AROMATIC DIAZONIUM SALTS In diazotizing aniline and other aromatic amines the base is first converted into its salt, usually the hydrochloride, by the addition of excess of acid (3 molecular proportions of hydro chloric acid). The solution or suspension is cooled and aqueous sodium nitrite is added with stirring, when the nitrous acid liber ated substitutes one nitrogen atom for three hydrogen atoms present in the molecule of the organic salt : It is on account of this replacement of hydrogen by nitrogen, which becomes attached to the nitrogen atom already present in the molecule, that the products now containing a double portion of nitrogen (azote) are termed diazo-compounds. Benzenediazon ium chloride, represented by the above formula, is an extremely soluble salt of explosive character, and for ordinary synthetic and industrial purposes it is not necessary to isolate it. This and other diazonium salts have, however, been prepared in an an hydrous condition by diazotizing the amine salt in alcohol or glacial acetic acid with methyl, ethyl or amyl nitrite, when the diazonium salt is precipitated on the addition of ether (E. Knoevenagel, E. Bamberger, A. Hantzsch).
Diazonium salts have been obtained with a large variety of acid radicals but, being endothermic compounds, they are all liable to explode in the dry state ; and since the diazonium chromates and perchlorates detonate with considerable violence, these salts have been suggested for use as explosives. When a diamine not containing its amino-groups in either ortho- or peri positions is wholly diazotized a bisdiazonium salt results. If the two amino-groups are in the same nucleus a mixture of mono and bisdiazonium salts is frequently produced, as in the case of paraphenylenediamine hydrochloride. When the amino-groups are in different aromatic rings, the bisdiazonium salt is produced with much greater facility. Benzidine, dianisidine and tolidine are the most outstanding examples of readily diazotizable dia mines, and their bisdiazonium salts are largely employed in the manufacture of certain azo-dyes which, having considerable affinity for cotton fibre, are termed direct cotton dyes.
In addition to the employment of diazonium salts in colour making, these reactive compounds are utilized in many synthetic operations both in the laboratory and in the works. Their appli cation to synthesis depends on the fact that through the diazo reaction the amino-group originally present in an aromatic amine becomes replaceable by other elementary or compound radicals.
(3) Replacement of the diazo-group by halogen, cyanide, cyanate and thiocyanate radicals. Fluorine may be introduced into the aromatic nucleus by treating diazonium salts or diazo amino-compounds (see below) with hydrofluoric acid. A more convenient method depends on the use of diazonium borofluorides (G. Balz and G. Schiemann, 1927), which are readily obtainable in a solid state. When heated these salts decompose in the following manner and introduce fluorine into the aromatic nucleus: Chlorine and bromine radi cals are introduced into aromatic nuclei by warming diazonium salts with acid solutions of the corresponding cuprous salt (T. Sandmeyer, 1884) or with copper powder (L. Gattermann, 189o) . Iodine is readily substituted for the diazo-group by adding potas sium iodide to the solution of diazonium sulphate. Appropriate modifications of the Sandmeyer and Gattermann reactions fur nish means for the introduction of cyanide, cyanate, thiocyanate and selenocyanate radicals into the aromatic nucleus.
(4) Replacement of the diazo-group by sulphur and sulphur containing radicals. When copper powder is added to a solution of diazonium sulphate saturated with sulphur dioxide, nitrogen is eliminated and an aromatic sulphinic acid is produced. Aromatic thiophenols or disulphides are obtained by adding potassium xanthate, to diazonium salts with subsequent hydrolysis of the resulting aromatic xanthate (Leuckart, 1887).
(5) Replacement of the diazo-group by a nitro-group. Potas sium mercuric nitrite gives with aqueous benzenediazonium nitrate the double salt which on being warmed with copper powder yields nitrobenzene (A. Hantzsch, 1900). The treatment of a diazonium sulphate with cuprosocupric sulphite and alkali nitrite leads to exchange of the diazo-radical for the nitro-group.
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.
This conception of diazonium structure fails, however, to ex press one important point, namely, the dependence of the stabil ity of the diazo-group on its attachment to an unsaturated organic radical. Accordingly, J. C. Cain proposed an alternative para quinonoid formula for diazonium salts, and subsequently G. T. Morgan and F. M. G. Micklethwait 0908) developed the idea that diazonium salts may possess a dynamic constitution, Cain's structure representing the middle phase of the molecular oscilla tion, This idea of a rhythmic change of structure in diazonium salts is comparable with the dynamic conception of the benzene molecule advanced by Kekule in propounding his cyclic theory of the con stitution of aromatic hydrocarbons. These quinonoid formula tions are- supported by the fact that, although the simpler ben zenoid diazonium salts are colourless, yet in the diphenyl, naph thalene and carbazole series such salts are frequently coloured.
Non-aromatic Diazonium Salts.—The foregoing conception of the structure of diazonium salts, which bases their existence on the unsaturated nature of associated organic complex, derives additional support from the fact that many non-aromatic amines containing unsaturated organic groups are diazotizable. Among the simplest bases which display this property are aminotriazole (I.), the aminopyrazoles (II.) and aminoisoxazole (III.).
It will be noticed that the grouping Ni12.Cis common to all diazotizable amines.
Internal Diazonium Salts.—Sulphonated amines of the ben zene and naphthalene series give rise to industrially important diazo-derivatives which are erroneously termed diazo-sulphonic acids although actually internal diazonium sulphonates, formerly formulated as cyclic compounds (I.).
But in accordance with current views on the electronic nature of valency, such substances are more correctly represented as con taining two bound ions, the two groups N2 and SO3 being associ ated, not by a valency linking, but by the transfer of an electron from the diazonium group to the sulphonate ion (II.). Somewhat allied to these internal diazonium sulphonates are the internal diazonium carboxylates (III.) and the diazo-oxides derived from ortho- and para-aminophenols.
Internal Diazo-oxides.—These diazo-oxides have been for mulated either as cyclic derivatives (I. and III.) or as quinone diazides (II. and IV.).
Two outstanding facts support the foregoing quinone-diazide formulae. These diazo-oxides are all yellow substances and they are never obtained from meta-aminophenols in which quinonoid formulation would be impossible. A similar group of diazoimines and diazoimides has been recorded (G. T. Morgan and F. M. G. Micklethwait and others, 19°8-18). These substances may be rep resented by two alternative formulae (V. and VI.).
The latter formulation derives support from the circumstance that the diazoimines and diazoimides are yellow and obtainable only from para-diamines and not from meta-diamines.
Cyclic Diazo-compounds.—When nitrous acid reacts with the salt of an orthodiamine or a peridiamine (see NAPHTHALENE), cyclic diazo-compounds are produced, ortho-phenylenediamine yielding phenylene-o-diazoimine (aziminobenzene, Formula I.), and peri- or :8-naphthylenediamine giving rise to naphthylene :8-diazimine (I :8-aziminonaphthalene, Formula II.).
The nitrogenous rings in these cyclic diazo-compounds are very inert and the substances do not couple with phenols or amines to yield azo-dyes.
Metallic Derivatives of Diazo-compounds.—In 1894 Schraube and Schmidt found that by adding aqueous p-nitro benzenediazonium chloride to warm sodium hydroxide solution a well-defined salt separated in yellowish bronzy leaflets. Its com position corresponded with the formula NO,C,,HeNNa.NO,HD. When treated with dilute acetic acid this salt yields a nitro samine, NO,C.HeNH.NO, but with hydrochloric acid it reverts to the diazonium chloride, NO,C6H,N,C1. The yellow sodium salt is a very stable substance and has been employed industrially in the production of para-nitraniline red. Homologues of this metallic diazo-salt are used increasingly in conjunction with Naphthol AS (see NAPHTHALENE) for dyeing cotton in fast red shades.
These stable metallic diazo-salts are produced from diazonium salts and warm caustic alkali, but another less stable series of metallic diazo-salts is formed by the use of cold alkali. A. Hantzsch suggested that the two series were stereoisomeric, but this view is not universally held.
Diazo-cyanides.—This group of substances having the gen eral formula Ar.N2.CN, where Ar is an aromatic group, are diazo compounds in which the azo-group is attached on both sides to carbon. In this respect they represent an intermediate group be tween ordinary diazo-derivatives and azo-compounds (q.v.). Moreover, the diazo-cyanides are of interest as furnishing the best example of Hantzsch's theory of stereoisomeric diazo-compounds. Thus from p-anisidine he obtained three isomeric diazo-cyanides: the colourless diazonium cyanide, an electrolyte obtained in the form of a double salt, CH2O.C.H,N,CN,HCN,2H20, and two red dish insoluble diazo-cyanides, the unstable syn-form melting at 50, which changes gradually to the stable anti-form melting at 121°.
Diazonium cyanide syn-Diazocyanide anti-Diazocyanide Similar stereoisomeric relationships have been detected but with less certainty among diazo-sulphonates, Ar.N:N.So3K.