Isophthalic Acid: Formula, Properties, Preparation & uses

Isophthalic acid is an organic compound with the formula C8H6O4. It is a white crystalline solid at room temperature. As an organic compound it belongs to the class of aromatic dicarboxylic acids. It is structurally similar to terephthalic acid but has an isomeric arrangement of its carboxylic acid groups.

Isophthalic acid is one of three isomers of benzenedicarboxylic acid, the others being phthalic acid and terephthalic acid. These aromatic dicarboxylic acids are used as precursors (in the form of acylchlorides) to commercially important polymers. The high-performance polymer polybenzimidazole is produced from isophthalic acid.

Chemical Structure

Crystalline isophthalic acid is built up from molecules connected by hydrogen bonds, forming infinite chains. The molecular structure of isophthalic acid consists of a benzene ring with two carboxylic acid groups (COOH) attached at the meta positions (1,3) relative to each other.

Physical Properties of Isophthalic Acid

  • Chemical formula: C8H6O4
  • Molecular weight: Approximately 166.13 g/mol
  • Appearance: White crystalline solid
  • Odor: Typically odorless or may have a faint odor
  • Melting point: Approximately 334-338°C (633-640°F)
  • Boiling point: Decomposes before boiling
  • Solubility in water: Sparingly soluble, forms a clear solution
  • Solubility in organic solvents: Soluble in polar organic solvents such as acetone, ethanol, and dimethyl sulfoxide (DMSO)
  • Density: Approximately 1.49 g/cm3
  • pH: Isophthalic acid solutions are typically acidic in nature
  • Hygroscopicity: Has a tendency to absorb moisture from the atmosphere
  • Stability: Relatively stable under normal conditions, but may decompose at high temperatures
  • Partition coefficient (log P): Approximately 0.37 (indicating moderate lipophilicity).

Chemical Properties of Isophthalic Acid

  • Isophthalic acid undergoes reactions of carboxylic acids, including: Acid-Base Reactions: Reacts with bases to form water-soluble salts called isophthalates. Esterification: Can undergo esterification reactions with alcohols to form esters, a reaction commonly used in the synthesis of plasticizers and fragrance compounds. Amide Formation: Can react with primary amines to form amides.
  • Undergoes oxidation reactions under certain conditions, producing carbon dioxide and water.
  • Can undergo dehydration reactions to form anhydrides under suitable conditions, though this is less common compared to other dicarboxylic acids due to the steric hindrance caused by the proximity of the carboxylic acid groups.
  • Is susceptible to electrophilic aromatic substitution reactions due to the presence of the aromatic ring, allowing for functionalization of the benzene ring.
  • Isophthalic acid can participate in condensation reactions with diols, such as ethylene glycol, to form polyesters, particularly polyethylene terephthalate (PET) resins, which find extensive use in the production of fibers, films, and packaging materials.
  • Can undergo decarboxylation reactions under certain conditions, particularly at elevated temperatures. This results in the loss of carbon dioxide and formation of aromatic compounds.
  • Reacts with different reagents to introduce functional groups onto the benzene ring. This allows for the synthesis of derivatives with tailored properties for specific applications.
  • Exhibits similar chemical behavior to other aromatic dicarboxylic acids, such as terephthalic acid and phthalic acid because of its aromatic ring structure and carboxylic acid functionality.

Preparation of Isophthalic Acid

Isophthalic acid can be prepared through different methods, but one of the most common industrial processes involves the oxidation of meta-xylene.

Oxidation of Meta-Xylene

  • Meta-xylene (C8​H10​) is oxidized using air or oxygen as the oxidizing agent. This process involves a multi-step reaction sequence.
  • In the initial step, meta-xylene is oxidized to meta-xylene sulfonic acid (C8​H10​O3​S) using a catalyst such as sulfuric acid (H2​SO4​) and air.
  • Meta-xylene sulfonic acid is then further oxidized to meta-benzoic acid (C7​H6​O4​) under controlled conditions.
  • Finally, meta-benzoic acid undergoes decarboxylation to yield isophthalic acid (C8​H6​O4​).


  • After the isophthalic acid is formed, it is usually purified through processes such as crystallization or distillation to remove any impurities and obtain the desired product in high purity.

Alternative Methods

  • Isophthalic acid can also be prepared from other starting materials, such as terephthalic acid or dimethyl terephthalate, through selective hydrogenation and subsequent oxidation reactions.
  • Or can be obtained through the oxidation of meta-dialkylbenzenes or meta-alkylbenzenesulfonic acids followed by decarboxylation.

Uses of Isophthalic Acid

  • Isophthalic acid is a key raw material in the production of polymers, particularly polyethylene terephthalate (PET) resins. It is commonly used as a comonomer in the synthesis of PET to enhance the properties of the polymer, such as its mechanical strength, thermal stability, and chemical resistance.
  • It is used in the production of other polyester resins, including unsaturated polyester resins (UPRs), which find applications in composite materials, coatings and adhesives.
  • It is used in the production of synthetic fibers, such as polyester fibers. These fibers are used in textiles, apparel, carpets, and industrial applications.
  • It is used in the production of certain medical devices, including catheters, tubing, and implantable materials, due to its biocompatibility and chemical inertness.
  • It is used in the textile industry as a component of dyeing auxiliaries and finishing agents to improve the color fastness, dye uptake, and surface smoothness of textiles.

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