Phthalic acid

Phthalic acid
Phthalic acids
Ball-and-stick model of the phthalic acid molecule
Preferred IUPAC name
Benzene-1,2-dicarboxylic acid
Other names
1,2-Benzenedioic acid
Phthalic acid
Benzene-1,2-dioic acid
ortho-Phthalic acid
Carboxylbenzoic acid
1,2-Phenylenedicarboxylic acid
Phenylene-1,2-dicarboxylic acid
3D model (JSmol)
ECHA InfoCard 100.001.703 Edit this at Wikidata
EC Number
  • 201-873-2
  • InChI=1S/C8H6O4/c9-7(10)5-3-1-2-4-6(5)8(11)12/h1-4H,(H,9,10)(H,11,12) ☒N
  • InChI=1/C8H6O4/c9-7(10)5-3-1-2-4-6(5)8(11)12/h1-4H,(H,9,10)(H,11,12)
  • OC(=O)c1ccccc1C(=O)O
Molar mass 166.132 g/mol
Appearance white solid
Density 1.593 g/cm3, solid
Melting point 207 °C (405 °F; 480 K)
0.6 g / 100 mL
Acidity (pKa) 2.89, 5.51
-83.61·10−6 cm3/mol
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
Related compounds
Isophthalic acid
Terephthalic acid
Related compounds
Phthalic anhydride
Phthaloyl chloride
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

In organic chemistry, phthalic acid is an aromatic dicarboxylic acid, with formula C6H4(CO2H)2 and structure HO(O)C−C6H4C(O)OH. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large scale. Phthalic acid is one of three isomers of benzenedicarboxylic acid, the others being isophthalic acid and terephthalic acid.


Phthalic acid is produced by the catalytic oxidation of naphthalene or ortho-xylene directly to phthalic anhydride and a subsequent hydrolysis of the anhydride.

Phthalic acid was first obtained by French chemist Auguste Laurent in 1836 by oxidizing naphthalene tetrachloride. Believing the resulting substance to be a naphthalene derivative, he named it "naphthalic acid". After the Swiss chemist Jean Charles Galissard de Marignac determined its correct formula, Laurent gave it its present name. Manufacturing methods in the nineteenth century included oxidation of naphthalene tetrachloride with nitric acid, or, better, oxidation of the hydrocarbon with fuming sulfuric acid, using mercury or mercury(II) sulfate as a catalyst.


Naphthalene on oxidation with potassium permanganate or potassium dichromate gives phthalic acid.[citation needed]

Reactions and uses

Phthalic acid crystals

It is a dibasic acid, with pKas of 2.89 and 5.51. The monopotassium salt, potassium hydrogen phthalate is a standard acid in analytical chemistry. Typically phthalate esters are prepared from the widely available phthalic anhydride. Reduction of phthalic acid with sodium amalgam in the presence of water gives the 1,3-cyclohexadiene derivative.


The toxicity of phthalic acid is moderate with LD50 (mouse) of 550 mg/kg.


The bacteria Pseudomonas sp. P1 degrades phthalic acid.

See also

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