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Phthalic anhydride

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Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. This anhydride of phthalic acid, a colourless solid, is an important industrial chemical, especially for the large-scale production of plasticizers.

Synthesis and production

Phthalic anhydride was first reported in 1836 by Laurent. It is presently obtained by catalytic oxidation of ortho-xylene and naphthalene (Gibbs phthalic anhydride process):
C6H4(CH3)2 + 3 O2 → C6H4(CO)2O + 3 H2O
C10H8 + 4.5 O2 → C6H4(CO)2O + 2 H2O + 2CO2
When separating the phthalic anhydride from byproducts such as o-xylene in water, or maleic anhydride, a series of switch condensers is required.

Applications in industry and organic synthesis

Phthalic anhydride is a versatile intermediate in organic chemistry, in part because it is bifunctional and cheaply available. Most characteristically, 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.

Preparation of phthalate esters

The alcoholysis reaction is the basis of the manufacture of phthalate esters, which are widely used plasticizers. 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. The process begins with the reaction of phthalic anhydride with alcohols gives the mixed esters:
C6H4(CO)2O + ROH → C6H4(CO2H)CO2R
The second esterification is more difficult and requires removal of water:
C6H4(CO2H)CO2R + ROH $overrightarrow\left\{leftarrow\right\}$ C6H4(CO2R)2 + H2O
The most important diester is bis(2-ethylhexyl) phthalate ("DEHP"), used in the manufacture of polyvinyl chloride.

Organic synthesis

Phthalic anhydride is a precursor to a variety of reagents useful in organic synthesis. Important derivatives include phthalimide and its many derivatives. Chiral alcohols form half esters (see above), and these derivatives are often resolvable because they form diastereomeric salts with chiral amines such as brucine. A related ring-opening reaction involves peroxides to give the useful peroxy acid:
C6H4(CO)2O + H2O2 → C6H4(CO3H)CO2H

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.

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.