What Is Acetic Anhydride?
Acetic anhydride, with the molecular formula C4H6O3 and specific formula (CH3CO)2O, is a colorless liquid. It reacts with water to form acetic acid and has a strong acetic acid odor when exposed to air moisture.
Pure acetic acid and glacial acetic acid, which have low water content, differ from acetic anhydride. Acetic anhydride is the simplest carboxylic anhydride to isolate and is a key raw material in organic synthesis, forming esters with alcohols and cellulose.
Properties of Acetic Anhydride
Acetic anhydride has a higher boiling point (139.8°C) than acetic acid (118°C) and can cause irritation and tearing when inhaled or if it contacts the skin. It should be handled with care, especially when heated.
Acetic anhydride hydrolyzes to form acetic acid, fully miscible in water:
(CH3CO)2O + H2O → 2 CH3COOH
Acetic Anhydride Manufacturing Process
First synthesized in 1852 by Charles Frédéric Gerhardt, acetic anhydride is now industrially produced by reacting ketene with acetic acid, or through the carbonylation of methyl acetate.
H2C=C=O + CH3COOH → (CH3CO)2O
CH3CO2CH3 + CO → (CH3CO)2O
Uses of Acetic Anhydride
Acetic anhydride is essential for acetylation in organic chemistry, introducing acetyl groups into various compounds.
Acetylation of Alcohols and Cellulose
Reacting with alcohols, it forms acetic esters (e.g., ethyl acetate from ethanol), and with cellulose, it yields acetyl cellulose used in a range of applications including cigarette filters and synthetic fibers.
(CH3CO)2O + CH3CH2OH → CH3CO2CH2CH3 + CH3COOH
Acetylation of Aromatic Rings
Acetic anhydride acetylates aromatic rings in the presence of an acid catalyst, as seen in the conversion of ferrocene to acetylferrocene.
(C5H5)2Fe + (CH3CO)2O → (C5H5)Fe(C5H4COCH3) + CH3COOH
Preparation of Other Acid Anhydrides
Acetic anhydride is also used to convert dicarboxylic acids into their corresponding anhydrides.
Additionally, acetic anhydride’s applications have expanded into new areas like polarizer protection film for LCDs, liquid crystal polymers, and urethane elastic fibers.