Pertaining to, or designating, a dibasic acid obtained by the oxidation of naphthalene and allied substances.
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Phthalic acid
Encyclopedia Britannica
PHTHALIC ACID. This name was given by Laurent to a di-basic acid, C 8 H 6 O 4 , which he obtained by the oxidation of naphthalin or its tetra-chloride with nitric acid. Schunck subsequently obtained the same acid by boiling alizarin with nitric acid, but failed to recognize its identity with Laurent’s.
One part of naphthalin is mixed with two parts of chlorate of potash, and the mixture added cautiously to ten parts of crude muriatic acid. The product, C] 0 H 8 .Cl 4 , is washed with water and then with “ligroin” (the more volatile fraction of petroleum). The chloride thus purified is oxidized by boiling it with ten parts of (gradually added) nitric acid of 1 '45 specific gravity, evaporated to dryness, and the residue distilled to obtain the anhydride C 8 H 4 O 3 , or
CO o rationally C 6 H 4 ^θ>O, long colourless needles fusing at 128° C., —the boiling point being 276°. When boiled with water it becomes
COOH
phthalic acid, rat. formula C 6 H 4 ^θθ∩, rhombic c: ystals, fusing at 184° C. with transformation into anhydride, very slightly soluble in water (100 parts at 11° dissolve 0'77 parts), more soluble in alcohol (100 of absolute dissolve 10‘1 parts at 15°). Phthalic acid, when heated to redness with lime, breaks up into CO 2 and benzol; the lime salt when mixed with one equivalent cáOH of lime, and kept at 330° to 350° C., yields carbonate and benzoate—
C 6 H 4 ∞θ⅛ + cáOH = CaCO 3 + C β H 5 COOca (Ca=2ca = 40).
Hence phthalic acid should be obtainable by the oxidation of di-derivatives, C 6 H 4 R'R", of benzol (R = CH 3 , C 2 i⅛ 5 , &c.), and indeed two acids, C 6 H 4 (COOH) 2 , can be thus produced, but neither is identical with phthalic.
Terephthalic acid is obtained by the oxidation of ordinary cymol, C H...
pC 6 H 4 'CH 7 ’ 01 ol b er similar “para” bodies with bichromate of potash and sulphuric acid. It is a white powder, quite insoluble in water, sublimable without fusion or dehydration.
Isophthalie acid is obtained similarly from “meta” derivatives, C 6 H 4 R 2 , of benzol, hair-fine needles fusing above 300°, almost insoluble in water, but pretty easily soluble in alcohol.
Ortho-bi-derivatives of benzol ought to give “ortho,” i.e., Laurent’s phthalic acid; but this acid itself is oxidized by the bichrome mixture into CO 2 and H 2 O.
Phthaleins are a most interesting family of coloured derivatives of phthalic anhydride, which were discovered by Baeyer, and soon found their way into the colour industry. As an example we quote phenol-phthalein, obtained by the union of the anhydride with phenol, C 6 H 6 OH = H + C 6 H 4 . OH = H + “Phen.” The phthalein is υ β tl4 C(Phen) 2 ’
Phthalic rest.
and, as will easily be understood, something widely different from the di-phenyl-phthalic ether. Phthalic anhydride and resorcin— one of the three di-hydroxyl derivatives, C 6 H 4 (OH) 2 , of benzol— unite into “fluorescin,” distinguished by the strong fluorescence of its solutions. Tetra-brom-fluorescin, a beautiful red colour, is used industrially as eosin (from Greek iγhs, the morning-red).
9th ed. Encyclopædia Britannica, vol. 18, p. 855
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