NEWS

of the Tomsk Polytechnic Institute named after S. M. Kirov, holder of the Order of the October Revolution and the Order of the Red Banner of Labor

Volume 257 — 1973

INVESTIGATION IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

82. KINETICS OF THE ACETYLATION OF 9-METHYLCARBAZOLE

M. I. Sukhoroslova, V. P. Lopatinskii, V. V. Bochkarev

(Presented by the Scientific-Methodological Seminar of the organic departments and laboratories)

The investigation we carried out earlier on the acetylation of 9-alkylcarbazoles in the presence of small amounts of catalysts showed that the main acetylation products are 3-acetyl-9-alkylcarbazoles [1]. However, during chromatographic separation on silica gel (solvent—chloroform), resin, 3,6-diacetylcarbazole, and two unknown substances were obtained as impurities; the infrared spectra of these substances showed the presence of carbonyl groups. Data on the chromatographic separation on silica gel in thin layers of the products of acetylation of 9-methylcarbazole with acetic anhydride at 135°C in the presence of sulfuric acid (molar ratio 9-methylcarbazole : acetic anhydride : sulfuric acid = \(1:20:1.17\cdot10^{-3}\)) and with a duration of 5 min are given in Table 1.

Since the reaction products contain impurities of resin, 3,6-diacetyl-9-methylcarbazole, and two unknown substances containing a carbonyl group, in our investigations we calculated the concentration of the carbonyl group, and not the concentration of 3-acetyl-9-methylcarbazole. Quantitative determination of the carbonyl group was carried out by the oximation method [2].

Processing of the results was carried out by the integral method. The acetylation of 9-methylcarbazole in the presence of sulfuric acid (Table 2), hydrochloric acid (Table 3), and magnesium perchlorate (anhydrone) (Table 4) was studied. In this, the influence of temperature, the initial concentration of 9-methylcarbazole, and the amount of catalyst on the yield of carbonyl compounds was investigated. The results of these studies-

... made it possible to determine the reaction order with respect to 9-methylcarbazole (Table 5), with respect to the catalyst (Table 6), the activation energy and pre-exponential factor (Table 7), and also to derive equations for the reaction rate and the rate constant (Table 8).

As can be seen from the tables, the true reaction order with respect to 9-methylcarbazole for all three catalysts is equal to unity, whereas the apparent reaction order with respect to 9-methylcarbazole differs from the true one and depends on the process conditions. Thus, for example, when the acetylation reaction is carried out in the presence of sulfuric acid at different temperatures, the apparent reaction order changes from 2.8 at 100°C to 5.1 at 135°C; in the presence of perchloric acid, from 1.7 at 60°C to 5.8 at 90°C.

Table 1

Chromatographic separation of the acetylation products of 9-methylcarbazole

* The melting point after crystallization from benzene is given.

** The melting point after crystallization from ethanol is given.

*** The presence of a carbonyl group was determined by a qualitative reaction with a solution of 2,4-dinitrophenylhydrazine.

Table 2

Acetylation of 9-methylcarbazole in the presence of sulfuric acid

* Molar ratio of reagents: 9-methylcarbazole:acetic anhydride:sulfuric acid = 1:20:1,17·10⁻³.

Table 3

Acetylation of 9-methylcarbazole in the presence of hydrochloric acid

* Molar ratio of reagents: 9-methylcarbazole : acetic anhydride : \(HClO_4 = 1:10:1{,}17 \cdot 10^{-3}\).

Table 4

Acetylation of 9-methylcarbazole in the presence of anhydrone

Table 5

Kinetic data for the acetylation reaction of 9-methylcarbazole with acetic anhydride

Table 6

Table 7

Kinetic data for the acetylation reaction of 9-methylcarbazole with acetic anhydride

Table 8

in the presence of magnesium perchlorate from 1.0 at \(70^\circ\mathrm{C}\) to 1.6 at \(99^\circ\mathrm{C}\) (Table 7). The value of the time order of the reaction is affected by the amount of catalyst (Table 6), as well as by the initial concentration of 9-methyl-

carbazole (Table 5).

The discrepancy between the true reaction order with respect to 9-methylcarbazole and the time order can be explained by the occurrence of secondary processes involving the initial 9-methylcarbazole.

The occurrence of secondary processes during the acetylation of 9-methylcarbazole is confirmed by the formation of resin-like products; moreover, in the presence of sulfuric and perchloric acids, resin formation is observed to a greater extent than when magnesium perchlorate is used as the catalyst. This is consistent with the fact that, when magnesium perchlorate is used as the catalyst, the time order with respect to 9-methylcarbazole was found to be lower than when sulfuric and perchloric acids are used as catalysts.

Taking the activation energy as a measure of catalyst activity, the catalysts studied may be arranged in the following series in order of increasing catalytic activity:

\[ \mathrm{H_2SO_4 < Mg(ClO_4)_2 < HClO_4} \]

This series differs from the series obtained in our preceding communication [1], where the activity of the catalysts was compared on the basis of the yield of carbonyl compounds obtained under identical acetylation conditions. Perchloric acid in that case showed lower activity owing to the occurrence of side condensation processes of the carbonyl compounds.

CONCLUSIONS

1. The kinetics of the acetylation of 9-methylcarbazole with acetic anhydride in the presence of sulfuric acid, perchloric acid, and magnesium perchlorate has been studied.

2. Equations for the reaction rate and rate constants have been obtained.

REFERENCES

1. M. M. Sukhoroslova, V. P. Lopatinskii, E. E. Sirotkina, T. D. Pirogova. Izvestiya TPI. Vol. 250, in press.

2. Houben–Weyl. Methods of Organic Chemistry. Vol. II, Moscow, Goskhimizdat, 1963.