Acetic anhydride, or acetic anhydride, is a compound with the chemical formula (CH3CO)2O. Often abbreviated as AcO, it is the simplest isolatable carboxylic anhydride and is widely used as a reagent in organic synthesis. It is a colorless liquid with a strong acetic acid odor and is formed by reacting with moisture in the air.

Acetic anhydride, like most acetic anhydride molar mass, is a flexible molecule with a non-planar structure. [4] Compared to the dipole-dipole repulsion between the two carbonyl oxygens, the π-system linkage through the central oxygen provides very weak resonance stability. The energy barrier to bond rotation between each optimal non-planar conformation is very low. [5]

Like most anhydrides, the carbonyl carbon atom of acetic anhydride has an electrophilic character because the leaving group is a carboxylate. Internal asymmetry may contribute to the strong electrophilicity of acetic anhydride, as the asymmetric geometry makes one side of the carbonyl carbon more reactive than the other, and in doing so tends to shift the electropositivity of the carbonyl carbon Consolidate to one side (see electron density map).
production [edit]
Acetic anhydride was first synthesized in 1852 by the French chemist Charles Frédéric Gerhardt (1816-1856) by heating potassium acetate with benzoyl chloride. [6]

Carbonylation of methyl acetate to produce acetic anhydride: [7]

CH3CO2CH3 + CO → (CH3CO)2O
The Tennessee Eastman Acetic Anhydride process involves the conversion of methyl acetate to methyl iodide and acetate. Carbonylation of methyl iodide in turn yields acetyl iodide, which reacts with acetate or acetic acid to yield the product. Rhodium chloride was used as catalyst in the presence of lithium iodide. Since acetic anhydride is unstable in water, the transformation was carried out under anhydrous conditions.

To a lesser extent, acetic anhydride can also be prepared by reacting enones (ketenes) with acetic acid at 45–55 °C and low pressure (0.05–0.2 bar). [8]

H2C=C=O + CH3COOH → (CH3CO)2O
(ΔH = -63 kJ/mol)
Wacker Chemie developed the route for the production of acetic anhydride from acetic acid via ketene in 1922,[9] 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.