# 1911 Encyclopædia Britannica/Phenazine

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​PHENAZINE (Azophenylene), C$_{12}$H$_8$N$_2$, , in organic chemistry, the parent substance of many dyestuffs, e.g. the eurhodines, toluylene red, indulines and safranines. It is a dibenzoparadiazine having the formula given below. It may be obtained by distilling barium azobenzoate (A. Claus, Ber., 1873, 6, p. 723); by passing aniline vapour over lead oxide, or by the oxidation of dihydrophenazine, which is prepared by heating pyrocatechin with orthophenylene diamine (C. Ris, Ber., 1886, 19, p. 2206). It is also formed when ortho-aminodiphenylamine is distilled over lead peroxide (O. Fischer and E. Hepp). It crystallizes in yellow needles which melt at 171° C., and are only sparingly soluble in alcohol. Sulphuric acid dissolves it, forming a deep-red solution. The more complex phenazines, such as the naphthophenazines, naphthazines and naphthotolazines, may be prepared by condensing ortho-diamines with ortho-quinones (O. Hinsberg, Ann., 1887, 237, p. 340); by the oxidation of an ortho-diamine in the presence of α-naphthol (O. Witt), and by ​the decomposition of ortho-anilido-(-toluidido- &c.)-azo compounds with dilute acids. If alkyl or aryl-ortho-diamines be used azonium bases are obtained. The azines are mostly yellow in colour, distil unchanged and are stable to oxidants. They add on alkyl iodides readily, forming alkyl azonium salts.
By the entrance of amino or hydroxyl groups into the molecule dyestuffs are formed. The mono-amino derivatives or eurhodines are obtained when the arylmonamines are condensed with ortho-amino -zo compounds; by condensing quinone dichlorimide or para-nitrosodimethyl aniline with monamines containing a free, para position, or by oxidizing ortho-hydroxydiaminodiphenylamines (R. Nietzki, Ber., 1895, 28, p. 2976; O. Fischer, ibid., 1896, 29, p. 1874) The are yellowish-red solids, which behave as weak bases, their salts undergoing hydrolytic dissociation in aqueous solution When heated with concentrated hydrochloric acid the amino group is replaced by the hydroxyl group and the phenolic eurhodols are produced.
The symmetrical diaminophenazine is the parent substance of the important dyestuff toluylene red or dimethyldiaminotoluphenazine It is obtained by the oxidation of orthophenylene diamine with ferric chloride; when a mixture of para-aminodimethylaniline and meta-toluylenediamine is oxidized in the cold, toluylene blue, an indamine, being formed as an intermediate product and passing into the red when boiled; and also by the oxidation of dimethylparaphenylene diamine with metatoluylene diamine. It crystallizes in orange-red needles and its alcoholic solution fluoresces strongly. It dyes silk and mordanted cotton a fine scarlet. It is known commercially as neutral red. For the phenazonium salts see Safranine.
Phenazone is an isomer of phenazine, to which it bears the same relation that plienanthrene bears to anthracene. It is formed by reducing diortho-dinitrodiphenyl with sodium amalgam and methyl alcohol, or by heating diphenylene-ortho-dihydrazine with hydrochloric acid to 150° C. It crystallizes in needles which melt at 156° C. Potassium permanganate oxidizes it to pyridazine tetracarboxylic acid.
* Phenazine. Phenazone.