Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid . This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2002, approximately 4.6 billion kilograms were produced.
Phthalic anhydride is a versatile intermediate in organic chemistry , in part because it is bifunctional and cheaply available. It undergoes hydrolysis and alcoholysis.Hydrolysis by hot water forms ortho-phthalic acid . This process is reversible: Phthalic anhydride re-forms upon heating the acid above 180 °C. Hydrolysis of anhydrides is not typically a reversible process. However, phthalic acid is easily dehydrated to form phthalic anhydride due to the creation of a thermodynamically favorable 5-membered ring.
- Phthalic anhydride is an important chemical intermediate in the plastics industry from which are derived numerous phthalate esters that function as plasticizers in synthetic resins. Phthalic anhydride itself is used as a monomer for synthetic resins such as glyptal, the alkyd resins, and the polyester resins.
- Phthalic anhydride is also used as a precursor of anthraquinone, phthalein, rhodamine, phthalocyanine, fluorescein, and xanthene dyes.
- Phthalic anhydride is used in the synthesis of primary amines, the agricultural fungicide phaltan, and thalidomide. Other reactions with phthalic anhydride yield phenolphthalein, benzoic acid, phthalylsulfathiazole (an intestinal antimicrobial agent), and orthophthalic acid.