Isophthalic acid

Isophthalic acid
Ball-and-stick model of the isophthalic acid molecule
Preferred IUPAC name
Benzene-1,3-dicarboxylic acid
Other names
Isophthalic acid
meta-Phthalic acid
121-91-5 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:30802 YesY
ChemSpider 8182 YesY
ECHA InfoCard 100.004.098
PubChem 8496
Molar mass 166.14 g/mol
Appearance White crystalline solid
Density 1.526 g/cm3, Solid
Insoluble in water
Acidity (pKa) 3.46, 4.46[1]
not listed
Related compounds
Benzoic acid
Phthalic acid (ortho)
Terephthalic acid (para)
Trimesic acid (benzene-1,3,5-tricarboxyllic acid)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Isophthalic acid is an organic compound with the formula C6H4(CO2H)2. This colourless solid is an isomer of phthalic acid and terephthalic acid.


Isophthalic acid is produced on the billion kilogram per year scale by oxidizing meta-xylene using oxygen.[2] The process employs a cobalt-manganese catalyst. In the laboratory, chromic acid can be used as the oxidant. It also arises by fusing potassium meta-sulphobenzoate, or meta-brombenzoate with potassium formate (terephthalic acid is also formed in the last case).

The barium salt (as its hexahydrate) is very soluble (a distinction between phthalic and terephthalic acids). Uvitic acid, 5-methylisophthalic acid, is obtained by oxidizing mesitylene or by condensing pyroracemic acid with baryta water.


Aromatic dicarboxylic acids are used as precursors (in form of acyl chlorides) to commercially important polymers, e.g. the fire-resistant material Nomex. Mixed with terephthalic acid, isophthalic acid is used in the production of resins for drink plastic bottles. The high-performance polymer polybenzimidazole is produced from isophthalic acid.[2] Also, the acid is used as an important input to produce insulation materials.


  1. Brown, H.C., et al., in Baude, E.A. and Nachod, F.C., Determination of Organic Structures by Physical Methods, Academic Press, New York, 1955.
  2. 1 2 Richard J. Sheehan "Terephthalic Acid, Dimethyl Terephthalate, and Isophthalic Acid" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.doi:10.1002/14356007.a26_193
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