Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

AZO COMPOUNDS

Online Encyclopedia
Originally appearing in Volume V03, Page 82 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

See also:

AZO COMPOUNDS , organic substances of the type R•N:N•R' (where R = an aryl See also:radical and R' = a substituted alkyl, or aryl radical). They may be prepared by the reduction of nitro compounds in alkaline See also:solution (using See also:zinc dust and See also:alkali, or a solution of an alkaline See also:stannite as a reducing See also:agent); by oxidation of hydrazo compounds; or by the coupling of a diazotized amine and any See also:compound of a phenolic or aminic type, providedthat there is a See also:free See also: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 See also:parent See also:base and its hydrochloride. This latter method of formation has been studied by H. See also:Goldschmidt and R. U. Reinders (See also:Bee., 1896, 29, p. 1369), who found that the reaction is monomolecular, and that the velocity See also:constant of the reaction is proportional to the amount of the hydrochloride of the base See also:present and also to the temperature, but is See also: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 See also:salt-forming, See also:acid or basic See also:groups (see See also:DYEING). By oxidizing agents they are converted into azoxy compounds, and by reducing agents into hydrazo compounds or See also:amines. Azo-See also:benzene, C6H5N:NC6H5, discovered by E. See also:Mitscherlich in 1834, may be prepared by reducing See also:nitrobenzene in alcoholic solution with zinc dust and See also:caustic soda; by the condensation of nitrosobenzene with See also:aniline in hot glacial acetic acid solution; or by the oxidation of aniline with See also:sodium hypobromite.

It crystallizes from See also:

alcohol in See also:orange red plates which melt at 68° C. and See also: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 See also:brown or red in See also: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 See also:indulines (q.v.). Amino-azo-benzene, C6H6•N2.C6H4NH2, 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 See also:meta-phenylene diamine. Diamino-azo-benzene (chrysoidine), See also:C6H6.N2•C,H3(See also:NH2)2, first prepared by O. Witt (Ber., 1877, to, p. 656), is obtained by coupling phenyl diazonium chloride with meta-phenylene diamine. It crystallizes in red octahedra and dyes See also:silk and See also:wool yellow.

Triamino-azo-benzene (meta-aminobenzene-azo-meta-phenylene diamine or See also:

Bismarck brown, phenylene brown, vesuvine, See also:Manchester brown), NH2•C6H4•N2•C6H3(NH2)2, is prepared by the See also:action of nitrous acid on meta-phenylene diamine. It forms brown crystals which are readily soluble in hot See also:water, and it dyes mordanted See also:cotton a dark brown. On the See also:composition of the commercial Bismarck brown see E. Tauber 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 See also:group. In these cases it has been shown by H. Goldschmidt and A. Merz (Ber., 1897, 30, p. 67o) 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 See also:tertiary amine with diazobenzene sulphonic acid the reaction takes See also:place between the acid and the base set free by the hydrolytic See also: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. See also:

Buss, Ber., 1897, 30, p. 2075). Methyl orange (helianthin, See also:gold orange, See also:Mandarin orange), (See also:CH3)2N•CsH4 N2•C6H4SO3Na, is the sodium salt of paradimethylaminobenzene-azo-benzene sulphonic acid. It is an orange crystalline See also: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 See also: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 See also: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 See also:attention, some chemists holding that they are true azophenols of the type R•N2•Ri.011, while others look upon them as having a quinonoid structure, i.e. as being quinone hydrazones, type R•NH•N:R1:0.

The first to attack the purely chemical See also:

side were Th. Zincke (Ber., 1883,16, p. 2929; 1884, 17, p. 3026; 1887, 20, p. 3171). and R. Meldola (Jour. Chem. See also:Sac., 1889. 55, pp. 114, 603). Th. Zincke found that the products obtained by coupling a diazonium salt with a-naphthol, and by condensing phenyl-See also:hydrazine with a-naphthoquinone, were identical; whilst Meldola acetylated the azophenols, and split the acetyl See also:pro-ducts by reduction in acid solution, but obtained no satisfactory results.

K. Auwers (Zeit. f. phys. Chem., 1896, 21, p. 355; B-er., 1900, 33, p. 13o2)examined the questionfromthe 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 See also:

act as such. A. Hantzsch (Ber., 1899, 32, pp. 590, 3088) considers that the oxyazo compounds are to be classed as pseudo-acids, possessing in the free See also:condition the configuration of quinone hydrazones, their salts, however, being of the normal phenolic type. J. T. See also:Hewitt (Jour. Cliem.

See also:

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 See also: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 See also:system, a brombenzene-azo-phenol is formed, whilst if it be removed (by the addition of sodium acetate) bromination takes place in the phenolic See also:nucleus; consequently the presence of the See also:mineral acid gives the azo compound a pseudo-quinonoid See also:character, which it does not possess if the mineral acid be removed from the See also:sphere of the reaction. Para-oxyazobenzene (benzene-azo-phenol), C6H5N: N(1)•C6H4• 011(4), is prepared by coupling diazotized aniline with phenol in alkaline solution. It is an orange-red crystalline compound which melts at 1 54° C. Ortho-oxyazobenzene, C61-15N: N (r) See also:C6H4 • OH(2), was obtained in small quantity by E. See also:Bamberger (Ber., 'goo, 33, p. 3189) simultaneously with the para compound, from which it may be separated by See also:distillation in a current of See also: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, C6115N: N(1)C6H4.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-azometa-anisole, from which meta-oxyazobenzene was obtained by See also:

hydrolysis with See also: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.NHR1, are obtained by the action of See also:primary amines on diazonium salts; by the action of nitrous acid on a free primary amine, an isodiazohydroxide 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, See also:nitrogen and an amine. Acid anhydrides replace the imino-See also:hydrogen See also:atom by acidyl radicals, and boiling with water converts them into phenols. They combine with phenyl isocyanate to See also:form See also: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, C6H5•N : N•NHC6H5, was first obtained by P. Griess (See also: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,. See also:

ether and benzene. Diazoimino benzene, CeH5N3, is also known. It may be pre-pared by the action of See also:ammonia on diazobenzene perbromide; by the action of See also:hydroxylamine on a diazonium sulphate (K. Heumann and L. 0economides,Ber., 1887, 20, p. 372); and by the action of phenyihydrazine 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 See also: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 See also:

iron gives azo compounds, and warming with ammonium sulphide gives hydrazo compounds. Concentrated sulphuric acid converts azox'ybenzene into oxyazobenzene (O. Wallach, Ber., 188o, 13, p. 525). Azoxybenzene, (C6H5N)20, 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 See also:united with an aromatic radical on the one See also:hand, and with a radical of the aliphatic See also:series on the other. The most easily obtained mixed azo, compounds are those formed by the See also:union of a diazonium salt with the See also:potassium or sodium salt of a nitroparaffin (V. See also:Meyer, Ber., 1876, 9, p. 384): C6H5N2•NO,+CH3.CH(NC,)K_ = KNO3+C6H5N2•CH(NO2)CH3. Benzene-azo-nitro-ethane.

Those not containing a nitro group may be prepared by the oxidation of the corresponding mixed hydrazo compounds with mercuric See also:

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: C51-I5NH•N : C(NO2)CH,+NaOII=NaNO,+C,H5NH•NH•CO•CH,. Benzene-azo-methane, C,H,.N2•CH3, is a yellow oil which boils at 15o° C. and is readily volatile in steam. Benzene-azoethane, C6H5•N2•C2H5, is a yellow oil which boils at about 18o° C. with more or less decomposition. On See also:standing with 6o % sulphuric acid for some See also:time, it is converted into the isomeric acetaldehyde-phenylhydrazone,C6H5NH•N: CH•CH3 (Ber., 1896, 29, P. 794). The diazo cyanides, C6H5N2•CN, and carboxylic acids, C6H5• N2.COOH, may also be considered as mixed azo derivatives. Diazobenzenecyanide, C6H5N2.CN, is an unstable oil, formed when potassium See also:cyanide is added to a solution of a diazonium salt. Phenyl-azo-carboxylic acid, C61-15•N2.000H, 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 See also:

amide, phenyl-azo-carbonamide, C6H5N2-CONH2, also results from the oxidation of phenylsemicarbazide (Thiele, loc. cit.), and forms reddish-yellow needles which melt at 114 C. When heated with See also:benzaldehyde to 12o° C. it yields diphenyloxytriazole, (C6I15)2CN3C(OH).

End of Article: AZO COMPOUNDS

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
AZO (c. 1150-1230) 		[next]
AZOIMIDE

Site © 2006 - Net Industries