Complex Formation of Nitro Derivatives and Halonitro Derivatives of the Aromatic Series with Amines

V. M. Morozova

(Presented by Professor, Doctor of Chemical Sciences B. V. Tronov)

The aim of the present work was to elucidate the regularities in the influence of substituents on the complex-forming ability of nitro and amino groups.

Amines as complex-forming agents have been well studied. They are complex-forming agents of the electron-donor type.

A feature of nitro compounds as complex-forming agents is their dual character, associated with the structure of the nitro group.

In the formation of complexes between nitro compounds and amines, the complex bond arises through attraction of the positively charged nitrogen of the nitro group to the unshared pair of electrons of the amino group [1].

Experimental Part

Research methods. Both preparative methods and methods of physicochemical analysis (colorimetric analysis and cryoscopy) were used in the work.

Procedure for carrying out colorimetric analysis. A KM-1 colorimeter was used in the work. Benzene served as the solvent. The concentration of the solutions was chosen depending on the solubility of the starting substances and with allowance for obtaining a color intensity convenient for carrying out colorimetric analysis. Solutions with concentrations of 0.5 M, 0.15 M, and 0.1 M were taken. From solutions of the initial components, mixtures were prepared differing in composition by ten mole percent. The color of most solutions of the complexes was yellow-green and, in some cases, yellow, orange, or red.

In the colorimetric analysis of each individual system, the color of a mixture of solutions of the starting substances with a molecular ratio of 1:1 was taken as the standard.

For the systems halonitrobenzene—amine, analyses for ionized halogen were carried out. In all cases the analysis gave negative results.

Colorimetric analysis was carried out for 35 binary systems of nitro compounds (nitrobenzene, o- and p-nitrotoluenes, p-nitroanisole, m-nitrobenzaldehyde, m- and p-dinitrobenzenes, o-, m- and p-chloro- and o-, m- and p-bromonitrobenzenes, 1,2,4-chloro— and 1,2,4-bromodinitrobenzenes) with amines (aniline, dimethylaniline, o- and p-toluidines, o- and p-anisidines, diphenylamine). The presence in a number of systems of the same shade of coloration made it possible for us to compare the color intensities of such systems.

For complexes of isomeric chloro- and bromonitrobenzenes, 1,2,4-chloro— and 1,2,4-bromodinitrobenzenes with p-toluidine, the degree of dissociation of the complex was determined by the cryoscopic method in a benzene solution. For all the selected complexes the degree of dissociation proved to be equal to one.

Isolation and investigation of the complex p-dinitrobenzene—benzidine. An equimolecular mixture of the starting dry substances was dissolved in a small amount of benzene with heating (35–40°). On cooling, crystals of the complex precipitated from the solution in the form of dark-violet plates. For recrystallization, the complex was dissolved in benzene at room temperature, and then part of the benzene was allowed to evaporate. The melting point of the complex was 108.5–109°. Recrystallization from heated benzene led to partial decomposition of the complex.

Decomposition of the complex was carried out by dissolving a weighed portion of it in dilute hydrochloric acid. The benzidine hydrochloride that passed into solution was filtered off, and the benzidine was liberated by the action of alkali. p-Dinitrobenzene remained on the filter.

Results of the analysis of the complex p-dinitrobenzene—benzidine.

The results of the analysis show that the complex obtained has a molecular composition of 1:1.

Comparison of the complex-forming ability of certain nitro compounds by the method of the action of vapors of the more volatile complex-forming agent on crystals of the less volatile substance. The complex-forming ability of certain nitro compounds was compared with respect to p-toluidine. Weighing bottles containing weighed portions of nitro compounds (0.001 gram-molecule) were placed in a desiccator, on the bottom of which a volatile complex-forming agent had been poured. Weighings were carried out at definite intervals; from the gain in weight judgments were made about the complex-forming ability of the nitro compound (the contents of the weighing bottles were regularly mixed). The temperature of the room in which the desiccators were located varied from 16 to 27°. The total duration of the experiments was up to three years.

Comparison of the complex-forming ability with respect to p-toluidine was carried out for the following nitro compounds: m- and p-dinitrobenzenes, 1,2,4-dinitroanisole, 1,2,4-, 1,2,5- and 1,2,6-dinitrophenols, and symmetrical trinitrobenzene.

The isomeric chloro- and bromonitrobenzenes and nitrophenols gave a colored, spreading mass in p-toluidine vapors.

For 1,2,4-chloro- and 1,2,4-bromodinitrobenzenes, a condensation reaction with elimination of hydrogen chloride proceeded in p-toluidine vapors.

m-Dinitrobenzene, on standing in an atmosphere of p-toluidine vapors, acquired a brown coloration already during the first day, but then the spreading of the m-dinitrobenzene gradually occurred.

p-Dinitrobenzene and 1,2,4-dinitroanisole did not change color and did not gain weight.

Discussion of the Results

Complex-forming ability was studied mainly by the colorimetric method.

The complexes of a series of nitro compounds with certain amines in benzene solution had the same shade of color when compared in a colorimeter, but differed in its intensity. The same shade of color ...

we explained by the same mechanism of complex formation, and the differing intensity of the coloration was taken as indicating a greater or lesser tendency of the groups participating in the interaction toward complex formation.

The method of comparing the intensity of coloration of systems whose complex solutions have the same hue made it possible for us to compare the complex-forming ability of individual amines and nitro compounds and to draw particular conclusions about the influence of substituents on the complex-forming ability of amino and nitro groups.

On the basis of the data of colorimetric analysis, it may be concluded that the introduction into aniline, as a second substituent, of a methyl group or a methoxy group weakens the withdrawal by the benzene ring of the electron pair from the amino nitrogen.

The influence of the second substituent is manifested more strongly when it is in the para position than when it is in the ortho position.

The methoxy group enhances the electron-donor properties of the amino group to a greater extent than does the methyl radical.

According to the data of colorimetric analysis, the amines selected for the study, in order of increasing electron-donor properties, may be arranged in the following series: aniline, o-toluidine, p-toluidine, o-anisidine, p-anisidine.

By colorimetric analysis, in all the selected systems, the formation of complexes with a component ratio of 1:1 was again found or confirmed.

Comparison of the intensity of coloration of solutions of complexes of certain nitro compounds with respect to the selected amines makes it possible to arrange the nitro compounds in the following series in order of decreasing electron-acceptor properties.

Comparison of these data shows that a methoxyl group in the p-position to the nitro group weakens the electron-acceptor functions of the nitro group.

A methyl group in the o-position to the nitro group also decreases the electron-acceptor functions of the latter. To a lesser extent, weakening of the electron-acceptor functions of the nitro group is observed under the influence of a methyl group located in the p-position; and with respect to such amines as o- and p-anisidines, a methyl group in the p-position even enhances the electron-acceptor functions of the nitro group.

A second nitro group, both in the m- and in the p-position, enhances the electron-acceptor functions of the nitro group. For m-dinitrobenzene, a comparatively large number of complexes with amines has been found and isolated. The complexes of m-dinitrobenzene with amines are more stable than those for nitrobenzene.

A second nitro group in the p-position also enhances the electron-acceptor functions of the nitro group and, according to the data of colorimetric analysis performed by us, enhances them to a greater extent than a second nitro group in the m-position.

A complex of m-dinitrobenzene—benzidine has been described in the literature [3], [1]. By precipitation from benzene solutions we obtained a complex of p-dinitrobenzene—benzidine. The former is thermally more stable; it can be recrystallized from hot benzene, whereas the latter decomposes under these conditions into its components.

The behavior of halogen-substituted nitrobenzenes as complex formers of the electron-acceptor type is confirmed by the work of Menshutkin [2], who, by thermal analysis, found complexes of the isomeric chloronitrobenzenes with aluminum chloride and aluminum bromide. Thermal analysis of binary systems of the isomeric chloronitrobenzenes with aniline gave a simple eutectic.

The identical color shades of the complexes for a whole series of halonitrobenzene systems with amines made it possible to compare the color intensity of these systems and, on this basis, to draw conclusions about the influence of the halogen on the reactivity of the nitro group.

Below are series in which the compounds are arranged in order of decreasing electron-acceptor properties with respect to the corresponding amine.

* o-Chloronitrobenzene and o-bromonitrobenzene differed in color.

Comparison of these series shows that the introduction of a halogen undoubtedly affects the complex-forming ability of the nitro group, more often enhancing the electron-acceptor functions of the latter and less often weakening them. At the same time, both the nature of the introduced halogen and its position relative to the nitro group have an effect.

With respect to amines of comparatively simple structure (aniline, o- and p-toluidines), the electron-acceptor properties of the nitro group were enhanced by the introduction of chlorine; the greatest enhancement was observed when chlorine was introduced in the m-position, a smaller enhancement when chlorine occupied the para position, and still smaller when chlorine occupied the ortho position.

For bromine the regularity obtained was less distinct.

The indications obtained by the cryoscopic method of complete dissociation (which corresponds to the absence of complex formation) of complexes of halonitrobenzenes, 1,2,4-chloro- and 1,2,4-bromodinitrobenzenes indicate the lower sensitivity of the cryoscopic method in comparison with the colorimetric method.

By the method involving the action of vapors of the more volatile complex former on crystals of the less volatile one, it was possible to compare the reactivity of 1,2,4-, 1,2,5-, 1,2,6-dinitrophenols and symmetrical trinitrobenzene toward p-toluidine.

The reactivity of these complex formers decreased in the following series: 1,2,5-dinitrophenol, 1,2,6-dinitrophenol, symmetrical trinitrobenzene, 1,2,4-dinitrophenol.

The broad use of this method for comparing the complex-forming ability of various nitro compounds is hindered, on the one hand, by the formation by some nitro compounds of liquid eutectics and, on the other hand, by the high density of packing of molecules in the crystals of solid components, which makes it difficult for the amine to penetrate deep into the crystals.

became. Thus, an attempt to determine the activity of various solid nitro compounds from their interaction with vapors of 1-naphthylamine did not yield positive results, since the increase in the weight of the solid component due to absorption of 1-naphthylamine ceased very rapidly.

The absence of interaction of crystals of p-dinitrobenzene with vapors of p-toluidine should be explained by the especially dense packing of the molecules of p-dinitrobenzene in the crystal lattice.

In benzene solution, for the system p-dinitrobenzene—p-toluidine, a complex with a component ratio of 1:1 was detected by the colorimetric method.

The system 1,2,4-dinitroanisole—p-toluidine was not studied colorimetrically, since benzene solutions of 1,2,4-dinitroanisole are themselves intensely colored. Therefore, the absence of complex formation between crystals of 1,2,4-dinitroanisole and vapors of p-toluidine, which is presumably attributed to the influence of the methoxyl group on the nitro groups, has not been confirmed by additional studies. A certain confirmation of the lowering of the electron-acceptor properties of the nitro group in dinitroanisole under the influence of the methoxyl group is the same kind of influence of the latter on the complex-forming ability of the nitro group in p-nitroanisole.

Conclusions

1. By the method of colorimetric analysis, 82 new complexes of aromatic nitro and dinitro compounds (electron acceptors) with aromatic amines, with a component ratio of 1:1, were detected.

2. By the precipitation method, the complex p-dinitrobenzene—benzidine was isolated.

3. Considerations are presented on the influence of chlorine, bromine, nitro, carboxyl, methyl, and methoxyl groups, as substituents, on the complex-forming ability of the nitro group.

4. Considerations are presented on the influence of methyl and methoxyl groups on the complex-forming ability of the amino group.

Literature

1. Tronov B. V. and Strelnikova N. D. Izv. TPI, 71, 63–99, Tomsk, 1952.
2. Meisenheimer B. N. Chem. Zbl. 164 and 1240, 1910.
3. C. A. Buehler and A. G. Heap. Chem. Zbl. 1013, 1927.