Acid Anhydrides

An acid anhydride is formally two acyl groups joined through a single shared oxygen, (RCO)2O -- structurally, and often literally, the dehydration product of two carboxylic aci …

(1) Heating a carboxylic acid with P2O5: this is the SAME dehydration already introduced in Section 12.12.C.5, restated here from the anhydride's own point of view -- 2 acetic acid + P2O5 gives acetic anhydride + H2O. (2) Reaction of an acid HALIDE with the SODIUM SALT of a (the same or a different) carboxylic acid: this is the more general, mixed-anhydride-capable route, since the two acyl fragments need not be identical. Example: acetyl chloride + sodium acetate gives acetic anhydride + NaCl -- the carboxylate oxygen acts as the …

An acid anhydride reacts with the same family of nucleophiles as an acid chloride (water, alcohols, ammonia, PCl5), but more SLOWLY, since -OCOR is a poorer leaving group than -Cl (Section 12.14's reactivity order) -- each reaction below simply expels one acyl group AS the corresponding free carboxylic acid, while the other acyl group is converted to the new product. (1) HYDROLYSIS: acetic anhydride, slowly hydrolysed by water, gives 2 acetic acid. (2) Reaction with ALCOHOL gives an ESTER (plus one equivalent of the free acid): acetic anhydride + ethanol gives ethyl acetate + acetic acid. (3) Reaction with AMMONIA gives an AMIDE (plus one equivalent of the free acid): acetic anhydride + NH3 gives acetamide + acetic a …