An acid anhydride is a member of a class of organic compounds derived formally (but not in practise) from carboxylic acids by the elimination of water. Acid anhydrides have the general formula RCOOCOR' and are prepared by reaction of an acid chloride with a carboxylate. Chemically, acid anhydrides resemble acid chlorides but are less violently reactive. They are used in the Friedel-Crafts reaction and Diels-Alder reaction and to make esters.