Physicochemical Analysis of the Aniline—Nitrobenzene System

A. S. Naumova and K. A. Prokop’eva.

Mixing aniline with nitrobenzene causes a reddish-brown coloration to appear, the intensity of which is different at different ratios of the substances. The appearance of such coloration and its constancy when the composition is changed can be explained by chemical interaction of the components of the solution.

Alekseev [1] studied, for this system, the change in heats of mixing and heat capacities with concentration. It is assumed [2] that the formation of mixtures of aniline and nitrobenzene is accompanied by two simultaneous processes: the decomposition of associated molecules, on the one hand, and weak chemical interaction of the components of the system, on the other.

I. Ostromyslenskii [3], who studied this system spectrophotometrically, also points to the chemical interaction of aniline and nitrobenzene with the formation of an equimolecular compound.

It is of interest to trace whether the indicated processes—i.e., chemical interaction and the decomposition of associated molecules—will be reflected in diagrams of other properties. The present investigation was undertaken for this purpose.

Experimental Part and Discussion of Results

The substances used were purified by repeated distillation. Both were previously dried with caustic potash. In addition, aniline was allowed to stand over metallic sodium, over which it was also distilled. For the investigation, the fraction of aniline boiling at 184° (766 mm pressure) and of nitrobenzene at 210.5° (765 mm pressure) was taken.

Tests of electrical conductivity showed that solutions containing aniline and nitrobenzene practically do not conduct electric current.

The system was studied colorimetrically, by the method of thermal analysis, and by the methods of viscosity and density. The last two properties were determined not only for the binary system, but also upon introduction into it of an indifferent solvent.

Viscosity and Density

The viscosity was determined at temperatures of 0, 25, and 50° in a viscometer for volatile liquids.

The data obtained in the study of the viscosity and density of the binary system aniline—nitrobenzene are given in Table 1. The diagram expressing the dependence of viscosity on composition (Fig. 1) gives no indication of the presence of chemical interaction between the components. It follows from Fig. 1 that the viscosity isotherms are represented by smooth curves, convex toward the axis

Table 1

Density and viscosity of the aniline—nitrobenzene system

composition, revealing a rather considerable negative deviation from additivity, especially at the lower temperature (0°). This course of the curves indicates that, upon mixing the components of the system, their associated molecules break down [4].

Fig. 1. Viscosity isotherms of the aniline—nitrobenzene system.

We have calculated the absolute temperature coefficient of viscosity (Table 2, Fig. 2), whose diagram likewise does not reflect chemical interaction.

Table 2

Temperature coefficient of viscosity

Fig. 2. Temperature coefficient of viscosity of the aniline—nitrobenzene system.

Fig. 3. Density isotherms of the aniline—nitrobenzene system.

The density isotherms (Fig. 3) are represented by straight lines, providing some indications of the presence of chemism between the components.

It is known [5; 6] that, when studying one or another property of a system in the presence of an indifferent solvent, it is possible to establish the pres-

chemical interaction even in the case when diagrams of this same property, but obtained for binary systems, give no indication of interaction.

To test this possibility in our system, we determined the viscosity and density in the presence of an indifferent substance. Isopropyl alcohol was taken as the latter, since in it, unlike the other liquids we tested, aniline and nitrobenzene dissolved, giving colorless mixtures. In addition, their dissolution in alcohol was accompanied by a considerable decrease in temperature, which indicated the dissociation of associated molecules of the components.

Fig. 4. Viscosity of the isoconcentrate of 50 mol.% isopropyl alcohol.

Fig. 5. Density of the isoconcentrate of 50 mol.% isopropyl alcohol.

The viscosity and density of the aniline—nitrobenzene—isopropyl alcohol system were measured at one alcohol concentration (50 mol.%) at a temperature of 25°. The results obtained are given in Table 3.

The dependence of viscosity on composition is shown in Fig. 4. It follows from Fig. 4 that the viscosity isotherm is expressed by a smooth curve, slightly convex toward the composition axis. Thus, the addition of alcohol to the two-component system does not change the form of the viscosity curve.

As Fig. 5 shows, the density isotherm, upon introduction of alcohol into the binary system, changes from a straight line into a curve slightly convex away from the composition axis. Such a course of the curve gives some indication of the presence of chemical interaction in the system.

Melting Point

Thermal analysis was carried out visually by the polythermal method.

The crystallization temperature of the solutions was measured with a mercury thermometer with a scale from 35 to 40°C.

The outer test tube was cooled with liquid air.

Most of the solutions exhibited considerable supercooling, i.e., gave a poorly crystallizing mass, especially in the region of 35–45 mol.% nitrobenzene. This greatly hampered observation.

The results of the thermal analysis are summarized in Table 4 and represented graphically in Fig. 6.

The liquidus curve, expressing the dependence of the crystallization temperature on the composition of the system, has a eutectic (at 39 mol.% nitrobenzene) with a crystallization temperature equal to −31°. This curve also passes through two points of inflection, occurring at 49 and 63 mol.% nitro-

Table 3

Isoconcentrate of isopropyl alcohol, 50 mol. %, at 25°C.

Table 4

Crystallization temperature of the aniline–nitrobenzene system

benzene. The points of inflection correspond, respectively, to the following crystallization temperatures: −23,5° and −16°.

The presence of points of inflection on the crystallization line indicates, as is known, that when the components are mixed, chemical interaction occurs with the formation of thermally unstable molecular compounds, which, upon melting, partially dissociate into their constituent parts. These compounds apparently have the following composition:

\[ \mathrm{C_6H_5NH_2 \cdot C_6H_5NO_2} \quad \text{and} \quad \mathrm{C_6H_5NH_2 \cdot 2C_6H_5NO_2}. \]

Colorimetric analysis

When preparing mixtures of different composition, it was not difficult to notice by eye that, with a gradual increase in the aniline content in the sol-

in the melt the intensity of the color changes. The most intensely colored appear to be solutions containing equal amounts of aniline and nitrobenzene. The variability of the color intensity, as already noted, also gives some indication of the chemical interaction of the components of the system.

It was of interest to trace, as far as possible in greater detail, the change in the color intensity of the solutions as their composition changed, and this was done with the aid of a Duboscq colorimeter.

For the colorimetric measurements a series of solutions of different composition was prepared, 10 cm³ each. The mixtures contained an increasing (per 1 cm³) number of cm³ of one component and a decreasing number of the other. In order to know precisely in what molecular ratios the components were present in the mixture, they were weighed.

Fig. 6. Liquidus curve of the aniline–nitrobenzene system.

Fig. 7. Colorimetric data for the aniline–nitrobenzene system.

Into the colorimeter cups were poured 10 cm³ each of the standard and of the solution under investigation. As the standard there was taken a solution containing 48 mol. % nitrobenzene, since among the prepared solutions it appeared to be the most intensely colored.

To equalize the color intensity of the mixtures—the standard and the comparison solution containing 89 mol. % nitrobenzene—it was necessary to change the height of the layer (in the colorimeter) of both solutions, since with a constant layer thickness of the standard solution equalization was not achieved.

The results of the colorimetric measurements are summarized in Table 5 and shown graphically as a composition–property diagram in Fig. 7. To construct the diagram, the ratios of the heights of the layers (in the colorimeter) of the standard and comparison solutions were used, i.e., quantities proportional to the color intensity.

Fig. 7 shows that the maximum color intensity corresponds to a solution containing about 50 mol. % nitrobenzene, which corresponds to the abscissa of the compound of composition C₆H₅NO₂·C₆H₅NH₂.

Thus, our colorimetric determinations are in agreement with the spectrophotometric measurements of Ostromyslenskii [3].

Table 5

Colorimetric data

Conclusions

1. Testing mixtures of aniline with nitrobenzene for electrical conductivity showed that the system practically does not conduct electric current.

2. At temperatures of 0, 25, and 45°, the viscosity and density of the binary system \(C_6H_5NO_2—C_6H_5NH_2\) were measured.

3. At 25°, the viscosity and density of the ternary system aniline—nitrobenzene—isopropyl alcohol were studied at an isoconcentrate of 50 mol. % alcohol.

4. The viscosity isotherms of both the binary and ternary systems give no indication of the presence of chemical interaction in the system.

5. When an indifferent solvent is introduced into a system consisting of aniline and nitrobenzene, the density isotherm changes from a straight line into a smooth curve, slightly convex away from the composition axis.

The form of the density diagrams indicates the presence of weakly expressed chemical interaction between the solution components.

1. The binary aniline—nitrobenzene system was studied by the method of thermal analysis.

2. The liquidus curve has one eutectic and two points of inflection. The presence of the latter on the fusibility diagram gives grounds to suppose that unstable complex compounds of the following compositions are formed in the system: \(C_6H_5NH_2 \cdot C_6H_5NO_2\) and \(C_6H_5NH_2 \cdot 2C_6H_5NO_2\).

3. Colorimetric analysis was also carried out for the binary aniline—nitrobenzene system.

The constructed composition–property diagram indicates the existence of an equimolecular compound in the system.

Literature

1. Alexejew, Wied. Ann., 38, 301, 1889.

2. Kurnakov N. S. Introduction to Physicochemical Analysis, Moscow—Leningrad, pp. 303–325, 1940.

3. Iwan Ostromisslensky, Ber., 1, 268, 1911.

4. Kurnakov N. S. Introduction to Physicochemical Analysis, Moscow—Leningrad, pp. 326–341, 1940.

5. Udovenko V. V. and Airapetova R. P. Journal of General Chemistry, 19, 1796, 1939.

6. Kremann. Mechanische Eigenschaften flüssiger Stoffe, p. 298, Leipzig, 1928.