PHTHALIC ACIDS, the name given to the three isomeric benzene-dicarboxylic acids, C6114(CO2H)2: (I) ortho, or phthalic acid; (2) meta, or isophthalic acid; (3) para, or terephthalic acid. The most important is the first, as it forms the starting material in the production of many synthetic dyes and of the synthetic resin, glyptal, glyceryl phthalate.
Phthalic acid was obtained by Laurent in 1836 by oxidizing naphthalene tetrachloride, and, believing it to be a naphthalene derivative, he named it naphthalenic acid; Marignac showed Lau rent's supposition to be incorrect, upon which Laurent gave it its present name. It was formerly manufactured by oxidizing naph thalene tetrachloride (prepared from naphthalene, potassium chlorate and hydrochloric acid) with nitric acid, or by oxidizing the hydrocarbon with fuming sulphuric acid, using mercury or mercuric sulphate as a catalyst (German pat. 9,202). These methods have been superseded by a catalytic process in which air and naphthalene vapour are passed over heated vanadium pent oxide ; the yield is upwards of 8o% of phthalic anhydride, which is readily separated by fractional sublimation from unchanged naphthalene. Phthalic acid forms white crystals, melting at with decomposition into water and phthalic anhydride ; the latter forms long white needles, melting at and boiling at Phthalyl chloride, C6H4(COC1)2 or (C0)0, formed by heating the anhydride with phosphorus pentachloride, is an oil solidifying at and boiling at In some reactions it behaves as having the first formula, in others as having the second.
Phthalyl chloride with phosphorus pentachloride gives two phthalylene tetrachlorides, one melting at and the other at They cannot be changed into one another, and have been given the formulae and C6H4(CC12)20.
Isophthalic acid is obtained by oxidizing meta-xylene with chromic acid, or by fusing potassium meta-sulphobenzoate, or meta-bromobenzoate with potassium formate (terephthalic acid is also formed in the last case). It melts above and dis solves in 7,800 parts of cold water and in 46o of boiling.
Terephthalic acid, formed by oxidizing para-derivatives of benzene, or best by oxidizing caraway oil, a mixture of cymene and cuminol, with chromic acid, is almost insoluble in water, alcohol and ether ; it sublimes without melting when heated.