetic anhydride reacts with ethanol to form ethyl acetate:

(CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3COOH

Usually a base such as pyridine is added to act as a catalyst. Lewis acidic scandium salts have also proven to be effective catalysts in specialized applications. [12]

Acetylation of aromatic rings

The aromatic ring is acetylated with acetic anhydride. Acid catalysts are often used to speed up the reaction. Examples are the conversion of benzene to acetophenone [13] and ferrocene to acetylferrocene: [14]

(C5H5)2Fe + (CH3CO)2O → (C5H5)Fe(C5H4COCH3) + CH3CO2H