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.

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. 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 mercuric sulphate 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.

Mostly based on the 1911 Encyclopedia.