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Anhydrides

Anhydrides, R–CO–O–CO–R′, result from the condensation of two carboxylic acids with loss of water. Their reactivity is similar to that of acyl halides, but each reaction uses only one of the two halves: the other leaves as a carboxylate.

Acetic anhydride

Nomenclature

Bond lengths of acetic anhydride: C=O 1.229 Å and C–O 1.393 Å

Physical and Bond Properties

Protonation of an acid derivative R–CO–L at the carbonyl oxygen and resonance forms of the cation, with the positive charge on the oxygen, the carbon and the L group

Acid-Base Properties

Formation of cyclic anhydrides on heating: succinic acid gives succinic anhydride and phthalic acid gives phthalic anhydride, with loss of water

Preparation

Reactions of anhydrides: hydrolysis to two carboxylic acids, reaction with phenol in pyridine to give the ester, and with ammonia or amines to give primary, secondary or tertiary amides plus the ammonium salt of the acid

Reactivity