acid anhydride
/ AN-hy-dryde /
Take two carboxylic acid molecules, squeeze out one molecule of water between them, and join the leftovers — that is an acid anhydride. The name says it plainly: "anhydride" means "without water," a dehydrated double-acid. Acetic anhydride, made from two acetic acids, is one of the most-used reagents in all of organic chemistry.
An anhydride has the structure R-CO-O-CO-R: two acyl groups (each a C=O) sharing a single bridging oxygen between them. When a nucleophile attacks one of the carbonyls, the bridging oxygen leaves as a carboxylate ion — a decent leaving group, though not as good as chloride. So the anhydride sits just below the acyl chloride in reactivity: more reactive than esters and amides, less reactive than acid chlorides. Symmetrical anhydrides (both halves the same, like acetic anhydride) are common; mixed anhydrides exist too and play important roles in biology.
Anhydrides are prized as gentle, controllable acylating agents. Acetic anhydride acetylates alcohols and amines cleanly — it is the reagent that puts the acetyl group onto salicylic acid to make aspirin, and onto sugars and amines in countless syntheses. Because the leaving group is a carboxylate rather than corrosive HCl, anhydrides are often milder and easier to handle than acyl chlorides while still being plenty reactive.
Acetic anhydride, (CH3CO)2O, reacts with the -OH of salicylic acid to install an acetyl group, producing acetylsalicylic acid (aspirin) and releasing one acetic acid as the byproduct (the leaving carboxylate, protonated).
Acetic anhydride is the classic acetylating reagent, as in the synthesis of aspirin.
An acid anhydride is not the same as an acid hydrate or a simple ester; the defining feature is the C(=O)-O-C(=O) linkage joining two acyl groups through one oxygen.