Phthalic acid

Phthalic acid

Phthalic acid (IUPAC systematic name: benzene-1,2-dicarboxylic acid) is an aromatic dicarboxylic acid, with formula C6H4(COOH)2. It is an isomer of isophthalic acid and terephthalic acid.


Phthalic acid 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. Manufacturing methods in the nineteenth century included oxidation of naphthalene tetrachloride (prepared from naphthalene, potassium chlorate and hydrochloric acid) with nitric acid, or, better, oxidation of the hydrocarbon with fuming sulfuric acid, using mercury or mercury(II) sulfate as a catalyst.

The catalytic oxidation of naphthalene directly to phthalic anhydride and a subsequent hydrolysis of the anhydride is one of the new production methods.


It forms white crystals, melting at 210 °C with decomposition into water and phthalic anhydride. Heating with an excess of lime produces benzene. The acid (and anhydride) are largely used in the color industry (see phenolphthalein).


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 Isophthalic acid Terephthalic acid

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