Research in the Chemistry of Carbazole Derivatives
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This paper examines side reactions occurring during the dehydration of 9-(2-hydroxy)ethylcarbazole with caustic potash to produce 9-vinylcarbazole. It identifies cleavage of the oxyethyl group as a source of carbazole contamination and argues that omitting the usual water treatment allows carbazole to remain bound as nonvolatile potassium carbazole during vacuum distillation. A revised preparative procedure is described, giving 9-vinylcarbazole in about 64 percent total yield after recrystallization, with purity close to 100 percent by hydrolytic oxidation. The paper also notes formation of 9-carbazolylacetic acid as an additional side reaction that can be useful synthetically but does not significantly affect monomer purity.
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RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES
66. On Side Reactions during the Dehydration of 9-(2-Hydroxy)ethylcarbazole in the Presence of Caustic Potash
I. P. ZHEREBTSOV, V. P. LOPATINSKII
(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)
The dehydration of 9-(2-hydroxy)ethylcarbazole over caustic potash, leading to the preparation of 9-vinylcarbazole, which is of practical interest, was described long ago (1). Certain conditions for this synthesis were subsequently refined (1, 2, 3, 4), since no details were given in the original communication [(1)].
A more detailed study of the factors determining the purity of the monomer obtained in this process showed that the dehydration of 9-(2-hydroxy)ethylcarbazole is accompanied by side reactions [(5)]. One of the side reactions consists in cleavage of the oxyethyl group from 9-(2-hydroxy)ethylcarbazole, with formation of acetaldehyde and carbazole. The previously published procedures recommend treating the mixture obtained after dehydration with water in order to separate the alkali, and then isolating 9-vinylcarbazole by vacuum distillation. In this procedure, the carbazole formed in the side reaction enters the final product and contaminates it. However, it turned out that when a mixture of 9-vinylcarbazole with carbazole and caustic potash is heated, carbazole can be bound into nonvolatile potassium carbazole, and during vacuum distillation the 9-vinylcarbazole is not contaminated with carbazole. Therefore, in order to obtain pure 9-vinylcarbazole, the reaction mixture formed during dehydration should not be treated with water, so as to avoid decomposition of potassium carbazole, but should be subjected to distillation immediately after dehydration. This procedure makes it possible to bind the carbazole formed during the cleavage of 9-(2-hydroxy)ethylcarbazole into nonvolatile potassium carbazole and to prevent contamination of 9-vinylcarbazole with carbazole.
It should be noted that, at the same time, one more side reaction is observed: the conversion of 9-(2-hydroxy)ethylcarbazole into 9-carbazolylacetic acid, which under certain conditions may serve as a method for obtaining it [(6)], although it does not affect the purity of the monomer obtained.
The recommended procedure for preparing 9-vinylcarbazole, taking into account the side reaction of carbazole formation, is given below.
9-Vinylcarbazole
A mixture of 50 g of 9-(2-hydroxy)ethylcarbazole (m.p. 81–82°C) with 8–9 g of granulated caustic potash in a 150–200 ml Würtz flask is melted by heating on a water bath and then heated on a metal bath to 270° until intensive evolution of water begins, which requires 3–5 min; heating is then continued at the indicated temperature for 25 min. The hot mixture is transferred to a 200 ml Claisen flask with a side arm whose diameter is not less than 10 mm, and 9-vinylcarbazole is distilled off at 180–190° (at a pressure of 10–12 mm Hg). The average yield of crude 9-vinylcarbazole is 32.4 g. The substance melts at 59–62°C and contains traces of unreacted 9-(2-hydroxy)ethylcarbazole. After recrystallization from methanol, the yield of 9-vinylcarbazole with m.p. 63–64° is 27 g (60%). The methanol must be distilled over caustic potash (about 0.1 g of caustic potash is added per 100 ml of methanol). From the filtrate, by additional partial distillation of the solvent and repeated recrystallization of the precipitated solid, about 2.2 g of 9-vinylcarbazole with m.p. 63–64° is obtained. The total yield of 9-vinylcarbazole is 64% of theory. The content of pure substance, determined by the hydrolytic oxidation method, in this preparation of 9-vinylcarbazole is close to 100%.
From the residue after distillation, carbazole and 9-carbazolylacetic acid can be isolated [(5, 6)].
LITERATURE
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- V. P. Lopatinskii, I. P. Zherebtsov. Author’s certificate 216732, B. I. 15, 1968.