THERMAL ANALYSIS OF FORMIC ACID—AMINE SYSTEMS

A. S. NAUMOVA, Kh. A. LELCHUK

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

Continuing work on the study of the interaction of formic acid with amines by methods of physicochemical analysis, we carried out a thermal analysis of binary systems containing formic acid and aromatic amines: orthotoluidine, dimethylaniline, and diphenylamine.

The substances required for the work, after preliminary purification, had the following constants: formic acid crystallized at 8°; orthotoluidine boiled at 197° and 758 mm pressure, melted at 22°; dimethylaniline boiled at 194° and 757 mm pressure, melted at 2°; diphenylamine melted at 53°, boiled at 302° and 759 mm pressure.

The thermal analysis of the systems was carried out by the visual-polythermal method. The liquidus line was constructed from the temperatures at which the first crystals appeared from the solutions. Each mixture was prepared separately. The seeding method was not used.

Since some mixtures crystallized at temperatures below zero degrees, we used liquid air for cooling them.

Formic acid—orthotoluidine system

This system was studied by one of us, together with Kartushinskaya [1], by the method of electrical conductivity and viscosity. The results obtained indicate the presence of chemical interaction between formic acid and orthotoluidine.

The results of measuring the crystallization temperature are shown graphically in Fig. 1. From Fig. 1 it follows that, upon addition of the first portions (approximately up to 5%) of formic acid to orthotoluidine, a sharp rise in the crystallization temperature is observed. Further, up to approximately 45 mol.% acid, this rise proceeds more slowly, and finally, in the region of solutions from 45 to 60 mol.% acid, the crystallization temperature remains almost constant. A further increase in the acid content in the system causes a sharp decrease in temperature down to the crystallization temperature of the eutectic (approximately −31°), which contains about 85 mol.% acid, after which the temperature again rises to the crystallization temperature of the acid.

The increase in the crystallization temperature upon addition of formic acid to orthotoluidine indicates the presence of chemical interaction of the components and the formation of a compound melting at

congruently at approximately \(62^\circ\). Thus, in the formic acid—orthotoluidine system a well-crystallizing compound is formed, whose melting point is much higher than the melting points of the pure components.

Fig. 1. Crystallization curve of the formic acid—orthotoluidine system.

From the liquidus curve it is difficult to draw a definite conclusion about the composition of the compound.

The maximum value of the viscosity [1] occurs at 60 mole % acid, that is, corresponds to a compound of the following composition:

\[ 3\,\mathrm{HCOOH}\cdot 2\,\mathrm{C}_6\mathrm{H}_4 \begin{matrix} \mathrm{NH}_2\\[-2pt] \diagup\\[-8pt] \\[-8pt] \diagdown\\[-2pt] \mathrm{CH}_3 \end{matrix} \]

But the diffuseness of the maximum in the crystallization diagram indicates the thermal instability of the compound; therefore it is possible that the viscosity maximum proved to be displaced from the abscissa of the compound and that the compound has a simpler composition.

Formic Acid—Dimethylaniline System

N. A. Pushin et al. [2] studied the refractive index of this system. On the basis of the data obtained, the authors come to the conclusion that mixing of the components is accompanied by contraction and that in the liquid phase there exists a complex composed of two molecules of formic acid and one molecule of dimethylaniline.

When preparing mixtures containing 40 or more mole percent acid, we observed heating. Solutions with an acid concentration from 73 to 89 mole % could not be crystallized. At temperatures beginning at \(20^\circ\) and below, these solutions had a very high viscosity and at approximately \(-40^\circ\) solidified into a glassy mass. The results of measurements of the temperature at which the first crystals appeared are shown in Fig. 2.

The liquidus curve obtained is characterized by the presence of a point of inflection corresponding to 33.3 mole percent acid with a melting point of about \(2.5^\circ\), and of a eutectic located in the region of higher—

Fig. 2. Crystallization curve of the formic acid—dimethylaniline system.

content of acid. It was impossible to establish the position of the eutectic and its melting temperature, because, as already noted, mixtures with an acid content from 73 to 89 mole % did not crystallize. Thus, judging from the form of the crystallization curve, chemical interaction occurs between the components. It is possible that the compound formed in this case melts congruently at an intermediate point [3], since the point of inflection corresponds to a component ratio of 1:2, which corresponds to a compound of composition

\[ \mathrm{HCOOH}\cdot 2\mathrm{C}_6\mathrm{H}_5\mathrm{N}(\mathrm{CH}_3)_2 . \]

It cannot be stated with certainty that the compound formed in the system has precisely this composition. There are cases [4, 5, 6] in which the point of inflection corresponds to remarkable points on diagrams of other properties and thus indicates the composition of the compound. But we had no possibility of comparing the crystallization diagram with curves of other properties, since this system, so far as we know, has been studied only by refractive index. The isotherms of the latter indicate a compound of another composition \([2\mathrm{HCOOH}\cdot \mathrm{C}_6\mathrm{H}_5\mathrm{N}(\mathrm{CH}_3)_2]\), which was not reflected in the crystallization curve. It is possible that this system is an example of systems [7, 8, 9, 10] in which the composition of the compound existing in the liquid phase differs from the composition of the compound formed by the same substances in the crystalline phase.

The formic acid—diphenylamine system

The data obtained in measuring the crystallization temperature of the system are presented graphically in Fig. 3. Fig. 3 shows that on the liquidus line there is a point of inflection at 55 mole % acid with a melting temperature of approximately 25.2° and a eutectic located in the region of higher formic-acid content. Such a course of the crystallization diagram indicates that the components interact chemically. As a result of the interaction, a thermally unstable compound is formed, melting incongruently¹.

Since the system has not been studied with respect to other properties, one can only suppose that the newly obtained compound has the composition

\[ \mathrm{HCOOH}\cdot(\mathrm{C}_6\mathrm{H}_5)_2\mathrm{NH}. \]

Fig. 3. Crystallization curve of the formic acid–diphenylamine system.

The presence of chemical interaction in the system is also indicated by heating and by a change in color accompanying the formation of mixtures of various composition. Beginning at approximately 60 to 90 mole % formic acid, the color changes from light yellow to red, passing through dark yellow, red, and dark red.

Thus, when formic acid is mixed with o-toluidine, dimethylaniline, and diphenylamine, interaction takes place. From the standpoint of the acid–base theories of Brønsted [11], Lewis [12], and Usanovich [13], this interaction is acid–base in character. The role of the base is played by the amines; moreover, the strongest base with respect to formic acid is apparently o-toluidine, which forms with it an easily crystallizing, high-melting, comparatively stable compound. The presence of this compound in the system found fairly clear expression in the crystallization diagram.

Conclusions

1. By the visual-polythermal method, crystallization diagrams have been studied for systems containing formic acid with o-toluidine, dimethylaniline, and diphenylamine.

2. The liquidus curves of all the systems studied indicate the presence of chemical interaction between the components.

3. In the formic acid—o-toluidine system a compound is formed with a congruent melting temperature. It is possible that the compound formed when dimethylaniline is mixed with formic acid also melts congruently. Diphenylamine with formic acid gives an incongruently melting compound.

Literature

1. Naumova A. S. Electrical conductivity, viscosity, and density of formic acid—toluidine systems (article included in this collection).

2. Pushin N. A., Matavul P., Rykovskii I. I., Nenadovich M. Zhurnal Obshchei Khimii 18, 1573, 1948.

3. Novikov I. I. Dokl. AN SSSR 100, 1119, 1955.

4. Bergman A. G., Kuznetsova M. N. Zhurnal Obshchei Khimii 9, 631, 1939.

5. Elfgort M. S. ZhRKhO 61, 947, 1929.

6. Kirillova A. D. and Dionis’ev D. E. Zhurnal Obshchei Khimii 23, 1107, 1953.

7. Kurnakov N. S. and Shternina E. B. Izv. SFA AN SSSR 13, 153, 1940.

8. Rudenko N. Z. and Dionis’ev D. E. Zhurnal Obshchei Khimii 23, 556, 1953; 25, 265, 1955.

9. Udovenko V. V. and Airapetova R. P. Zhurnal Obshchei Khimii 17, 425, 1947.

10. Tamman, Pillsbury. Z. anal. chem. 172, 243, 1928.

11. J. Brönsted. Ber. Dtsch. chem. Ges. 42, 718, 1923.

12. G. N. Zewis. Frankl. Jnst. 226, 293, 1938.

13. Usanovich M. Zhurnal Obshchei Khimii 9, 182, 1939.

1. The compound formed in the system is a complex, not a valence compound, since it is comparatively unstable. Solutions having the most intense coloration, when placed in a mixture of benzene and alkali solution, are decolorized, since, as a result of decomposition of the complex compound, the liberated diphenylamine dissolves in the benzene layer, and the acid—in the alkali solution. ↩