Phthalic acid

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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)[3]
0.6 g / 100 mL [1]
Acidity (pKa) 2.89, 5.51[2]
-83.61·10−6 cm3/mol
NFPA 704 (fire diamond)
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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Phthalic acid is an aromatic dicarboxylic acid, with formula C6H4(CO2H)2. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large scale.[4] 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.[4]

Phthalic acid was first obtained by French chemist Auguste Laurent in 1836 by oxidizing naphthalene tetrachloride.[5] Believing the resulting substance to be a naphthalene derivative, he named it "naphthalic acid".[5][6][7] After the Swiss chemist Jean Charles Galissard de Marignac determined its correct formula,[8] Laurent gave it its present name.[5][9][10] 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.[5]


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

Reactions and uses[edit]

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.[11]


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


The bacteria Pseudomonas sp. P1 degrades phthalic acid.[12]

See also[edit]


  2. ^ 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.
  3. ^ Several melting points are reported, for example: (i) 480. K (NIST website), (ii) 210−211 °C with decomposition (Sigma-Aldrich on-line), (iii) 191 °C in a sealed tube (Ullmann's Encyclopedia of Industrial Chemistry), (iv) 230 °C with conversion to phthalic anhydride and water (J.T.Baker MSDS).
  4. ^ a b Lorz, Peter M.; Towae, Friedrich K.; Enke, Walter; et al. (2007). "Phthalic Acid and Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a20_181.pub2.
  5. ^ a b c d Chisholm, Hugh, ed. (1911). "Phthalic Acids" . Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. pp. 545–546.
  6. ^ Auguste Laurent (1836) "Sur l'acide naphtalique et ses combinaisons" (On naphthalic acid and its compounds), Annales de Chimie et de Physique, 61 : 113-125. (Note: The empirical formulae of the compounds that were analyzed in this article are incorrect, in part because, during this period, chemists used incorrect atomic masses for carbon (6 instead of 12) and other elements.)
  7. ^ Reprinted in German as: Auguste Laurent (1836) "Ueber Naphthalinsäure und ihre Verbindungen" (On naphthalenic acid and its compounds), Annalen der Pharmacie, 19 (1) : 38-50; for the preparation of phthalic acid, see page 41.
  8. ^ C. de Marignac (1841) "Ueber die Naphtalinsäure und ein bei ihrer Darstellung entstehendes flüchtiges Produkt" ("On naphthalinic acid and a volatile product that arises during its preparation"), Annalen der Chemie und Pharmacie, 38 (1) : 13-20. (Note: Again, Marignac's empirical formulae are wrong because chemists at this time used incorrect atomic masses.)
  9. ^ Auguste Laurent (1841) "Sur de nouvelles combinaisons nitrogénées de la naphtaline et sur les acides phtalique et nitrophtalique" (On new nitrogenous compounds of naphthalene, and on phthalic acid and nitrophthalic acid), Revue Scientifique et Industrielle, 6 : 76-99; on page 92, Laurent coins the name "acide phtalique" (phthalic acid) and admits that his earlier empirical formula for phthalic acid was wrong.
  10. ^ Reprinted in German as: Auguste Laurent (1842) "Ueber neue stickstoffhaltige Verbindungen des Naphtalins, über Phtalinsäure und Nitrophtalinsäure" (On new nitrogenous compounds of naphthalene, on phthalic acid and nitrophthalic acid), Annalen der Chemie und Pharmacie, 41 (1) : 98-114; on page 108, Laurent coins the name "Phtalinsäure" (phthalic acid).
  11. ^ Richard N. McDonald and Charles E. Reineke (1988). "trans-1,2-Dihydrophthalic Acid". Organic Syntheses.; Collective Volume, vol. 6, p. 461
  12. ^ Ishtiaq Ali, Muhammad (2011). Microbial degradation of polyvinyl chloride plastics (PDF) (Ph.D.). Quaid-i-Azam University. p. 47.
  • Merck Index, 9th ed, #7178

External links[edit]