What Is Phthalic Anhydride?
Phthalic anhydride, also known as 1,3-isobenzofurandione, is an acid anhydride derived from the dehydration-condensation of two carboxylic (carboxyl) groups in phthalic acid. This compound transforms back into phthalic acid upon exposure to moisture in the air.
Uses of Phthalic Anhydride
Phthalic anhydride is primarily used to produce phthalic acid diesters, such as dioctyl phthalate and dibutyl phthalate, which serve as plasticizers to soften plastics. It is also a key ingredient in the manufacturing of polyester resins, alkyd resins for synthetic resin paints, dyes like phenolphthalein, fluorescein, eosin, and raw materials for pharmaceuticals. These compounds are extensively utilized in products ranging from wallpaper and floor coverings to electrical wire insulation and PVC leather for furniture. In 2017, Japan produced over 160,000 tons, exporting about 50,000 tons internationally.
Properties of Phthalic Anhydride
With a molecular formula of C8H4O3, phthalic anhydride is a pure white, needle-like crystalline powder or flake. It emits a slight pungent odor and has a density of 1.527 g/cm3, melting point of 130.8°C, boiling point of 284.5°C, flash point of 151.7°C, and ignition point of 570°C. It is soluble in alcohol, marginally soluble in ether and boiling water.
Other Information on Phthalic Anhydride
1. Production of Phthalic Anhydride
Phthalic anhydride is produced by the intramolecular dehydration of phthalic acid or by oxidizing naphthalene or ortho-xylene in the gas phase using vanadium pentoxide as a catalyst. The process can be categorized into fluidized-bed or fixed-bed methods, involving reaction, condensation, and distillation stages. The efficiency of this process results in approximately 85% yield of phthalic anhydride.
- Using Naphthalene: C10H8 + 4.5O2 → C8H4O3 + 2H2O + 2CO2
- Using Ortho-Xylene: C6H4(CH3)2 + 3O2 → C8H4O3 + 3H2O
Vanadium-based catalysts are commonly used, supported on materials like silica, alumina, silicon carbide, or pumice stone, with the reaction occurring within 0.1 to 0.5 seconds at temperatures between 400 to 500℃.
2. Reactions Involving Phthalic Anhydride
Phthalic anhydride is instrumental in synthesizing phenolphthalein, a widely used acid-base indicator, through the intermolecular dehydration reaction of phthalic acid and phenol. It also serves as a protecting group for primary amine groups in organic synthesis.
3. Toxicity of Phthalate Plasticizers
The health and environmental impacts of phthalate plasticizers have been scrutinized, particularly in the 1990s and early 2000s. However, after over half a century of global usage, including in medical applications, the risks posed by phthalate plasticizers are considered manageable, with no immediate need for further regulation beyond current measures.