Phthalic acid

In organic chemistry, phthalic acid is an aromatic dicarboxylic acid, with formula C₆H₄(CO₂H)₂ and structure HO(O)C−C₆H₄−C(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.^[4] Phthalic acid is one of three isomers of benzenedicarboxylic acid, the others being isophthalic acid and terephthalic acid.

Phthalic acid

Phthalic acids
Names
Preferred IUPAC name Benzene-1,2-dicarboxylic acid
Other names 1,2-Benzenedioic acid Phthalic acid Benzene-1,2-dioic acid ortho-Phthalic acid
Identifiers
CAS Number	88-99-3 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29069 N
ChemSpider	992 N
ECHA InfoCard	100.001.703
EC Number	201-873-2
PubChem CID	1017
UNII	6O7F7IX66E Y
CompTox Dashboard (EPA)	DTXSID8021484
InChI 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 Key: XNGIFLGASWRNHJ-UHFFFAOYSA-N 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) Key: XNGIFLGASWRNHJ-UHFFFAOYAX
SMILES OC(=O)c1ccccc1C(=O)O
Properties
Chemical formula	C8H6O4
Molar mass	166.132 g/mol
Appearance	white solid
Density	1.593 g/cm3, solid
Melting point	207 °C (405 °F; 480 K)[1]
Solubility in water	0.6 g / 100 mL [2]
Acidity (pKa)	2.89, 5.51[3]
Magnetic susceptibility (χ)	−83.61·10−6 cm3/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H315, H318, H319, H335
Precautionary statements	P261, P264, P264+P265, P271, P280, P302+P352, P304+P340, P305+P351+P338, P305+P354+P338, P317, P319, P321, P332+P317, P337+P317, P362+P364, P403+P233, P405, P501
NFPA 704 (fire diamond)	2 0 0
Related compounds
Related carboxylic acids	Isophthalic acid Terephthalic acid
Related compounds	Phthalic anhydride Phthalimide Phthalhydrazide Phthaloyl chloride Benzene-1,2- dicarboxaldehyde
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Production

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.

Synthesis

Naphthalene, on oxidation with potassium permanganate or potassium dichromate, gives phthalic anhydride,^{[citation needed]} which, through hydrolysis with hot water, gives phthalic acid.^[11] A more standard procedure is to oxidize napthalene with air, but this reaction proceeds explosively unless conditions are set up very accurately.^[12]

Uses

Phthalic acid in the form of phthalic anhydride is an important industrial chemical, used for making phthalates (esters of phthalic acid) that are used as plasticizers. However, phthalic anhydride is usually not made by dehydration of phthalic acid but from p-xylene or naphthalene.

Reactions

Phthalic acid crystals

It is a dibasic acid, with pK_as of 2.89 and 5.51. The monopotassium salt, potassium hydrogen phthalate is a standard acid in analytical chemistry. Phthalate esters are typically 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.^[13]

Safety

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

Biodegradation

The bacteria Pseudomonas sp. P1 degrades phthalic acid.^[14]

See also

• Isophthalic acid
• Phthalate
• Phthalic anhydride
• Potassium hydrogen phthalate, a primary standard for acid–base titrations
• Terephthalic acid