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Originally appearing in Volume V08, Page 174 of the 1911 Encyclopedia Britannica.
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DIAZO COMPOUNDS, in organic chemistry, compounds of the type R•N•2•X (where R = a hydrocarbon radical, and X = an acid radical or a hydroxyl group). These compounds may be divided into two classes, namely, the true diazo compounds, characterized by the grouping —N=N—, and the diazonium compounds, characterized by the grouping N:N<. The diazonium compounds were first discovered by P. Griess (Ann., 1858, 106, pp. 123 et seq.), and may be prepared by the action of nitrous fumes on a well-cooled solution of a salt of a primary amine, C6H5NH2•HNO3 + HNO2 = C6H5N2.NO3 + 2H2O, or, as is more usually the case (since the diazonium salts themselves are generally used only in aqueous solution) by the addition of a well-cooled solution of potassium or sodium nitrite to a well-cooled dilute acid solution of the primary amine. In order to isolate the anhydrous diazonium salts, the method of E. Knoevenagel (Ber., 1890, 23, p. 2094) may be employed. In this process the amine salt is dissolved in absolute alcohol and diazotized by the addition of amyl nitrite; a crystalline pre- cipitate of the diazonium salt is formed on standing, or on the addition of a small quantity of ether: The diazonium salts are also formed by the action of zinc-dust and acids on the nitrates of primary amines (R. Mohlau,Ber., 1883, 16, p. 3080), and by the action of hydroxylamine on nitrosobenzenes. They are colourless crystalline solids which turn brown on exposure. They dissolve easily in water, but only to a slight extent in alcohol and ether. They are very unstable, exploding violently when heated or rubbed. Benzene diazonium nitrate, C6HsN(NO3):N, crystallizes in long silky needles. The sulphate and chloride are similar, but they are not quite so unstable as the nitrate. The bromide may be prepared by the addition of bromine to an ethereal solution of diazo-amino-benzene (tribromaniline remaining in solution). By the addition of potassium bromide and bromine water to diazonium salts they are converted into a perbromide, e.g. C6H5N2Br3, which crystallizes in yellow plates. The diazonium salts are characterized by their great reactivity and consequently are important reagents in synthetical processes, since by their agency the amino group in a primary amine may be exchanged for other elements or radicals. The chief reactions are as follows: 1. Replacement of—NI-I, by—OH:—The amine is diazotized and the aqueous solution of the diazonium salt is heated, nitrogen being eliminated and a phenol formed. 2. Replacement of —NH2 by halogens and by the — CN and CNO groups:—The diazonium salt is warmed with an acid solution of the corresponding cuprous salt (T. Sandmeyer, Bee., 1884, 17, p. 2650), or with copper powder (L. Gattermann, Ber., 1890, 23, p. 1218; 1892, 25, p. 1074). In the case of iodine, the substitution is effected by adding a warm solution of potassium iodide to the diazonium solution, no copper or cuprous salt being necessary; whilst for the production of nitriles a solution of potassium cuprous cyanide is used. This reaction (the so-called Sandmeyer " reaction) has been investigated by A. Hantzsch and J. W. Blagden (Ber.,1900,33,p•254 ), who consider that three simultaneous reactions occur, namely, the formation of labile double salts which decompose in such a fashion that the radical attached to the copper atom wanders to the aromatic nucleus; a catalytic action, in which nitrogen is eliminated and the acid radical attaches itself to the aromatic nucleus; and finally, the formation of azo compounds. 3. Replacement of — NH2 by — NO2:—A well-cooled concentrated solution of potassium mercuric nitrate is added to a cooled solution of benzene diazonium nitrate, when the crystalline salt 2C6H5N2•NO3, Hg(NO2)2 is precipitated. On warming this with copper powder, it gives a quantitative yield of nitrobenzene (A. Hantzsch, Ber., 1900, 33, p. 2551). 4. Replacement of —NH2 by hydrogen:—This exchange is brought about, in some cases, by boiling the diazonium salt with alcohol; but I. Remsen and his pupils (Amer. Chem. Journ., 1888, 9, pp. 389 et seq.) have shown that the main product of this reaction is usually a phenolic ether. This reaction has also been investigated by A. Hantzsch and E. Jochem (Ber., 1901, 34, p. 3337), who arrived at the conclusion that the normal decomposition of diazonium salts by alcohols results in the formation of phenolic ethers, but that an increase in the molecular weight of the alcohol, or the accumulation of negative groups in the aromatic nucleus, diminishes the yield of the ether and increases the amount of the hydrocarbon formed. The replacement is more readily brought about by the use of sodium stannite (P. Friedlander, Ber., 1889, 22, p. 587), or by the use of a concentrated solution of hypophosphorous acid (J. Mai, Ber., 1902, 35, p. 162). A. Hantzsch (Ber., 1896,29,p. 947 ;1898, 31, p. 1253) has shown that the chlor- and brom- diazoniumthiocyanates, when dissolved in alcohol containing a trace of hydrochloric acid, become converted into the isomeric thiocyanbenzene diazonium chlorides and bromides. This change only occurs when the halogen atom is in the ortho- or para- position to the — N2— group. Metallic Diazo Derivatives.—Benzene diazonium chloride is decomposed by silver oxide in aqueous solution, with the formation of benzene diazonium hydroxide, C,H5.N(OH)i N. This hydroxide, although possessing powerful basic properties, is unstable in the presence of alkalis and neutralizes them, being converted first into the isomeric benzene-diazotic acid, the potassium salt of which is obtained when the diazonium chloride is added to an excess of cold concentrated potash (A. Hantzsch and W. B. Davidson, Bee., 1898, 31, p. 1612). Potassium benzene diazotate, CeH6N2.OK, crystallizes in colourless silky needles. The free acid is not known ; by the addition of the potassium salt to 50% acetic acid at — 20° C., the acid anhydride, benzene diazo oxide, (CsH5N2)2O, is obtained as a very unstable, yellow, insoluble compound, exploding spontaneously at o° C. Strong acids convert it into a diazonium salt, and potash converts it into the diazotate. On the constitution, of these anhydrides see E. Bamberger, Bee., 1896, 29, p. 446, and A. Hantzsch, Ber., 1896, 29, p. 1067 ; 1898, 31, p. 636. By the addition of the diazonium salts to a hot concentrated solution of a caustic alkali, C. Schraube and C. Schmidt(Ber., 1894, 27, p. 52o)obtained an isomer of potassium benzene diazotate. These iso-diazotates are formed much more readily when the aromatic nucleus in the diazonium salt contains negative radicals. Potassium benzene iso-diazotate resembles the normal salt, but is more stable, and is more highly ionized. Car-bon dioxide converts it into phenyl nitrosamine, C6H6NH•NO (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: potassium salt —~ R•N(CH3).NO2 nitramine. R•NH'NO2 silver salt R'N: N'O'OCH3 diazoate. 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 thediazocompoundswereformedby the addition of complex groupings of the type C6H4N2— to the inorganic acids; whilst A. Kekule (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 (C6H5N2CI)2•PtC14; similar gold salts, C6H5N2Cl•AuC13, 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 diazohydroxide 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: R.N R•N HON N•OH Normal hydroxide Iso hydroxide (Syn-compound) (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 parachlorbenzonitrile 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-methoxybenzenediazonium hydroxide in the presence of an excess of hydro-cyanic acid at ordinary temperatures. This salt is a colourless crystalline substance of composition CH 30.C6H4.N2.CN•HCN•2H20, 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 synand 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 forma- tion 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:
DIAZOMATA (Gr. 6t4coµa, a girdle)

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