Phthalic acid

Phthalic acid
Phthalic acid
CAS number 88-99-3 YesY
PubChem 1017
EC number 201-873-2
Jmol-3D images Image 1
Molecular formula C8H6O4
Molar mass 166.14 g/mol
Appearance white solid
Density 1.593 g/cm3, solid
Melting point

191 - 230 °C[1]

Solubility in water 0.6 g / 100 mL [2][3]
Acidity (pKa) 2.98, 5.28[4]
NFPA 704
NFPA 704.svg
Related compounds
Related carboxylic acids Isophthalic acid
Terephthalic acid
Related compounds Phthalic anhydride
Phthaloyl chloride
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Phthalic acid is an aromatic dicarboxylic acid, with formula C6H4(CO2H)2. It is an isomer of isophthalic acid and terephthalic acid. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large scale.[5]



Phthalic acid is produced by the catalytic oxidation of naphthalene directly to phthalic anhydride and a subsequent hydrolysis of the anhydride.[5]

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 naphthalenic 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.

Reactions and uses

Phthalic acid crystals

It is a dibasic acid, with pKa's 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.[6]


Phthalic acid is one of three isomers of benzenedicarboxylic acid, the others being isophthalic acid and terephthalic acid. Sometimes the term "phthalic acids" is used to refer to this family of isomers, but in the singular, "phthalic acid", refers exclusively to the ortho- isomer.

Phthalic-acid-2D-skeletal.png Isophthalic-acid-2D-skeletal.png Terephthalic-acid-2D-skeletal.png
  phthalic acid     isophthalic acid     terephthalic acid
  (ortho-phthalic acid)     (meta-phthalic acid)     (para-phthalic acid)


The toxicity of phthalic acid is low with LD50 (mouse) of 550 mg/kg. However, many phthalate esters have been implicated as endocrine disruptors.

See also


  1. ^ Several melting points are reported, for example: (i) 210-211° C with decomposition (Sigma-Aldrich on-line), (ii) 191 °C in a sealed tube (Ullmann's Encyclopedia of Industrial Chemistry), (iii) 230 °C with conversion to phthalic anhydride and water (J.T.Baker MSDS).
  2. ^
  3. ^
  4. ^ 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.
  5. ^ a b Peter M. Lorz, Friedrich K. Towae, Walter Enke, Rudolf Jäckh, Naresh Bhargava, Wolfgang Hillesheim “Phthalic Acid and Derivatives” in Ullmann's Encyclopedia of Industrial Chemistry, 2007, Wiley-VCH, Weinheim. doi:10.1002/14356007.a20_181.pub2
  6. ^ Richard N. McDonald and Charles E. Reineke (1988), "trans-1,2-Dihydrophthalic Acid", Org. Synth., ; Coll. Vol. 6: 461 
  • Merck Index, 9th ed, #7178

External links

 This article incorporates text from a publication now in the public domainChisholm, Hugh, ed (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 

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Look at other dictionaries:

  • Phthalic acid — Phthalic Phthal ic, a. [Naphthalene + ic.] (Chem.) Pertaining to, or designating, a dibasic acid obtained by the oxidation of naphthalene and allied substances. [1913 Webster] {Phthalic acid} (Chem.), a white crystalline substance, {C6H4.(CO2H)2} …   The Collaborative International Dictionary of English

  • phthalic acid — [thal′ik, fthal′ik as′id] n. [(NA)PHTHAL(ENE) + IC] any of three isomeric acids, C6H4 (COOH) 2, esp. one ( ortho phthalic acid) obtained by oxidation of naphthalene or xylene and used in the manufacture of dyes, medicines, etc …   English World dictionary

  • phthalic acid — ftalio rūgštis statusas T sritis chemija formulė C₆H₄(COOH)₂ atitikmenys: angl. phthalic acid rus. фталевая кислота ryšiai: sinonimas – 1,2 benzendikarboksirūgštis …   Chemijos terminų aiškinamasis žodynas

  • phthalic acid — 1. Chem. any of three isomeric acids having the formula C8H6O4, esp. the ortho isomer (orthophthalic acid), a colorless, crystalline, slightly water soluble solid used chiefly in the manufacture of dyes, medicine, and perfume. 2. See isophthalic… …   Universalium

  • phthalic acid — o Benzenedicarboxylic acid. * * * phthal·ic acid .thal ik n any of three isomeric acids C8H6O4 obtained by oxidation of various benzene derivatives * * * phthal·ic ac·id (thalґik) any of the isomers, but usually the ortho isomer, of the… …   Medical dictionary

  • phthalic acid — [ (f)θalɪk] noun Chemistry a crystalline acid derived from benzene, with two carboxylic acid groups attached to the benzene ring. Origin C19: shortening of naphthalic (see naphthalene) …   English new terms dictionary

  • phthalic acid — noun Etymology: International Scientific Vocabulary, short for obsolete naphthalic acid, from naphthalene Date: 1857 any of three isomeric acids C8H6O4 obtained by oxidation of various benzene derivatives …   New Collegiate Dictionary

  • phthalic acid — noun Any of three isomeric aromatic organic dicarboxylic acids, CH(COOH); especially the ortho isomer manufactured from naphthalene, used in the synthesis of dyes, perfumes and other compounds …   Wiktionary

  • phthalic acid — phthal′ic ac′id n. chem. any of three colorless, crystalline, isomeric acids having the formula C8H6O4, used chiefly in the manufacture of dyes, medicine, and perfume • Etymology: 1855–60 …   From formal English to slang

  • phthalic acid — n. Chem. one of three isomeric dicarboxylic acids derived from benzene. Derivatives: phthalate n. Etymology: abbr. of NAPHTHALIC: see NAPHTHALENE …   Useful english dictionary

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