Acetic anhydride

Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH₃CO)₂O.Acetic anhydride was first synthesized in 1852 by French chemist Charles Frédéric Gerhardt (1816-1856). See: Charles Gerhardt (1852) Comptes Rendus, vol. 34, pages 755-758. 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.

Structure

Contrary to what its Lewis structure seems to predict, acetic anhydride, like many other acid anhydrides that are free to rotate, has experimentally been found to be aplanar. The pi system linkage through the central oxygen offers very weak resonance stabilization compared to the dipole-dipole repulsion between the two carbonyl oxygens. However, the energy barriers to bond rotation between each of the optimal aplanar conformations are quite low.

Like most acid anhydrides, the carbonyl carbon of acetic anhydride is a potent electrophile as the leaving group for each carbonyl carbon (a carboxylate) is a good electron-withdrawing leaving group. The internal asymmetry may contribute to acetic anhydride's potent electrophilicity as the...
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