Phthalic anhydride

Phthalic anhydride is the anhydride of phthalic acid. It dissolves in alcohol and some other organic solvents.

Phthalic anhydride is obtained either by catalytic oxidation of ortho-xylene with O2 (oxygen) or by catalytic oxidation of naphthalene (called the Gibbs phthalic anhydride process 1918). The byproducts of the naphthalene reaction are carbon dioxide and water.

It is hydrolyzed by hot water, forming ortho-phthalic acid. This process is reversible, with phthalic anhydride being re-formed from the acid above 180 &deg;C.

When separating the phthalic anhydride from byproducts such as o-xylene in water, or maleic anhydride, a series of switch condensers is required, rather than a complicated setup of distillation columns that never converge in many industrial process simulation programs such as AspenTech.

Phthalic anhydride is widely used in industry in the production of dyes (rhodamine, anthraquinone derivatives), insecticides, plasticizers and pharmaceuticals. It also finds use in analytical chemistry.