Research in the Chemistry of Carbazole Derivatives

XXI. Synthesis of 3,6-Diacetylcarbazole

V. P. Lopatinskii, M. M. Anosova, L. P. Konkina

(Presented by the scientific seminar of the problem laboratories of the Faculty of Chemical Technology)

The subject of the present work was the study of the acylation reaction of carbazole with the aim of obtaining 3,6-diacetylcarbazole, which is an intermediate product in the synthesis of 3,6-divinylcarbazole, a new monomer based on carbazole.

According to the literature, 3,6-diacetylcarbazole was obtained by Plant, Rogers, and Williams [1] by treating a mixture of carbazole and anhydrous aluminum chloride in carbon disulfide with acetyl bromide, with heating under a reflux condenser for one hour; also by Buu-Hoï and Rua [2] by treating a mixture of carbazole and anhydrous aluminum chloride with a solution of acetyl chloride in carbon disulfide; and by Mitchell and Plant [3] by acetylating carbazole with acetic anhydride in nitrobenzene. The authors do not indicate the yields of the product in any of the cited works.

We obtained 3,6-diacetylcarbazole by treating a mixture of carbazole and anhydrous aluminum chloride in dry benzene with acetyl chloride (with the addition of 0.1 mole of acetyl bromide) for 2 hours under cooling. In this connection, the influence on the yield was studied of: a) the amount of acetyl chloride; b) the amount of aluminum chloride; c) the duration of the reaction; d) the change in temperature; e) the amount of solvent (benzene). The results of some experiments are given in Table 1. As a result of the study, optimal conditions were found under which the yield of crude product reaches 90% of the theoretical. Pure 3,6-diacetylcarbazole is obtained after two recrystallizations from ethanol, as a result of which the yield decreases to 53%. The optimal amounts of reagents (per mole of carbazole) are: 2.5 moles of acetyl chloride, 2.5 moles of aluminum chloride, and 19 moles of benzene at a temperature of 5°C and a reaction duration of 2–2.5 hours.

Experimental Part

Starting reagents: benzene, dry, cryoscopic; acetyl chloride and acetyl bromide, “pure” grade; aluminum chloride, anhydrous, “purified” grade. Carbazole, “pure” grade, was additionally recrystallized from ethanol and had m.p. 241°C.

Table 1

*) The amount of acetyl chloride, aluminum chloride, and benzene is given in moles per mole of carbazole.

3,6-Diacetylcarbazole

To a mixture of 5.0 g of carbazole and 50 ml of benzene, 9.99 g of anhydrous aluminum chloride was added. The mixture was cooled in a snow bath, and at a temperature of 5°C, with vigorous stirring, 6.1 g of acetyl chloride and 0.17 g of acetyl bromide were added dropwise over 15–20 minutes. The reaction was continued at 5°C for 2–2.5 hours with periodic stirring. After completion of the reaction, ice was added to the reaction mass; the precipitate that formed was filtered off, washed with water to neutral reaction, and recrystallized twice from ethanol. The yield of 3,6-diacetylcarbazole, m.p. 232–233°C, was 3.95 g (53% of theory). 3,6-Diacetylcarbazole crystallizes in the form of colorless scales or needles, dissolves moderately in hot ethanol, and is almost insoluble in benzene. It forms a yellow picrate, m.p. 155–156.5°C.

Analytical results: found, %: C 76.76; H 5.23; N 5.67.
$\mathrm{C}_{16}\mathrm{H}_{13}\mathrm{O}_{2}\mathrm{N}$. Calculated, %: C 76.47; H 5.21; N 5.57.

Conclusions

1. The influence of the principal factors on the yield of 3,6-diacetylcarbazole in the acetylation of carbazole with acetyl chloride (with addition of acetyl bromide) in benzene in the presence of anhydrous aluminum chloride was studied.

2. It was shown that, under optimal conditions, 3,6-diacetylcarbazole can be isolated in a yield of 53% of theory.

References

1. S. Plant, R. Rogers, S. Williams, J. Chem. Soc., 1935, 741.
2. N. Buu-Hoi, R. Royer, Rec. trav. chim., 66, 533, 1947; C. A. 42, 5014, 1948.
3. D. Mitchell, S. Plant, J. Chem. Soc., 1936, 1295.