Acid anhydrides are a type of organic compound that can be thought of as being formed from two carboxylic acid molecules by the removal of a water molecule. They contain two acyl (–CO–) groups joined by an oxygen atom, giving them the general structure R–CO–O–CO–R.
Key Points about Acid Anhydrides
- They are derivatives of carboxylic acids.
- The name “anhydride” comes from “without water,” since they can be formed by dehydration of acids.
- They may be symmetrical (when both acyl groups are the same, e.g., acetic anhydride) or mixed/unsymmetrical (when two different carboxylic acids combine).
Preparation of Acid Anhydrides
- By heating carboxylic acids together, water is eliminated, and the anhydride is formed.
- In the lab, acetic anhydride (the most common example) is often prepared by controlled dehydration of acetic acid or by reaction of acyl chlorides with carboxylate salts.
Uses and Importance
- Acid anhydrides are very reactive and are used in the synthesis of esters and amides.
- Acetic anhydride is widely used in industry to make cellulose acetate (used in textiles and films), aspirin, and other pharmaceuticals.