Study in the Chemistry of Carbazole Derivatives
Abstract Generated abstract
This study examines alternative catalysts for the acetylation of carbazole with acetic anhydride, motivated by the hazards of using hydrochloric acid despite its high activity. The authors compare anhydrous salts of hydrochloric, phosphoric, and sulfuric acids under varied conditions and assess formation of 9-acetylcarbazole by yield and melting point. Several salts showed no catalytic effect, while ammonium perchlorate, acid sulfates, ammonium sulfate, and ammonium hydrogen phosphate gave near quantitative conversion under harsher conditions. Magnesium perchlorate was the most active salt catalyst, producing approximately quantitative 9-acetylcarbazole in 5 minutes at 40°C without resinification, and the crude product could be purified by recrystallization alone.
Full Text
RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES
IV. ACETYLATION OF CARBAZOLE WITH ACETIC ANHYDRIDE IN THE PRESENCE OF SALTS OF HYDROCHLORIC ACID AND OTHER CATALYSTS
V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova1
(Presented by Prof. G. P. Kutepov, Doctor of Chemical Sciences)
In previous studies it was shown that the most active catalysts for the acetylation of carbazole with acetic anhydride are hydrochloric acid [1] and an aqueous solution of zinc chloride [2]. Quantitative conversion of carbazole into 9-acetylcarbazole is achieved in the presence of 0.4–0.8% (by weight) hydrochloric acid (in the form of a 30% aqueous solution) in 5 minutes at 20°C, or in the presence of 2% (by weight) zinc chloride in 10 minutes at 80°C. Since working even with aqueous solutions of hydrochloric acid is unsafe, especially in the presence of organic substances, we set ourselves the task of investigating other catalysts for the acetylation of carbazole.
In the present work we give the results of a study of the catalytic activity of a series of salts of hydrochloric, phosphoric, and sulfuric acids in the reaction of acetylation of carbazole with acetic anhydride (NH₄ClO₄, KClO₄, Mg(ClO₄)₂, Na₂SO₄, K₂SO₄, (NH₄)₂SO₄, NaHSO₄, KHSO₄, Na₃PO₄, Na₂HPO₄, NaH₂PO₄, (NH₄)₂HPO₄). Anhydrous salts were used as catalysts, with the exception of Mg(ClO₄)₂, which showed high catalytic activity also in the form of the crystalline hydrate. With aqueous solutions of salts, acetylation did not take place. It was established that in the presence of certain salts (KClO₄, Na₂SO₄, K₂SO₄, Na₃PO₄, Na₂HPO₄, NaH₂PO₄) 9-acetylcarbazole is not formed even upon many hours of boiling carbazole in an excess of acetic anhydride. Data on the catalytic activity of the other salts are given in Table 1.
As the data of Table 1 show, magnesium perchlorate exhibits the greatest activity; in its presence (in an amount of 2% of the weight of carbazole), at 40°C, quantitative conversion of carbazole into 9-acetylcarbazole can be achieved in 5 minutes. Thus, in its catalytic activity magnesium perchlorate approaches the activity of hydrochloric acid and is more convenient to handle. It should
Table 1
Results of experiments on the acetylation of carbazole with acetic anhydride
in the presence of catalysts
| Exp. no. | Catalyst | Amount of catalyst, % by weight of carbazole | Molar ratio of acetic anhydride to carbazole | Reaction temperature, °C | Reaction time, min | Yield of crude 9-acetylcarbazole, % of theoretical | Melting point of crude 9-acetylcarbazole, °C |
|---|---|---|---|---|---|---|---|
| 1 | NaHSO₄ | 5 | 4 | 100 | 180 | ∼100 | 68—69 |
| 2 | KHSO₄ | 4 | 4 | 100 | 120 | ∼100 | 69—70 |
| 3 | (NH₄)₂SO₄ | 5 | 4 | 100 | 90 | ∼100 | 68—69 |
| 4 | (NH₄)₂HPO₄ | 5 | 4 | 100 | 60 | ∼100 | 68—69 |
| 5 | NH₄ClO₄ | 2 | 4 | 50 | 15 | ∼100 | 69—70 |
| 6 | Mg(ClO₄)₂ | 2 | 4 | 40 | 5 | ∼100 | 69—70 |
It should be noted that in the presence of all the salt catalysts listed in Table 1, acetylation is not accompanied by resinification, in contrast to a number of other catalysts proposed earlier [1—3]. Therefore, crude 9-acetylcarbazole obtained in the presence of the salts investigated does not require boiling with animal charcoal during purification and can be brought to the correct melting point only by recrystallization.
Experimental Part
Starting Materials
Carbazole was isolated from crude anthracene from the Kemerovo Coke-Chemical Plant by means of paraformaldehyde [4] and had m.p. 244°C. Acetic anhydride and all catalysts were of “chemically pure” grade.
9-Acetylcarbazole
Acetic anhydride, the catalyst, and carbazole were charged into a round-bottom flask fitted with a thermometer and stirrer. The reaction mixture was rapidly brought to the required temperature with the stirrer operating. After dissolution of the carbazole, heating was continued for another 1—2 minutes, after which the contents of the flask were poured into water and stirred until the precipitated product had solidified. The precipitate was filtered off, washed on the filter with water until the acetic acid was removed, and dried over CaCl₂ and soda lime in a vacuum desiccator to constant weight. For purification, crude 9-acetylcarbazole was recrystallized from ethanol. An example of the synthesis of 9-acetylcarbazole in the presence of Mg(ClO₄)₂ is given below. A mixture of 11.20 ml of acetic anhydride, 5.0 g of carbazole, and 0.01 g of Mg(ClO₄)₂ was stirred. Heating at 40°C was continued for 5 minutes. After treatment of the reaction mass with water, filtration, washing, and drying of the precipitate, 6.24 g of 9-acetylcarbazole with m.p. 69—70°C was obtained (yield about 100% of theoretical). Recrystallization from ethanol raised the m.p. to 73—74.5°C; repeated recrystallization gave a substance with m.p. 74.5—75.5°C (according to the literature, 9-acetylcarbazole has m.p. 69—70° [5], 75° [6], 76° [3, 7, 8]).
Conclusions
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The catalytic activity of a number of salts of hydrochloric, phosphoric, and sulfuric acids in the acetylation reaction of carbazole with acetic anhydride has been investigated.
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It has been shown that magnesium perchlorate possesses the greatest catalytic activity.
Literature
- A. A. Berlin. ZhOKh, 14, 438, 1944.
- V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova. Izvestiya Tomskogo politekhnicheskogo instituta, 111, 1961.
- J. Böeseken, Rec. trav. chim., 31, 350, 1912.
- V. P. Lopatinskii, E. E. Sirotkina. Izvestiya Tomskogo politekhnicheskogo instituta, 111, 1961.
- S. M. Sherlin, A. Ya. Berlin. ZhOKh, 7, 2275, 1937.
- W. Perkin, S. Plant, J. Chem. Soc., 123, 676, 1923.
- A. W. Blom, Helv. chim. acta, 4, 625, 1921.
- B. Barclay, N. Campbell, J. Chem. Soc., 997, 1946.
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The following students took part in the experimental portion of the work: G. V. Sonina, T. V. Kamarenko, T. I. Taran, V. D. Tsyganova, A. G. ↩