# 1911 Encyclopædia Britannica/Azo Compounds

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​AZO COMPOUNDS, organic substances of the type R·N:N·R′ (where R = an aryl radical and R′ = a substituted alkyl, or aryl radical). They may be prepared by the reduction of nitro compounds in alkaline solution (using zinc dust and alkali, or a solution of an alkaline stannite as a reducing agent); by oxidation of hydrazo compounds; or by the coupling of a diazotized amine and any compound of a phenolic or aminic type, provided that there is a free para position in the amine or phenol. They may also be obtained by the molecular rearrangement of the diazoamines, when these are warmed with the parent base and its hydrochloride. This latter method of formation has been studied by H. Goldschmidt and R. U. Reinders (Ber., 1896, 29, p. 1369), who found that the reaction is monomolecular, and that the velocity constant of the reaction is proportional to the amount of the hydrochloride of the base present and also to the temperature, but is independent of the concentration of the diazoamine. The azo compounds are intensely coloured, but are not capable of being used as dyestuffs unless they contain salt-forming, acid or basic groups (see Dyeing). By oxidizing agents they are converted into azoxy compounds, and by reducing agents into hydrazo compounds or amines.
Azo-benzene, C$_6$H$_5$N:NC$_6$H$_5$, discovered by E. Mitscherlich in 1834, may be prepared by reducing nitrobenzene in alcoholic solution with zinc dust and caustic soda; by the condensation of nitrosobenzene with aniline in hot glacial acetic acid solution; or by the oxidation of aniline with sodium hypobromite. It crystallizes from alcohol in orange red plates which melt at 68° C. and boil at 293° C. It does not react with acids or alkalis, but on reduction with zinc dust in acetic acid solution yields aniline.
Amino-azo Compounds may be prepared as shown above. They are usually yellowish brown or red in colour, the presence of more amino groups leading to browner shades, whilst the introduction of alkylated amino groups gives redder shades. They usually crystallize well and are readily reduced. When heated with aniline and aniline hydrochloride they yield indulines (q.v.). Amino-azo-benzene, C$_6$H$_5$·N$_2$·C$_6$H$_4$NH$_2$, crystallizes in yellow plates or needles and melts at 126° C. Its constitution is determined by the facts that it may be prepared by reducing nitro-azo-benzene by ammonium sulphide and that by reduction with stannous chloride it yields aniline and meta-phenylene diamine. Diamino-azo-benzene (chrysoidine), C$_6$H$_5$·N$_2$·C$_6$H$_3$(NH$_2$)$_2$, first prepared by O. Witt (Ber., 1877, 10, p. 656), is obtained by coupling phenyl diazonium chloride with meta-phenylene diamine. It crystallizes in red octahedra and dyes silk and wool yellow. Triamino-azo-benzene (meta-aminobenzene-azo-meta-phenylene diamine or Bismarck brown, phenylene brown, vesuvine, Manchester brown), NH$_2$·C$_6$H$_4$·N$_2$·C$_6$H$_3$(NH$_2$)$_2$, is prepared by the action of nitrous acid on meta-phenylene diamine. It forms brown crystals which are readily soluble in hot water, and it dyes mordanted cotton a dark brown. On the composition of the commercial Bismarck brown see E. Täuber and F. Walder (Ber., 1897, 30, pp. 2111, 2899; 1900, 33, p. 2116). Alkylated amino-azo-benzenes are also known, and are formed by the coupling of diazonium salts with alkylated amines, provided they contain a free para position with respect to the amino group. In these cases it has been shown by H. Goldschmidt and A. Merz (Ber., 1897, 30, p. 670) that the velocity of formation of the amino-azo compound depends only on the nature of the reagents and not on the concentration, and that in coupling the hydrochloride of a tertiary amine with diazobenzene sulphonic acid the reaction takes place between the acid and the base set free by the hydrolytic dissociation of its salt, for the formation of the amino-azo compound, when carried out in the presence of different acids, takes place most rapidly with the weakest acid (H. Goldschmidt and F. Buss, Ber., 1897, 30, p. 2075).
Methyl orange (helianthin, gold orange, Mandarin orange), (CH$_3$)$_2$N·C$_6$H$_4$·N$_2$·C$_6$H$_4$SO$_3$Na, is the sodium salt of para-dimethylaminobenzene-azo-benzene sulphonic acid. It is an orange crystalline powder which is soluble in water, forming a yellow solution. The free acid is intensely red in colour. Methyl orange is used largely as an indicator. The constitution of methyl orange follows from the fact that on reduction by stannous chloride in hydrochloric acid solution it yields sulphanilic acid and para-aminodimethyl aniline.
Oxyazo Compounds.—The oxyazo compounds are prepared by adding a solution of a diazonium salt to a cold slightly alkaline solution of a phenol. The diazo group takes up the para position ​with regard to the hydroxyl group, and if this be prevented it then goes into the ortho position. It never goes directly into the meta position.
The constitution of the oxyazo compounds has attracted much attention, some chemists holding that they are true azophenols of the type R·N$_2$·R$_1$·OH, while others look upon them as having a quinonoid structure, i.e. as being quinone hydrazones, type R·NH·N:R$_1$:O. The first to attack the purely chemical side were Th. Zincke (Ber., 1883,16, p. 2929; 1884, 17, p. 3026; 1887, 20, p. 3171) and R. Meldola (Jour. Chem. Soc., 1889, 55, pp. 114, 603). Th. Zincke found that the products obtained by coupling a diazonium salt with α-naphthol, and by condensing phenyl-hydrazine with α-naphthoquinone, were identical; whilst Meldola acetylated the azophenols, and split the acetyl products by reduction in acid solution, but obtained no satisfactory results. K. Auwers (Zeit. f. phys. Chem., 1896, 21, p. 355; Ber., 1900, 33, p. 1302) examined the question from the physico-chemical standpoint by determining the freezing-point depressions, the result being that the para-oxyazo compounds give abnormal depressions and the ortho-oxyazo compounds give normal depressions; Auwers then concluded that the para compounds are phenolic and the ortho compounds are quinone hydrazones or act as such. A. Hantzsch (Ber., 1899, 32, pp. 590, 3089) considers that the oxyazo compounds are to be classed as pseudo-acids, possessing in the free condition the configuration of quinone hydrazones, their salts, however, being of the normal phenolic type. J. T. Hewitt (Jour. Chem. Soc., 1900, 77, pp. 99 et seq.) nitrated para-oxyazobenzene with dilute nitric acid and found that it gave a benzene azo-ortho-nitrophenol, whereas quinones are not attacked by dilute nitric acid. Hewitt has also attacked the problem by brominating the oxyazobenzenes, and has shown that when the hydrobromic acid produced in the reaction is allowed to remain in the system, a brombenzene-azo-phenol is formed, whilst if it be removed (by the addition of sodium acetate) bromination takes place in the phenolic nucleus; consequently the presence of the mineral acid gives the azo compound a pseudo-quinonoid character, which it does not possess if the mineral acid be removed from the sphere of the reaction.
Para-oxyazobenzene (benzene-azo-phenol), C$_6$H$_5$N:N(1)·C$_6$H$_4$·OH(4), is prepared by coupling diazotized aniline with phenol in alkaline solution. It is an orange-red crystalline compound which melts at 154° C. Ortho-oxyazobenzene, C$_6$H$_5$N:N(1)C$_6$H$_4$·OH(2), was obtained in small quantity by E. Bamberger (Ber., 1900, 33, p. 3189) simultaneously with the para compound, from which it may be separated by distillation in a current of steam, the ortho compound passing over with the steam. It crystallizes in orange-red needles which melt at 82.5–83° C. On reduction with zinc dust in dilute sal-ammoniac solution, it yields ortho-aminophenol and aniline. Meta-oxyazobenzene, C$_6$H$_5$N:N(1)C$_6$H$_4$·OH(3), was obtained in 1903 by P. Jacobson (Ber., 1903, 36, p. 4093) by condensing ortho-anisidine with diazo benzene, the resulting compound being then diazotized and reduced by alcohol to benzene-azo-meta-anisole, from which meta-oxyazobenzene was obtained by hydrolysis with aluminium chloride. It melts at 112–114° C. and is easily reduced to the corresponding hydrazo compound.
Diazo-Amines.—The diazo-amines, R·N:N·NHR$_1$, are obtained by the action of primary amines on diazonium salts; by the action of nitrous acid on a free primary amine, an iso-diazohydroxide being formed as an intermediate product which then condenses with the amine; and by the action of nitrosamines on primary amines. They are crystalline solids, usually of a yellow colour, which do not unite with acids; they are readily converted into amino-azo compounds (see above) and are decomposed by the concentrated halogen acids, yielding haloid benzenes, nitrogen and an amine. Acid anhydrides replace the imino-hydrogen atom by acidyl radicals, and boiling with water converts them into phenols. They combine with phenyl isocyanate to form urea derivatives (H. Goldschmidt, Ber., 1888, 21, p. 2578), and on reduction with zinc dust (preferably in alcoholic acetic acid solution) they yield usually a hydrazine and an amine. Diazoamino benzene, C$_6$H$_5$·N:N·NHC$_6$H$_5$, was first obtained by P. Griess (Ann., 1862, 121, p. 258). It crystallizes in yellow laminae, which melt at 96° C. and explode at slightly higher temperatures. It is readily soluble in alcohol, ether and benzene.
Diazoimino benzene, C$_6$H$_5$N$_3$, is also known. It may be prepared by the action of ammonia on diazobenzene perbromide; by the action of hydroxylamine on a diazonium sulphate (K. Heumann and L. Oeconomides, Ber., 1887, 20, p. 372); and by the action of phenylhydrazine on a diazonium sulphate. It is a yellow oil which boils at 59° C. (12 mm.), and possesses a stupefying odour. It explodes when heated. Hydrochloric acid converts it into chloraniline, nitrogen being eliminated; whilst boiling sulphuric acid converts it into aminophenol.
Azoxy Compounds, R·N·O·N·R′, are usually yellow or red crystalline solids which result from the reduction of nitro or nitroso compounds by heating them with alcoholic potash (preferably using methyl alcohol). They may also be obtained by the oxidation of azo compounds. When reduced (in acid solution) they yield amines; distillation with reduced iron gives azo compounds, and warming with ammonium sulphide gives hydrazo compounds. Concentrated sulphuric acid converts azoxybenzene into oxyazobenzene (O. Wallach, Ber., 1880, 13, p. 525). Azoxybenzene, (C$_6$H$_5$N)$_2$O, crystallizes from alcohol in yellow needles, which melt at 36° C. On distillation, it yields aniline and azobenzene. Azoxybenzene is also found among the electro-reduction products of nitrobenzene, when the reduction is carried out in alcoholic-alkaline solution.
The mixed azo compounds are those in which the azo group ·N:N· is united with an aromatic radical on the one hand, and with a radical of the aliphatic series on the other. The most easily obtained mixed azo compounds are those formed by the union of a diazonium salt with the potassium or sodium salt of a nitroparaffin (V. Meyer, Ber., 1876, 9, p. 384):
* C$_6$H$_5$N$_2$·NO$_3$ + CH$_3$·CH(NO$_2$)K＝KNO$_3$ + C$_6$H$_5$N$_2$·CH(NO$_2$)CH$_3$.
* Benzene-azo-nitro-ethane.
Those not containing a nitro group may be prepared by the oxidation of the corresponding mixed hydrazo compounds with mercuric oxide. E. Bamberger (Ber., 1898, 31, p. 455) has shown that the nitro-alkyl derivatives behave as though they possess the constitution of hydrazones, for on heating with dilute alkalies they split more or less readily into an alkaline nitrite and an acid hydrazide:
C$_6$H$_5$NH·N:C(NO$_2$)CH$_3$ + NaOH＝NaNO$_2$ + C$_6$H$_5$NH·NH·CO·CH$_3$.
Benzene-azo-methane, C$_6$H$_5$·N$_2$·CH$_3$, is a yellow oil which boils at 150° C. and is readily volatile in steam. Benzene-azo-ethane, C$_6$H$_5$·N$_2$·C$_2$H$_5$, is a yellow oil which boils at about 180° C. with more or less decomposition. On standing with 60% sulphuric acid for some time, it is converted into the isomeric acetaldehyde-phenylhydrazone, C$_6$H$_5$NH·N:CH·CH$_3$ (Ber., 1896, 29, p. 794).
The diazo cyanides, C$_6$H$_5$N$_2$·CN, and carboxylic acids, C$_6$H$_5$·N$_2$·COOH, may also be considered as mixed azo derivatives. Diazobenzenecyanide, C$_6$H$_5$N$_2$·CN, is an unstable oil, formed when potassium cyanide is added to a solution of a diazonium salt. Phenyl-azo-carboxylic acid, C$_6$H$_5$·N$_2$·COOH, is obtained in the form of its potassium salt when phenylsemicarbazide is oxidized with potassium permanganate in alkaline solution (J. Thiele, Ber., 1895, 28, p. 2600). It crystallizes in orange-red needles and is decomposed by water. The corresponding amide, phenyl-azo-carbonamide, C$_6$H$_5$N$_2$·CONH$_2$, also results from the oxidation of phenylsemicarbazide (Thiele, loc. cit.), and forms reddish-yellow needles which melt at 114° C. When heated with benzaldehyde to 120° C. it yields diphenyloxytriazole, (C$_6$H$_5$)$_2$CN$_3$C(OH).