Acetic Anhydride

Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH₃CO)₂O.^[1] Commonly abbreviated Ac₂O, it is the simplest isolatable acid anhydride and is a widely used reagent in organic synthesis. It is a colorless liquid that smells strongly of acetic acid, formed by its reaction with the moisture in the air.

Formic anhydride is an even simpler acid anhydride, but it spontaneously decomposes, especially once removed from solution.