Phthalic acids

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Phthalic acid
Image:Phthalic acids.jpg
General
Other names	Benzene-1,2- dicarboxylic acid
Molecular formula	C₆H₄(COOH)₂
SMILES	?
Molar mass	166.14 g/mol
Appearance	white solid
CAS number	[88-99-3]
Properties
Density and phase	1.593 g/cm³, solid
Solubility in water	Slightly soluble
Solubility in ethanol	Soluble
Melting point	210 °C decomp.
Acidity (pK_a)	?
Thermodynamic data
Standard enthalpy of formation Δ_fH°_solid	? kJ/mol
Standard enthalpy of combustion Δ_cH°_solid	? kJ/mol
Standard molar entropy S°_solid	? J.K⁻¹.mol⁻¹
Hazards
EU classification	not listed
NFPA 704	Image:Nfpa h0.png Image:Nfpa f1.png Image:Nfpa r1.png
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Regulatory data	Flash point, RTECS number, etc.
Related compounds
Related carboxylic acids	Isophthalic acid Terephthalic acid
Related compounds	Phthalic anhydride Phthalimide Phthalhydrazide Phthaloyl chloride Benzene-1,2- dicarboxaldehyde
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Phthalic acids, also known as benzene dicarboxylic acids, are organic acids with the chemical formula C₆H₄(COOH)₂. There are three isomers: ortho, or phthalic acid (1,2-benzenedicarboxylic acid); meta, or isophthalic acid (1,3-benzenedicarboxylic acid); para, or terephthalic acid (1,4-benzenedicarboxylic acid). Their chemical structures are shown here.

Image:Phthalic acid isomers.PNG

Phthalic acid (i.e., the ortho isomer) is used mainly in the form of the anhydride to produce other chemicals such as dyes, perfumes, saccharin, phthalates and many others.

Phthalic acid was obtained by French chemist Auguste Laurent in 1836 by oxidizing naphthalene tetrachloride, and, believing the resulting substance to be a naphthalene derivative, he named it naphthalenic acid. Swiss chemist Jean Charles Galissard de Marignac determined its formula and showed Laurent’s supposition to be incorrect, upon which Laurent gave it its present name. It can be manufactured by oxidizing naphthalene tetrachloride (prepared from naphthalene, potassium, chlorate and hydrochloric acid) with nitric acid, or, better, by oxidizing the hydrocarbon with fuming sulfuric acid, using mercury or mercury(II) sulfate as a catalyst.

It forms white crystals, melting at 213° with decomposition into water and phthalic anhydride; the latter forms long white needles, melting at 128° and boiling at 284°. Heating with an excess of lime produces benzene. The acid (and anhydride) are largely used in the color industry (see phenolphthalein).

Isophthalic acid can be obtained by oxidizing meta-xylene with chromic acid, or by fusing potassium meta-sulphobenzoate, or meta-brombenzoate with potassium formate (terephthalic acid is also formed in the last case). It melts above 300°. The barium salt (6H₂0) is very soluble (a distinction between phthalic and terephthalic acids). Uvitic acid, 5-methyl isophthalic acid, is obtained by oxidizing mesitylene or by condensing pyroracemic acid with baryta water.

Terephthalic acid can be formed by oxidizing para-diderivatives of benzene, or best by oxidizing caraway oil, a mixture of cymene and cuminol, with chromic acid, as almost insoluble in water, alcohol and ether; it sublimes without melting when heated. On an industrial scale, terephthalic acid is produced by oxidation of p-xylene by oxygen from air. The terephthalic acid is, in turn, often used as a monomer component in the production of polymers such as polyethylene terephthalate or PET (the commonest polyester).