1911 Encyclopædia Britannica/Phthalic Acids
PHTHALIC ACIDS, or Benzene Dicarboxylic Acids, C6H4(CO2H)2. There are three isomers: (1) ortho, or phthalic acid; (2) meta, or isophthalic acid; (3) para, or terephthalic acid.
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 determined its formula and showed Laurent’s supposition to be incorrect, upon which Laurent gave it its present name. It is manufactured by oxidizing naphthalene tetrachloride (prepared from naphthalene, potassium, chlorate and hydrochloric acid) with nitric acid, or, better, by oxidizing the hydrocarbon with fuming sulphuric acid, using mercury or mercuric sulphate as a catalyst (German pat. 91, 202). It also results on the oxidation of diderivatives of benzene. 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°. Heated with an excess of lime it gives benzene; calcium benzoate results when calcium phthalate is heated with one molecule of lime to 330°–350°. The acid (and anhydride) are largely used in the colour industry (see Fluorescein; Phenolphthalein).
Phthalyl chloride, C6H4(COCl)2 or C6H4(CCl2) (CO)O, formed by heating the anhydride with phosphorus chloride, is an oil which solidifies at 0° and boils at 275°. In some reactions it behaves as hav1ng the first formula, in others as having the second. Phthalyl chloride with phosphorus pentachloride gives two phthalylene tetrachlorides, one melting at 88° and the other at 47°. They cannot be changed into one another, and have been given the formulae C6H4(CCl3) (COCl) and C6H4(CCl2)2O. Phthalimide, C6H4(CO)2NH, is formed by heating phthalic anhydride or chloride in ammonia gas or by molecular rearrangement of ortho-cyanbenzoic acid. It forms N-metallic and alkyl salts. Bromine and potash give anthranilic acid, C6H4(NH2)(CO2H). (See Indigo.)
Isophthalic acid is 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°, and dissolves in 7800 parts of cold water and in 460 of boiling. The barium salt (+6H2O) 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, 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.
For the reduced phthalic acids see Polymethylenes.