(A. Hantzsch). The potassium salt of the iso-diazo hydroxide yields on methylation a nitrogen ether, R·N(CH3)·NO, whilst the silver salt yields an oxygen ether, R·N:N·OCH3. These results point to the conclusion that the iso-diazo hydroxide is a tautomeric substance. The same oxygen ether is formed by the methylation of the silver salt of the normal diazo hydroxide; this points to the conclusion that the isomeric hydroxides, corresponding with the silver derivatives, have the same structural formulae, namely, R·N:N·OH. These oxygen ethers contain the grouping -N:N-, since they couple very readily with the phenols in alkaline solution to form azo compounds (q.v.) (E. Bamberger, Ber., 1895, 28, p. 225); they are also explosive.
By oxidizing potassium benzene iso-diazotate with alkaline potassium ferricyanide, E. Bamberger (Ber., 1894, 27, p. 914) obtained the diazoic acids, R·NH·NO2, substances which he had previously prepared by similarly oxidizing the diazonium salts, by dehydrating the nitrates of primary amines with acetic anhydride, and by the action of nitric anhydride on the primary amines. Concentrated acids convert them into the isomeric nitro-amines, the -NO2 group going into the nucleus in the ortho- or para- position to the amine nitrogen; this appears to indicate that the compounds are nitramines. They behave, however, as tautomeric substances, since their alkali salts on methylation give nitrogen ethers, whilst their silver salts yield oxygen ethers:
Phenyl nitramine, C6H5NH·NO2, is a colourless crystalline solid, which melts at 46° C. Sodium amalgam in alkaline solution reduces it to phenylhydrazine.
Constitution of the Diazo Compounds.—P. Griess (Ann., 1866, 137, p. 39) considered that the diazo compounds were formed by the addition of complex groupings of the type C6H4N2- to the inorganic acids; whilst A. Kekulé (Zeit. f. Chemie, 1866, 2, p. 308), on account of their ready condensation to form azo compounds and their easy reduction to hydrazines, assumed that they were substances of the type R·N:N·Cl. The constitution of the diazonium group -N2·X, may be inferred from the following facts:—The group C6H5N2- behaves in many respects similarly to an alkali metal, and even more so to the ammonium group, since it is capable of forming colourless neutral salts with mineral acids, which in dilute aqueous solution are strongly ionized, but do not show any trace of hydrolytic dissociation (A. Hantzsch, Ber., 1895, 28, p. 1734). Again, the diazonium chlorides combine with platinic chloride to form difficultly soluble double platinum salts, such as (C6H5N2Cl)2·PtCl4; similar gold salts, C6H5N2Cl·AuCl3, are known. Determinations of the electrical conductivity of the diazonium chloride and nitrate also show that the diazonium radical is strictly comparable with other quaternary ammonium ions. For these reasons, one must assume the existence of pentavalent nitrogen in the diazonium salts, in order to account for their basic properties.
The constitution of the isomeric diazo hydroxides has given rise to much discussion. E. Bamberger (Ber., 1895, 28, pp. 444 et seq.) and C. W. Blomstrand (Journ. prakt. Chem., 1896, 53, pp. 169 et seq.) hold that the compounds are structurally different, the normal diazo-hydroxide being a diazonium derivative of the type R·N(∶N)·OH. The recent work of A. Hantzsch and his pupils seems to invalidate this view (Ber., 1894, 27, pp. 1702 et seq.; see also A. Hantzsch, Die Diazoverbindungen). According to Hantzsch the isomeric diazo hydroxides are structurally identical, and the differences in behaviour are due to stereo-chemical relations, the isomerism being comparable with that of the oximes (q.v.). On such a hypothesis, the relatively unstable normal diazo hydroxides would be the syn-compounds, since here the nitrogen atoms would be more easily eliminated, whilst the stable iso-diazo derivatives would be the anti-compounds, thus:
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| Normal hydroxide (Syn-compound) |
Iso hydroxide (Anti-compound) | |
In support of this theory, Hantzsch has succeeded in isolating a series of syn- and anti-diazo-cyanides and -sulphonates (Ber., 1895, 28, p. 666; 1900, 33, p. 2161; 1901, 34, p. 4166). By diazotizing para-chloraniline and adding a cold solution of potassium cyanide, a salt (melting at 29° C.) is obtained, which readily loses nitrogen, and forms para-chlorbenzonitrile on the addition of copper powder. By dissolving this diazocyanide in alcohol and reprecipitating it by water, it is converted into the isomeric diazocyanide (melting at 105-106° C.), which does not yield para-chlorbenzonitrile when treated with copper powder. Similar results have been obtained by using diazotized para-anisidine, a syn- and an anti- compound being formed, as well as a third isomeric cyanide, obtained by evaporating para-methoxy-benzenediazonium hydroxide in the presence of an excess of hydrocyanic acid at ordinary temperatures. This salt is a colourless crystalline substance of composition CH3O·C6H4·N2·CN·HCN·2H2O, and has the properties of a metallic salt; it is very soluble in water and its solution is an electrolyte, whereas the solutions of the syn- and anti- compounds are not electrolytes. The isolation of these compounds is a powerful argument in favour of the Hantzsch hypothesis which requires the existence of these three different types, whilst the Bamberger-Blomstrand view only accounts for the formation of two isomeric cyanides, namely, one of the normal diazonium type and one of the iso-diazocyanide type.
Benzene diazonium hydroxide, although a strong base, reacts with the alkaline hydroxides to form salts with the evolution of heat, and generally behaves as a weak acid. On mixing dilute solutions of the diazonium hydroxide and the alkali together, it is found that the molecular conductivity of the mixture is much less than the sum of the two electrical conductivities of the solutions separately, from which it follows that a portion of the ions present have changed to the non-ionized condition. This behaviour is explained by considering the non-ionized part of the diazonium hydroxide to exist in solution in a hydrated form, the equation of equilibrium being:
On adding the alkaline hydroxide to the solution, this hydrate is supposed to lose water, yielding the syn-diazo hydroxide, which then gives rise to a certain amount of the sodium salt (A. Hantzsch, Ber., 1898, 31, p. 1612),
This assumption also shows the relationship of the diazonium hydroxides to other quaternary ammonium compounds, for most of the quaternary ammonium hydroxides (except such as have the nitrogen atom attached to four saturated hydrocarbon radicals) are unstable, and readily pass over into compounds in which the hydroxyl group is no longer attached to the amine nitrogen; thus the syn-diazo hydroxides are to be regarded as pseudo-diazonium derivatives. (A. Hantzsch, Ber., 1899, 32, p. 3109; 1900, 33, p. 278.) It is generally accepted that the iso-diazo hydroxides possess the oxime structure R·N:N·OH.
Hantzsch explains the characteristic reactions of the diazonium compounds by the assumption that an addition compound is first formed, which breaks down with the elimination of the hydride of the acid radical, and the formation of an unstable syn-diazo compound, which, in its turn, decomposes with evolution of nitrogen (Ber., 1897, 30, p. 2548; 1898, 31, p. 2053).
J. Cain (Jour. Chem. Soc., 1907, 91, p. 1049) suggested a quinonoid formula for diazonium salts, which has been combated by Hantzsch (Ber., 1908, 41, pp. 3532 et seq.). G. T. Morgan and F. M. G. Micklethwaite (Jour. Chem. Soc., 1908, 93, p. 617; 1909, 95, p. 1319) have pointed out that the salts may possess a dynamic formula, Cain’s representing the middle stage, thus:
Diazoamines.—The diazoamines, R·N2·NHR, may be prepared by the action of the primary and secondary amines on the diazonium salts, or by the action of nitrous acid on the free primary amine. In the latter reaction it is assumed that the isodiazohydroxide first formed is immediately attacked by a second molecule of the amine. They are yellow crystalline solids, which do not unite with acids. Nitrous acid converts them, in acid solution, into diazonium salts.
C6H5N2·NHC6H5 + 2HCl + HNO2＝2C6H5N2Cl + 2H2O.
They are readily converted into the isomeric aminoazo compounds, either by standing in alcoholic solution, or by warming with a mixture of the parent base and its hydrochloride; the diazo group preferably going into the para-position to the amino group. When the para-position is occupied, the diazo group takes the ortho-position. H. Goldschmidt and R. U. Reinders (Ber., 1896, 29, p. 1369, 1899) have shown that the transformation is a monomolecular reaction, the velocity of transformation in moderately dilute solution being independent of the concentration, but proportional to the amount of the catalyst present (amine hydrochloride) and to the temperature. It has also been shown that when different salts of the amine are used, their catalytic influence varies in amount and is almost proportional to their degree of ionization in aqueous solution. Diazoaminobenzene, C6H5N2·NHC6H5, crystallizes in golden yellow laminae, which melt at 96° C. and explode at a slightly higher temperature. It is readily soluble in alcohol, ether and benzene. Concentrated hydrochloric acid converts it into chlorbenzene, aniline and nitrogen. Zinc dust and alcoholic acetic acid reduce it to aniline and phenylhydrazine.
Diazoimino compounds, R·N3, may be regarded as derivatives of azoimide (q.v.); they are formed by the action of ammonia on the diazoperbromides, or by the action of hydroxylamine on the diazonium sulphates (J. Mai, Ber., 1892, 25, p. 372; T. Curtius, Ber., 1893, 26, p. 1271). Diazobenzeneimide, C6H5N3, is a yellowish oil of stupefying odour. It boils at 59° C. (12 mm.), and explodes when heated. Concentrated hydrochloric acid decomposes it with formation of