V. O. Lukashevich, E. S. Lisitsyna

On the Planarity of Molecules of Diazoamino Compounds

(Presented by Academician V. A. Kazanskii, July 2, 1964)

Diazoamino compounds of the aromatic series (triazenes) are, as is known, colored substances characterized by maximum absorption of radiant energy in the region 350–365 mµ. In earlier works (¹, ²) it is mentioned that 2,4,6—2′,4′,6′-hexachloro- and 2,4,6—2′,4′,6′-hexabromodiazoaminobenzene are colorless. Starting from chloro- and bromoanilines, we synthesized a large number of diazoamino compounds (with identical benzene nuclei) and became convinced that, alongside colored compounds, there are also quite a few colorless compounds.

Fig. 1. Absorption spectra of diazoamino compounds in various solvents: solid curves—in n-hexane, dashed—in alcohol, dotted—in cyclohexane; 1, 2, 3—2,4,6—2′,4′,6′-hexabromodiazoaminobenzene; 4, 5, 6—2,3,4—2′,3′,4′-hexachlorodiazoaminobenzene.

It is interesting to note that in the colorless compounds the halogen atoms occupy all four possible o-positions with respect to both nitrogen atoms; the situation does not change when a larger number of substituents is present. When, however, 2 or all 4 o-positions with respect to both nitrogen atoms are free from substituents—halogens—the diazoamino compound has a more or less deep yellow color.

Their electronic absorption spectra are characteristic: whereas the colored substances have a clearly expressed absorption maximum in the region 350–365 mµ, as well as in the region 240–260 mµ, the colorless ones are represented by more or less smooth curves with a small maximum in the region 270–280 mµ, or none at all.

Fig. 2. Absorption spectra of colorless diazoamino compounds in n-hexane:
a—2,4,6—2′,4′,6′-hexachloro-,
b—2,3,4,6—2′,3′,4′,6′-octachloro-,
c—2,3,5,6—2′,3′,5′,6′-octachloro-,
d—2,4,6—2′,4′,6′-hexamethyl-,
e—2,6—2′,6′-tetrabromo-,
f—2,4,6—2′,4′,6′-hexabromodiazoaminobenzene.

do not have an absorption maximum. The spectra of individual diazoamino compounds were recorded in various solvents (alcohol, cyclohexane, n-hexane), and no sharp difference in the nature of the curves for one and the same compound was observed (see Fig. 1) \((^4)\).

Starting from mesidine, 2,4,6—2′,4′,6′-hexamethyldiazoaminobenzene was obtained, which also proved to be colorless (see Fig. 2). Thus, in the presence of bulky substituents in positions suitable with respect to the nitrogen atoms, the absorption of radiant energy by a given substance shifts into the short-wavelength region of the spectrum. Models of diazoamino compounds constructed by the Stuart–Briegleb method confirm our supposition that the absence of color is associated with sharp changes in the planarity of such molecules.

Fig. 3. Absorption spectra of colored diazoamino compounds: 1—2,4,5—2′,4′,5′-hexachloro-, 2—3,4,5—3′,4′,5′-hexachloro-, 3—2,3,4—2′,3′,4′-hexachloro-, 4—2,4—2′,4′-tetrachloro-, 5—2,4,5—2′,4′,5′-hexabromodiazoaminobenzene

It is known that, under the action of an excess of mineral acid, diazoamino compounds decompose with the formation of diazonium salts and amines; therefore diazoamino compounds are prepared in weakly acidic media. This also includes the method of obtaining them with amyl nitrite \((^{1,3})\) in ethanol in the presence of traces of HCl (see the preparation method B in Table 1). In this connection, the unusual stability of many halogen-substituted diazoamino compounds in a strongly acidic medium is of interest. On this basis, a method is proposed for obtaining a number of these compounds (see preparation method A in Table 1). Nitrosylsulfuric acid is mixed with a solution of the haloaniline in conc. \(\mathrm{H_2SO_4}\); upon completion of the reaction, the mixture is poured into water with ice, and the product precipitates almost immediately. The final concentration of the acid diluted with water is 10–12%. Some compounds could not be obtained in this way, for example 2,4,6—2′,4′,6′-hexachloro- and hexabromodiazoaminobenzene.

Fig. 4. Absorption spectra of colored diazoamino compounds in n-hexane: 1—diazoaminobenzene, 2—4,4′-dimethyldiazoaminobenzene, 3—3,4—3′,4′-tetramethyldiazoaminobenzene

It should be noted that there is no universal method for obtaining diazoamino compounds; for example, 3,4—3′,4′-tetramethyldiazoaminobenzene is obtained by the usual methods in a yield of 90% of theory \((^5)\),

Table 1

formation of the isomeric 2,6—2′,6′-tetramethyldiazoaminobenzene does not occur under the same conditions. Many halogen derivatives, for example 2,4,6—2′,4′,6′-hexahalo-substituted compounds, are readily obtained with the aid of ethyl nitrite in ethanol in the presence of H₂SO₄ (final concentration of H₂SO₄ 2%) at 18–20° (see method B in Table 1).

Experimental Part

2,3,5,6—2′,3′,5′,6′-Octachlorodiazoaminobenzene (I), method A. 4.6 g of symmetrical tetrachloroaniline in 35 ml of conc. H₂SO₄ was diazotized at 5–7° with nitrosylsulfuric acid (1.4 g of NaNO₂ in 20 ml of conc. H₂SO₄), stirred for 1 h, and 2.3 g of the same amine in 25 ml of conc. H₂SO₄ was introduced at 20°. After 3 h the mixture was poured into 1 l of water with ice, filtered, and washed with water. Colorless needles (from a mixture of benzene and ligroin).

2,3,4,6—2′,3′,4′,6′-Octachlorodiazoaminobenzene (II) was obtained analogously to I. 2.3 g of 2,3,4,6-tetrachloroaniline in 18 ml of H₂SO₄ and 0.7 g of NaNO₂ in 10 ml of H₂SO₄ were taken. 1.15 g of the same amine in 12 ml of H₂SO₄ was added. Colorless needles (from a mixture of ether and alcohol).

2,4,5—2′,4′,5′-Hexachlorodiazoaminobenzene (III) was obtained by method A, by the interaction of 3.9 g of 2,4,5-trichloroaniline in 25 ml of conc. H₂SO₄ with nitrosylsulfuric acid (0.7 g of NaNO₂ in 10 ml of H₂SO₄). Tufts of yellow needles (from acetone).

2,3,4—2′,3′,4′-Hexachlorodiazoaminobenzene (IV) was obtained analogously to III from 1.96 g of 2,3,4-trichloroaniline in 12 ml of H₂SO₄ and 0.35 g of NaNO₂ in 5 ml of H₂SO₄. Yellow needles (from benzene).

2,5—2′,5′-Tetrachlorodiazoaminobenzene (V), method A. From 1.6 g of 2,5-dichloroaniline in 12 ml of H₂SO₄ and 0.7 g of NaNO₂ in 10 ml of H₂SO₄.

Added 0.8 g of the same amine in 6 ml of H₂SO₄. Light-yellow needles (from ligroin).

3,4,5—3′,4′,5′-Hexachlorodiazoaminobenzene (VI) was obtained analogously to I by method A. Yellow needles (from benzene).

2,4,6—2′,4′,6′-Hexachlorodiazoaminobenzene (VII), method B. 1.96 g of symmetrical trichloroaniline, 1.9 g of conc. H₂SO₄, 17 ml of ethanol, and 0.7 g of NaNO₂ were stirred for 3 hours at 20°, after which the mixture was poured into 80 ml of water with ice, filtered, and washed with water and cold alcohol. Bundles of colorless needles (from ligroin). Molecular weight found: 396.0; 402.10; calculated: 403.93.

2,4,6—2′,4′,6′-Hexabromodiazoaminobenzene (VIII) was obtained by method B. Taken were 6.6 g of symmetrical tribromoaniline, 3.8 g of H₂SO₄, 70 ml of ethanol, and 0.7 g of NaNO₂. Bundles of colorless needles (from benzene).

2,4,5—2′,4′,5′-Hexabromodiazoaminobenzene (IX) was obtained analogously to VIII. Yellow needles (from benzene).

2,6—2′,6′-Tetrabromodiazoaminobenzene (X), method B. A solution of 5 g of 2,6-dibromoaniline in 70 ml of ethanol and 2.4 g of amylnitrite was stirred for 4 hours at 20°. On the following day the mixture, cooled to 0°, was filtered and washed with cold alcohol. Colorless needles (from ligroin).

2,4—2′,4′-Tetrachlorodiazoaminobenzene (XI) was obtained analogously to X. Straw-yellow needles (from ligroin).

2,4,6—2′,4′,6′-Hexamethyldiazoaminobenzene (XII). 4.0 g of mesidine, 5 ml of ether, and 3.5 g of amylnitrite were mixed and left to stand in an open vessel for 3 hours at 25—30°. The mixture was cooled in ice, filtered, and washed with cold alcohol. Bundles of colorless needles (from cyclohexane), elongated prisms (from a mixture of ether and alcohol). Molecular weight found: 273.6; 267.0; calculated: 281.39.

4,4′-Dimethyldiazoaminobenzene (XIII) was obtained analogously to XII. Light-yellow needles (from alcohol).

3,4—3′,4′-Tetramethyldiazoaminobenzene (XIV), method Г. A solution of 1.2 g of 3,4-dimethylaniline and 3.0 g of HCl in 1.5 ml of water was mixed with a diazonium solution prepared from 1.2 g of 3,4-dimethylaniline, 4.6 g of HCl in 10 ml of water, and 0.7 g of NaNO₂. After mixing, 4.8 g of sodium acetate in 15 ml of water was added immediately. Yellow needles (from benzene).

Diazoaminobenzene (XV) was obtained by method Г. Yellow needles (from benzene).

Diazoamino compounds I, III, V, VII, VIII were also obtained by another method. For example, VIII: at 5—7°, 3.3 g of 2,4,6-tribromoaniline in 300 ml of alcohol was mixed with 1.2 g of amylnitrite and 0.5 g of HCl and stirred for 3 hours at 20°. On the following day the cooled mixture was filtered and washed with cold alcohol. 2.7 g (82% of theory) of VIII was obtained.

Purification of diazoamino compounds sometimes presents considerable difficulties. Many of the diazoamino compounds described were purified by chromatographic adsorption on aluminum oxide, using solvents—acetone, benzene, n-hexane—after which they were recrystallized from various suitable solvents.

The electronic absorption spectra data were obtained on an SF-4 spectrophotometer in solutions of n-hexane, alcohol, and cyclohexane; the concentration of the solutions was \(10^{-4}\) mol/l.

We are grateful to L. G. Lummer, E. S. Levin, and I. E. Ovcharenko, who took a large part in recording the spectra, for consultation in the spectral analysis section.

Scientific-Research Institute
of Organic Intermediates and Dyes

Received
30 VI 1964

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