Anhydride phtalique

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. Early procedures involved liquid-phase mercury-catalyzed oxidation of naphthalene. The modern industrial variant process instead uses vanadium pentoxide (V2O5) as the catalyst in a gas-phase reaction with naphthalene using molecular oxygen. The overall process involves oxidative cleavage of one of the rings and loss of two of the carbon atoms as carbon dioxide. An alternative process involves oxidation of the two methyl groups of o-xylene, a more atom-economical process. This 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 1/2 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–Wohl naphthalene oxidation reaction) has declined relative to the o-xylene route. Phthalic anhydride can also be prepared from phthalic acid by simple thermal dehydration above 210°C. 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 million tonnes of these esters were produced annually, and the scale of production was increasing each year, all from phthalic anhydride.