AZO (c. 1150-1230), Italian jurist, was born at Bologna, studied under Joannes Bassianus, and became professor of civil law at Bologna. Azo occupied a very important position among the glossators, and his Summa Codicis and Apparatus ad codicim, which were collected by his pupil, Alessandro de Santo Aegidio, and completed by the additions of Hugolinus and Odofredus, formed a methodical exposition of Roman law, and were of such weight before the tribunals that it used to be said, "Chi non ha Azzo, non vada a palazzo." Azo numbered among his pupils Accursius and Jacobus Balduinus. He died about 1230.
Azo, whose name is sometimes written Azzo and Azzolenus, and who is occasionally described as Azo Soldanus, from the surname of his father, and as Azzo Porcius, is to be distinguished from two other famous Italians of the same name ; viz., Azo Lambertaccius, a canonist of the 13th century, professor of canon law at the University of Bologna, author of Questiones in jus canonicum, and Azo de Ramenghis, a canonist of the 14th century, also a professor of canon law at Bologna, and author of Repetitiones super libro Decretorum.
an important group of organic sub stances which may be prepared by the reduction of nitro-com pounds in alkaline solution (using zinc dust and alkali, or a solu tion of an alkaline stannite as a reducing agent) ; by oxidation of hydrazo-compounds ; or by condensation of a nitroso-compound and an amine, since the substances have the general formula R•N : N•R', where R is an aryl radical and R' a substituted alkyl or an aryl radical. The essential difference between these sub stances and diazo-compounds (q.v.) is that in the azo-compounds the group --N :N--- is attached by both its valencies to different carbon atoms.
Azomethane CH3•N:N•CH3, the simplest azo-compound, is produced by oxidizing dimethylhydrazine (hydrazomethane) with chromic acid (J. Thiele, 1909). It is a colourless gas, condensing to a pale yellow liquid (b.p. 1•5° C) .
The aromatic azo-compounds are intensely coloured, but are not capable of being used as dye-stuffs unless they contain salt forming acid or basic groups (see DYES, SYNTHETIC). By oxi dizing agents they are converted into azoxy-compounds, and by reducing agents into hydrazo-compounds or amines.
Azobenzene discovered by E. Mitscherlich in 1834, may be prepared by reducing nitrobenzene in alcoholic solu tion 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 crys tallizes 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 of the benzene series are prepared by the molecular rearrangement of the diazoamines (see DIAZO-coM POUNDS) when these are warmed with the parent base and its hy drochloride. 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-azobenzene crystal lizes in yellow plates or needles and melts at 126° C. Its con stitution is determined by the fact that by reduction with stan nous chloride it yields aniline and para-phenylenediamine. Diam ino-azobenzene (chrysoidine), first prepared by 0. Witt (1877), is obtained by coupling phenyldiazonium chloride with meta-phenylenediamine. It crystallizes in red octahedra and dyes silk and wool yellow. Bismarck brown (phenylene brown, vesuvine, Manchester brown), which contains the triamino-azobenzene, and a more complex disazo dye (E. Tauber and F. Walder, 1900) is prepared by the action of nitrous acid on meta-phenylenediamine. It forms brown crystals which are readily soluble in hot water, and it dyes mordanted cotton a dark brown. Alkylated amino-azobenzenes 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.
Methyl orange (helianthin, gold orange, Mandarin orange), is the sodium salt of para-dimethyl aminobenzene-azobenzene sulphonic acid. It is an orange crystal line 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. Its constitution follows from the fact that on reduction by stannous chloride in hydrochloric acid solu tion it yields sulphanilic acid and para-aminodimethylaniline.
Methyl red N the sodium salt of o—carboxybenzene-azo-dimethylaniline, is prepared by coupling diazotized anthranilic acid (benzene diazonium carboxylate) (see DIAZO-COMPOUNDS) with dimethylaniline. It is a useful indicator in acidimetry and alkalimetry.
Hydroxyazo-cornpounds.—The hydroxyazo-compounds are pre pared by adding a solution of a diazonium salt to a cold slightly alkaline solution of 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 di rectly into the meta position.
The constitution of the hyroxyazo-compounds has attracted much attention, some chemists holding that they are true azo phenols of the type while others look upon them as having a quinonoid structure, i.e., as being quinone hydrazones of the type R.NH.N :R :0. The first to study the problem on the purely chemical side was Th. Zincke (1883-87) who found that the products obtained by coupling a diazonium salt with a-naphthol and by condensing phenylhydrazine with a-naphthoquinone, were identical; whilst R. Meldola (1889) who acetylated the azo phenols, and split the acetyl products by reduction in acid solu tion obtained evidence of enolic structure. K. Auwers (1896 1900) , examined the question from the physicochemical stand point by determining the freezing-point depressions, the result being that the para-hydroxyazo-compounds gave abnormal de pressions ; and the ortho-hydroxyazo compounds gave normal depressions; Auwers then concluded that the para compounds are phenolic and the ortho compounds are quinone hydrazones or act as such. A. Hantzsch (1899) considered that the hydroxy azo-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, 190o, nitrated para-hydroxyazobenzene with dilute nitric acid and found that it gave a benzene-azo-ortho-nitrophenol, whereas quinones are not attacked by dilute nitric acid. He also found by bromination experiments that the presence of mineral acid gives the azo-compound a pseudo-quinonoid character, which it does not possess if by addition of sodium acetate, the mineral acid be removed from the sphere of the reaction.
Para-hydroxyazobenzene (benzene-azo-phenol) :N( ) (4), is prepared by coupling diazotized aniline with phenol in alkaline solution. It is an orange-red crys talline compound which melts at 1S4° C. Ortho-hydroxyazoben zene, : N (I) (2 ), was obtained in small quantity by E. Bamberger (1900) simultaneously with the para compound, from which it may be separated by its volatility in steam. It crys tallizes in orange-red needles which melt at 82.5-83° C. Meta hydroxyazobenzene :N (1) (3) melting at II 2 114° C. was obtained in 1903 by P. Jacobson by condensing ortho-anisidine with benzene diazonium chloride, the resulting compound then being diazotized and reduced by alcohol to ben zene-azo-meta-anisole, from which meta-oxyazobenzene was ob tained by hydrolysis with aluminium chloride.
Azoxy-compounds, R•N•O.N•R', are usually yellow or red crys talline 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 oxida tion 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 hy droxyazobenzene (0. Wallach, 188o). Azoxybenzene 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.
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 radi cal of the aliphatic series on the other. The most easily obtained mixed azo-compounds are those formed by the union of a diazo nium salt with the potassium or sodium salt of a nitroparaffin (V. Meyer, 1876) : Those not containing a nitro-group may be prepared by the oxida tion of the corresponding mixed hydrazo-compounds with mer curic oxide. E. Bamberger (1898) has shown that the nitro alkyl derivatives behave as though they possess the constitution of hydrazones, for on heating with dilute alkalis they split more or less readily into alkaline nitrite and an acid hydrazide : Benzene-azomethane, is a yellow oil which boils at 15o° C. and is readily volatile in steam. Benzene-azoethane, is a yellow oil which boils at about 180° C with more or less decomposition. On standing with 6o% sulphuric acid for some time, it is converted into the isomeric acetaldehyde phenylhydrazone, C6H5NH.N Phenyl-azocarboxylic acid, is obtained in the form of its potassium salt when phenylsemicarbazide is oxidized with potassium permanganate in alkaline solution (J. Thiele, 1895). It crystallizes in orange-red needles and is decomposed by water. (G. T. M.)