The key difference between terephthalic acid and isophthalic acid

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Terephthalic acid (TPA) and isophthalic acid (IPA) are both isomers of phthalic acid; they have the same chemical formula, C₈H₆O₄, and an identical relative molecular mass of 166.13. However, the positions of the two carboxyl groups on the benzene ring differ, which results in significant differences in their physical and chemical properties, reaction characteristics, and areas of application. They are key intermediates with diverse uses in the chemical industry. Structural differences are the most fundamental distinction between the two. The two carboxyl groups of terephthalic acid are located at the 1,4 positions on the benzene ring, giving it a symmetrical structure; strong intermolecular forces exist, which facilitates the formation of dense crystals ； The two carboxyl groups of isophthalic acid are located in the meta positions (1,3) relative to the benzene ring; its structure is asymmetric, the molecular arrangement is loose, and its crystallinity is low. These structural differences directly determine the differences in their physical properties. In terms of physical and chemical properties, terephthalic acid appears as white needle-like crystals or powder; it sublimes at around 402°C at atmospheric pressure, and melts at 425°C in a sealed tube. It is insoluble in water, ethanol, and diethyl ether, being soluble only in alkaline solutions. It possesses extremely high thermal stability. Isophthalic acid is a colorless crystal with a melting point of 345–348°C. It is slightly soluble in water and soluble in solvents such as methanol and acetone. Its sublimation temperature is lower than that of terephthalic acid, and it is more prone to reactions such as esterification and halogenation; its reactivity is slightly higher. The toxicity of both is similar; they are both low-toxicity substances. However, the industrial production process for terephthalic acid is more mature, and its output is far higher than that of isophthalic acid. The application area is the most obvious difference between the two. Terephthalic acid is a key raw material in the polyester industry, primarily used to produce polyethylene terephthalate (PET), which is widely applied in polyester fibers, beverage bottles, food packaging films, and engineering plastics. Thanks to its regular structure, it confers products with high strength and toughness; it is the most produced isomer of terephthalic acid. Isophthalic acid is focused on the field of high-performance materials; its main use is in the production of unsaturated polyester resins, which enhances the resins’ resistance to hydrolysis, weathering, and mechanical stress. These resins are used in anti-corrosion and weather-resistant products such as ships, chemical storage tanks, and fiberglass products. Furthermore, it can also be used as a PET copolymerization modifier to improve the transparency and heat-fillability of products, for use in high-end packaging ； It is also used in the production of powder coatings, alkyd resins, and meta-aramids, playing an important role in the fields of corrosion prevention and insulation. In summary, although both belong to the category of aromatic dicarboxylic acids, their differences in structural symmetry result in distinct characteristics in terms of physical and chemical properties as well as applications: terephthalic acid is used for producing large-scale, general-purpose polyester products, while isophthalic acid is employed in high-performance, specialized materials. Together, they contribute to the development of various industries such as chemicals, textiles, and packaging.