Acid anhydrides:
Acid anhydrides are made by treating acid chlorides along with a carboxylate salt. Carboxylic acids are not simply transformed to acid anhydrides directly. Though, five-membered and six-membered cyclic anhydrides can be synthesized from diacids through heating the acyclic structures to remove water shown in figure.
Figure: Synthesis of cyclic acid anhydrides from acyclic di-acids.