THERMAL ANALYSIS OF AMINE–PHENOL SYSTEMS

A. S. Naumova, K. A. Prokopieva

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

The aramination reaction, which plays a major role in the synthesis of dyes, is considerably accelerated in the presence of certain mineral and organic substances. V. V. Koyain studied the aramination reaction of certain oxy derivatives of anthraquinone in the presence of various additives. He established that this reaction is accelerated by substances that readily form complexes with aromatic amines, and that the catalytic activity of these substances is the greater, the less stable the complex formed in the reacting system.

Phenols and amines are distinguished by a great capacity for complex formation.

It was of interest to use the method of thermal analysis to determine whether the nature of the chemical interaction and the presence of complex compounds in systems composed of substances participating in the aramination reaction would be reflected in melting diagrams.

The substances used in this work were subjected to thorough purification. After repeated distillation with metallic sodium, aniline had a boiling point of 184° at a pressure of 757 mm. Ortho-toluidine was purified by double distillation. The fraction boiling at 197° and a pressure of 758 mm was taken. Paratoluidine was recrystallized from xylene, after which it melted at 45.5°. Resorcinol and quinizarin were also purified by recrystallization: the former from benzene (melting point 109.5–110°) and the latter from glacial acetic acid (melting point 194°). For thermal analysis, each mixture was prepared separately.

Thermal analysis was carried out visually and polythermally. The liquidus curves were constructed from the temperatures at which the first crystals separated from the solutions.

To determine low crystallization temperatures in the resorcinol–aniline system, the mixtures were cooled with liquid air.

The aramination reaction proceeds better at an elevated temperature (184°). In order to determine whether a change in temperature has any effect on the behavior of the system, measurements were made before and after treating the system by heating to 200°C. Heating was carried out in a glycerin bath so that the temperature changed uniformly by 4–5° per minute. At a temperature of 200°, each mixture was held for 5 min, and only after this was the crystallization temperature determined.

Resorcinol–p-Toluidine System

The results of measuring the temperatures at which the first crystals appear (before treatment of the system by heating) are shown graphically in Fig. 1.

Both liquidus curves consist of three branches: two branches of crystallization of the pure components and a branch of crystallization of the low-melting compound

\[ \mathrm{C_6H_4(OH)_2 \cdot 2C_6H_4<\begin{matrix}NH_2\\ CH_3\end{matrix}} \]

with a congruent melting point. The concentration region in which the compound exists is small (from 55 to 75 mol.% p-toluidine). The melting point of the compound is much lower than the melting points of the pure components. The eutectic points correspond to approximately 55 and 75 mol.% p-toluidine. Heating to \(200^\circ\) and subsequent cooling of the system do not produce substantial changes. The dystectic point (at 67 mol.% p-toluidine), corresponding to the composition of the compound, does not change its abscissa. However, the maximum on the liquidus curve becomes more diffuse. The complex compound formed after treatment of the system by heating becomes thermally still less stable; on melting it dissociates. Its melting point decreases somewhat (approximately from \(13\) to \(11^\circ\)). The melting point of the first eutectic, on the contrary, rises significantly (approximately from \(-14\) to \(0^\circ\)), while the second changes little (from \(10\) to \(9^\circ\)).

Fig. 1.

Resorcinol–o-Toluidine System

The crystallization diagrams before (Fig. 2) and after heating the system to \(200^\circ\), obtained by us\(^1\), are, in form, analogous to the fusibility diagrams studied by Kremann for the aniline–hydroquinone system [1] and by Shushunov with Zouzolkova for the p-toluidine–hydroquinone system [2]. The diagrams consist of two branches, one of which corresponds to crystallization of resorcinol, and the other to a compound formed by the interaction of resorcinol with o-toluidine. The dystectic point of the second branch, with a melting point of \(67^\circ\) (Fig. 2), corresponds to 50 mol.%, that is, to a compound of the composition:

\[ \mathrm{C_6H_4(OH)_2 : C_6H_4<\begin{matrix}NH_2\\ CH_3\end{matrix}} \]

\(^1\) We do not present the liquidus curve obtained after heating the system to \(200^\circ\), for the reasons indicated below.

The eutectic has a melting temperature of \(64^\circ\) and occurs at 60 mol.% resorcinol. After heat treatment of the system, the maximum is smoothed out, thus indicating the low thermal stability of the compound.

Quinizarin–p-toluidine system

The data obtained in determining the temperature of separation of the first crystals from solution are given in Fig. 3¹. Both liquidus curves repeat one another in their course. They have a eutectic containing 97.5 mol.% p-toluidine with a melting temperature of \(39\text{–}40^\circ\) and an inflection point at 42 mol.% p-toluidine. The presence of an inflection point on the crystallization curve indicates

Fig. 2.

Fig. 3.

the existence of chemical interaction in the system and the formation of an incongruently melting compound. The composition of the latter is difficult to determine, since other properties of this system have not been studied. It may be assumed that the hidden maximum occurs at 33.3 mol.% p-toluidine and that the compound has the following composition:

\[ 2\,\mathrm{C_6H_4} \begin{matrix} \diagup \mathrm{CO} \diagdown\\[-2pt] \diagdown \mathrm{CO} \diagup \end{matrix} \mathrm{C_6H_2(OH)_2}\cdot \mathrm{C_6H_4} \begin{matrix} \diagup \mathrm{NH_2}\\[-2pt] \diagdown \mathrm{CH_3} \end{matrix} \]

The data obtained show that mixtures containing more than 50 mol.% p-toluidine (the region of existence of the complex compound), after heat treatment, crystallize at a lower temperature than before such treatment. This, apparently, may be explained by the fact that the compound formed decomposes somewhat upon heating. The inflection point on the crystallization curve becomes less sharply expressed.

Resorcinol–Aniline System

The mixtures have a dark-brown coloration.

It was not possible to study the crystallization temperature of this system over the entire concentration range. In the region of solutions containing from 10 to 56 mol.% resorcinol, crystallization could not be induced even by introducing seed crystals. When the temperature was lowered, the mixtures became strongly supercooled and turned into a glassy mass. The crystallization diagrams obtained by us almost completely overlap one another (Fig. 4) and have two branches, one of which corresponds to crystallization of the amine, the other to that of resorcinol¹.

According to N. N. Efremov [3], thickening is a sign of the existence of a chemical compound in a system. Therefore it is possible that, when resorcinol is mixed with aniline, chemical interaction occurs with the formation of a complex compound possessing considerable viscosity, which hinders crystallization.

Fig. 4.

Thus, it may be considered that in all the systems studied by us chemical interaction occurs with the formation of a complex compound. From the standpoint of the acid and base theories of Brønsted [4], Lewis [5], and Usanovich [6], this interaction is acid–base in character. The phenols (resorcinol and quinizarin) play the role of the acid, while the amines (aniline, o- and p-toluidine) exhibit the properties of bases. The difference in the properties of the acid and the base is apparently manifested to a greater degree in the resorcinol–p-toluidine system, since the presence of chemical interaction in this system is expressed more clearly in the crystallization diagram.

Conclusions

1. By the visual-polythermal method, before and after heating to 200°, the crystallization curves were studied for systems containing quinizarin with p-toluidine and resorcinol with p-toluidine, o-toluidine, and aniline.

2. The liquidus diagrams obtained for the first three systems indicate the presence of chemical interaction between the components.

3. It was not possible to construct the complete crystallization diagram for the resorcinol–aniline system; therefore no definite conclusions can be drawn concerning the interaction of the components of the system.

4. Treatment of the systems studied by heating to 200°C does not cause significant changes in the systems.

Literature

1. Kremman R. and Rodinis, Monatsch., 27, 125, 1905.
2. Shushunov V. A. and Zouzolkova Z., Journal of Physical Chemistry, 22, 167, 1948.
3. Efremov N. N., Izvestiya SFKhA, Academy of Sciences of the USSR, 14, 63, 1941.
4. Brönsted, Ber. Dtsch. Chem. Ges., 42, 718, 1923.
5. G. N. Lewis, J. Franke Inst., 226, 293, 1938.
6. Usanovich M., Journal of General Chemistry, 9, 182, 1939.

1. Figure 4 shows the diagram of the system before its heating. ↩↩