From Wikipedia, the free encyclopedia. Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH 3 CO) 2 O. Commonly abbreviated Ac 2 O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis.ike 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 group.