Phthalic anhydride is the organic compound of the formula C₆H₄ (CO) ₂O. It is the anhydride of phthalic acid. This colorless solid is an important industrial chemical product, in particular for the large-scale production of plasticizers for plastics.


Phthalic anhydride is a versatile intermediate in organic chemistry, in part because it is bifunctional and inexpensive. It undergoes hydrolysis and alcoholysis. Hydrolysis by hot water produces orthophthalic acid. This process is reversible: phthalic anhydride is reformed by heating the acid above 180 ° C. 6 Hydrolysis of anhydrides is not normally a reversible process. However, phthalic acid is easily dehydrated to form phthalic anhydride due to the creation of a thermodynamically favorable 5-membered ring.