Research in the Chemistry of Carbazole Derivatives
Abstract Generated abstract
This study examines the reported failure of urea to react directly with secondary aromatic amines and proposes that the reaction can occur in a weakly alkaline medium rather than under the acidic or strongly alkaline conditions commonly used. Conductivity measurements of carbazole and aniline solutions in benzene are used to support the formation and thermal dissociation of ion pairs, suggesting conditions under which a carbazole anion may react with isocyanic acid generated from urea. Applying this hypothesis, the authors synthesize carbazole-9-carboxylic acid amide by heating carbazole and urea in aniline, obtaining crystalline product in high yield under optimized conditions. The work contributes a proposed route for direct interaction of urea with secondary aromatic amines and demonstrates it for carbazole.
Full Text
RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES
73. ON THE INTERACTION OF CARBAZOLE WITH UREA
V. P. Lopatinskii, V. V. Goldshtein
(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)
Although the reactions of urea with amines have been studied in detail, it is noted in the literature that it is incapable of interacting with secondary aromatic amines (1). A study of the patent literature also shows that the direct interaction of secondary amines with urea has not yet been carried out.
The reason for the inactivity of secondary aromatic amines in this reaction should, in our opinion, be sought in the fact that the process is traditionally carried out exclusively in an acidic medium (2—7), where dissociation of the N—H bond of the secondary aromatic amine, which is weakly acidic in character, is suppressed by the stronger acids—catalysts.
Carrying out the reaction in a neutral medium likewise does not lead to success, since dissociation of the N—H bond remains extremely slight, while at elevated temperature urea more readily forms products of thermal transformation than it reacts with the secondary amino group.
In a strongly alkaline medium, reactions of solvolysis of urea proceed predominantly, with formation of carbon dioxide, ammonia, and salts of cyanic acid (8—9).
Only carrying out the reaction in a weakly alkaline medium makes it possible to weaken the N—H bond of the amine and bring it into reaction with urea with formation of an unsymmetrical disubstituted urea. In a weakly alkaline medium, conditions arise for dissociation of the N—H bond, and at the same time this medium does not destroy the urea.
The experiments performed confirm the above. The electrical conductivity of solutions of carbazole (a secondary amine) and aniline (a weakly alkaline agent) in benzene (an indifferent solvent (10)) was measured.
Solutions of carbazole and aniline have a higher electrical conductivity than solutions of each of these substances separately; moreover, the solutions with a molar ratio carbazole : aniline of 1 : 1 possess the maximum conductivity. Increasing the voltage applied to the cell with the solution promotes an increase in electrical conductivity; the electrical conductivity becomes noticeable beginning at a voltage of 200 V and higher \((1 \cdot 10^{-6}—1.5 \cdot 10^{-6}\ \Omega)\). For this solution, at a voltage of 225 V, the influence of temperature (up to 62°) on the change in conductivity was studied. It was shown that, with increasing temperature, the conductivity increases, changing according to a logarithmic law, and for solutions
with a total molar concentration not higher than 0.1 mole/kg of solvent, the following equation is valid:
\[ \sigma = 0.1 C \cdot 1.5t, \]
where
\[ \begin{aligned} \sigma&\text{—conductivity, ohm}^{-1}\,\text{m}^{-1};\\ t&\text{—temperature, }^\circ\mathrm{C};\\ C&\text{—total concentration, mole/kg of solvent }(C\text{ of carbazole}\\ &\quad = C\text{ of aniline}). \end{aligned} \]
Judging from the data obtained, when aniline and carbazole are dissolved simultaneously, an interaction occurs between them, leading to the formation of more or less stable ion pairs.
High voltage (180–200 V and higher) and an increase in temperature promote the destruction of ion pairs into ions. It should be expected that at temperatures of 150–180° the ion concentration will become appreciable. Similar conditions can be created by dissolving carbazole in aniline and heating the solutions to the boiling temperature. If urea is present in the same solution, then an increase in temperature will lead to its thermal dissociation to isocyanic acid. The latter contains an electrophilic center (the carbon atom) capable of interacting with the carbazole anion. As a result of the interaction, the amide of carbazole-9-carboxylic acid will be formed.
Experimental Part
Into a flask equipped with a reflux condenser, aniline (20 ml), carbazole, and urea are introduced successively. The mixture is heated to gentle boiling; in this process carbazole and urea dissolve in the aniline. The reaction lasts 3–4 hours, counted from the beginning of boiling.
Then benzene is added to the reaction mixture, cooled to 60–70°, and stirring is continued until the reaction mixture has completely cooled. The precipitate that separates is filtered off on a Büchner funnel, washed with a small amount of benzene, and dried.
Needle-shaped crystals, stable in air, with a slight lilac tint and a melting point of 250–251°C are formed. Unlike carbazole, this substance does not sublime. Found molecular weight 209, calculated—210. Nitrogen found 13.02%, calculated 13.30%. The results of experiments to determine the optimal reaction conditions are given in Table 1. An attempt to wash the resulting product with a mixture of hydrochloric and acetic acids led to its decomposition to carbazole, which is consistent with data on the low energy of the N—C bond in molecules
Table 1
Optimal conditions for the synthesis of carbazole-9-carboxylic acid amide
| Carbazole content, mole | Carbazole content, g | Urea content, mole | Urea content, g | Reaction time, h | m.p., °C | Yield, % |
|---|---|---|---|---|---|---|
| 0.01 | 1.67 | 0.01 | 0.60 | 2.5 | 250 | 75–80 |
| 0.01 | 1.67 | 0.01 | 0.60 | 4.0 | 251 | 87–91 |
| 0.01 | 1.67 | 0.011 | 0.66 | 4.0 | 251 | 90–93 |
| 0.01 | 1.67 | 0.012 | 0.72 | 4.0 | 250 | 93–97 |
| 0.01 | 1.67 | 0.013 | 0.78 | 4.0 | 250 | ca. 100 |
| 0.01 | 1.67 | 0.013 | 0.78 | 2.5 | 248 | 83–70 |
9-acyl derivatives of carbazole [11]. Nitrous acid resinifies the reaction product.
Conclusions
-
The possibility has been proposed of carrying out the direct interaction of urea with secondary aromatic amines in a weakly alkaline solvent at elevated temperature.
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On the basis of the proposed hypothesis, the amide of carbazole-9-carboxylic acid was synthesized.
LITERATURE
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