Isolation of Carbazole from Crude Anthracene Using Paraformaldehyde
Abstract Generated abstract
This study examines the isolation of carbazole from crude anthracene by replacing aqueous formaldehyde with paraformaldehyde in the established conversion of carbazole to soluble 9-oxymethylcarbazole. Enriched anthracene from the Kemerovo coke-chemical plant was treated in ethanol at 80°C with paraformaldehyde and calcium hydroxide, followed by regeneration of carbazole from the intermediate using aqueous ammonia. Under the best reported conditions, carbazole was obtained in 86 to 90% yield relative to its content in enriched anthracene, with 93 to 96% purity before recrystallization and 98.7% purity after one recrystallization from chlorobenzene. The substitution of paraformaldehyde preserved product quality while reducing ethanol consumption by about half and formaldehyde consumption by a factor of five.
Full Text
Isolation of Carbazole from Crude Anthracene Using Paraformaldehyde
V. P. Lopatin, E. E. Sirotkina1
(Presented by Professor, Doctor of Chemical Sciences L. P. Kustov)
In previous studies [1–7] it was established that carbazole, unlike the other components of crude anthracene, reacts with formaldehyde to give 9-oxymethylcarbazole, which can readily be separated from anthracene owing to its good solubility in organic solvents. When boiled with aqueous alkali solutions, 9-oxymethylcarbazole decomposes back with regeneration of carbazole. This method can be used successfully only provided that the crude anthracene is first enriched, during which phenanthrene and oily impurities are removed from it; these, like 9-oxymethylcarbazole, are also readily soluble in aromatic hydrocarbons. The reaction of carbazole with formaldehyde gives the best results when it is carried out in an alcoholic medium in the presence of caustic or carbonic alkalis. A disadvantage of the formaldehyde method is the considerable consumption of solvent (alcohol) and the need to use large excesses of formaldehyde (10 moles per mole of carbazole).
In the present work it was found that good results in isolating carbazole from crude anthracene can be obtained by using paraformaldehyde as the reagent (instead of an aqueous formaldehyde solution) in ethanol, with calcium oxide hydrate as catalyst. The results of several experiments are given in Table 1.
In this work crude anthracene from the Kemerovo coke-chemical plant was used; according to our analyses, it contained 35% carbazole. To isolate carbazole, the crude anthracene was first freed from phenanthrene and oils by ordinary washing with aromatic solvents [6, 7], and then the enriched anthracene was treated with paraformaldehyde in an alcoholic medium. The reaction was carried out at a temperature of 80°C for 40 minutes in the presence of Ca(OH)\(_2\) (5% of the weight of carbazole). The best yields of the reaction product were obtained when two moles of formaldehyde in the form of paraformaldehyde and 8.5 moles of ethanol were used per mole of carbazole. The resulting solution of 9-oxymethylcarbazole was freed from the anthracene precipitate; the filtrate was boiled for 30 minutes with an aqueous ammonia solution, as a result of which carbazole was regenerated from 9-oxymethylcarbazole. The carbazole isolated in this way has
Results of several experiments on isolating carbazole from crude anthracene using paraformaldehyde
Table 1
| No. | Charge of enriched anthracene, g | Content of carbazole in the charge, % by weight of enriched anthracene | Reaction temperature, °C | Reaction duration, min | Ca(OH)₂, % by weight of all carbazole | HCOOH, g per 2 g-mol of carbazole | Paraformaldehyde, g per 2 g-mol of carbazole | C₂H₅OH in the mother liquors per mol of carbazole | Yield of carbazole, % of the content in enriched anthracene | Content of carbazole, % | Melting point, °C | Carbazole after recrystallization: content of carbazole, % | Carbazole after recrystallization: melting point, °C |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 50 | 56 | 80 | 45 | 10 | 300 | — | 17 | 90.2 | 91 | 236 | 97 | 240 |
| 2 | 50 | 56 | 80 | 45 | 5 | 300 | — | 17 | 95.1 | 90 | 236 | 96 | 239 |
| 3 | 50 | 63 | 65 | 45 | 10 | 300 | — | 17 | 90.0 | 87 | 235 | 95 | 238 |
| 4 | 50 | 63 | 65 | 50 | 5 | 300 | — | 17 | 80.7 | 90 | 236 | 97 | 240 |
| 5 | 200 | 60 | 80 | 40 | 5 | — | 60 | 8.5 | 85.3 | 95.9 | 237 | 98.8 | 242 |
| 6 | 200 | 60 | 80 | 40 | 5 | — | 60 | 8.5 | 86.6 | 90.4 | 237 | 98.7 | 242 |
| 7 | 240 | 60 | 80 | 40 | 5 | — | 60 | 8.5 | 89.3 | 93.3 | 237 | 98.7 | 242 |
| 8 | 1000 | 60 | 80 | 40 | 5 | — | 60 | 8.5 | 86.4 | 95.7 | 238 | 98.8 | 243 |
m.p. 237–238°C and contains 93–96% carbazole (by nitrogen, determined by the Kjeldahl method [8]). The yield of carbazole is 86–90% of its content in the enriched anthracene. A single recrystallization from chlorobenzene raises the carbazole content to 98.7%, and its melting point to 242–243°C.
Thus, the use of paraformaldehyde instead of formalin leads to the same results with respect to the quality of the isolated carbazole and has the advantage of making it possible to reduce the consumption of solvent by a factor of 2, and of formaldehyde by a factor of 5.
Conclusions
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The isolation of carbazole from crude anthracene from the Kemerovo Coke-Chemical Plant using paraformaldehyde has been studied.
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It has been found that replacing formalin with paraformaldehyde can reduce the consumption of solvent by a factor of 2 and that of formaldehyde by a factor of 5.
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V. G. Melnikova and I. B. Akhmetzyanova took part in the experimental part of the work. ↩