Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commercially. This white solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2000, the worldwide production volume was estimated to be about 3 million tonnes per year.
Phthalic anhydride was first reported in 1836 by Auguste Laurent. More contemporary routes include oxidation of naphthalene or ortho-xylene. Vanadium pentoxide (V2O5) is the active oxidant for several key steps of the processes, and is regenerated by molecular oxygen. Starting from o-xylene, the oxidation reaction is run at about 320–400 °C and has the following stoichiometry:
C6H4(CH3)2 + 3 O2 → C6H4(CO)2O + 3 H2O
The reaction proceeds with about 70% selectivity. About 10% of maleic anhydride is also produced: C6H4(CH3)2 + 7.5 O2 → C4H2O3 +4 H2O + 4 CO2
Phthalic anhydride and maleic anhydride are recovered by distillation by a series of switch condensers.
The naphthalene route (the Gibbs phthalic anhydride process or the Gibbs–Wohg naphthalene oxidation reaction), a process whose use has declined in compared to the o-xylene route, has the following mechanism:
Phthalic anhydride can also be prepared from phthalic acid by simple dehydration.
Phthalate esters plasticizers: The primary use of phthalic anhydride is a precursor to phthalate esters, used as plasticizers in vinyl chloride. Phthalate esters are derived from phthalic anhydride by the alcoholysis reaction. In the 1980s, approximately 6.5×109 kg of these esters were produced annually, and the scale of production was increasing each year, all from phthalic anhydride. Phthalic anhydride has another major use in the production of polyester resins and other minor uses in the production of alkyd resins used in paints and lacquers, certain guys, insect repellents, and urethane polyester polyols.
Precursor to dyestuffs: Phthalic anhydride is widely used in industry for the production of certain dyes. A well-known application of this reactivity is the preparation of the anthroquinone dye quinizarin by reaction with para-chlorophenol followed by hydrolysis of the chloride. Phenolphthalein can be synthesized by the condensation of phthalic anhydride with two equivalents of phenol under acidic conditions (hence the name).
Pharmaceuticals: Phthalic anhydride treated with cellulose acetate gives cellulose acetate phthalate (CAP), a common enteric coating excipient that has also been shown to have antiviral activity. Phthalic anhydride is a degradation product of CAP.