Research in the Chemistry of Carbazole Derivatives
Abstract Generated abstract
This study examines how hydrogen bonding with methanol affects the ultraviolet absorption spectra of carbazole and 3,6-dichlorocarbazole, with attention to shifts associated with the imino group as a proton donor. Spectra were recorded in isooctane, methanol, and mixed isooctane, methanol solvents, and changes in band position and molar extinction coefficients were compared under controlled concentrations. The results show bathochromic shifts and increased absorption intensity in methanol-containing media, with 3,6-dichlorocarbazole reaching the methanol-like spectral pattern more rapidly than carbazole. The authors conclude that 3,6-dichlorocarbazole has greater proton-donor activity, and hence greater acidity, than carbazole.
Full Text
Research in the Chemistry of Carbazole Derivatives
38. The Influence of Hydrogen Bonding on the Ultraviolet Absorption Spectra of Carbazole and 3,6-Dichlorocarbazole
I. P. Zherebtsov, T. A. Sagachenko, V. P. Lopatinskii
(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)
It is known that the $\pi \to \pi^{*}$ transition in the region around 290 m$\mu$ in the ultraviolet spectrum of carbazole is sensitive to hydrogen bonding arising through the hydrogen of the imino group with proton acceptors [1, 2]. It has been shown, in particular, that the indicated band undergoes a bathochromic shift [1]; moreover, from the magnitude of the shift one can estimate the relative proton-donor activity of carbazole and other secondary amines [2].
The aim of our work was to investigate the formation of hydrogen bonding between carbazole, 3,6-dichlorocarbazole, and methanol. The spectra of the indicated substances were recorded in isooctane, methanol, and in mixtures of these solvents. The spectra of carbazole and 3,6-dichlorocarbazole in isooctane are presented in Fig. 1.
Fig. 1. 1 — carbazole; 2 — 3,6-dichlorocarbazole
The introduction of substituents causes the appearance of a bathochromic shift, a blurred spectral pattern, and a certain increase in intens-
absorption. Considering the spectra in methanol (Fig. 2), it may be noted that the influence of hydrogen bonding is manifested in a stronger bathochromic shift of the absorption bands and in an increase in intensity. On the basis of these data alone one can already estimate the relative proton-donor activity of the indicated substances in the formation of a hydrogen bond.
Fig. 2. 1—carbazole; 2—3,6-dichlorocarbazole
of the indicated substances in the formation of a hydrogen bond. Table 1 presents the values of the logarithms of the molar extinction coefficients of solutions of carbazole and 3,6-dichlorocarbazole in a mixed solvent (isooctane plus methanol), at concentrations of the substances under study of \(0.35 \cdot 10^{-4}\ \mathrm{mol/l}\) and \(0.2 \cdot 10^{-4}\ \mathrm{mol/l}\), respectively.
As is seen from Table 1, the spectral pattern of carbazole and 3,6-dichlorocarbazole changes at different concentrations of methanol in isooctane.
Table 1
| Substance | Concentration of methanol in isooctane, \(\mathrm{mol/l}\cdot 10^{-4}\) | \(\lg \varepsilon\), \(\lambda\) 292 | \(\lg \varepsilon\), \(\lambda\) 323 | \(\lg \varepsilon\), \(\lambda\) 336 |
|---|---|---|---|---|
| Carbazole | 0.468 | 4.1792 | 3.4718 | 3.4118 |
| Carbazole | 0.624 | 4.1992 | 3.4972 | 3.4201 |
| Carbazole | 0.620 | 4.2021 | 3.5081 | 3.4217 |
| Substance | Concentration of methanol in isooctane, \(\mathrm{mol/l}\cdot 10^{-4}\) | \(\lg \varepsilon\), \(\lambda\) 302 | \(\lg \varepsilon\), \(\lambda\) 339 | \(\lg \varepsilon\), \(\lambda\) 350 |
|---|---|---|---|---|
| 3,6-Dichlorocarbazole | 0.272 | 3.3779 | 3.5421 | 3.5072 |
| 3,6-Dichlorocarbazole | 0.408 | 4.3881 | 3.5901 | 3.5518 |
| 3,6-Dichlorocarbazole | 0.544 | 4.3897 | 3.5861 | 3.5689 |
The spectrum of 3,6-dichlorocarbazole in the mixed solvent approaches the spectral pattern in methanol more rapidly. The results obtained indicate that 3,6-dichlorocarbazole exhibits greater proto-
proton-donor activity than carbazole, which also indicates its greater acidity.
Starting Reagents and Experimental Conditions
Carbazole had a melting point of 244°C. It was purified by recrystallizing the reagent-grade product from methanol. 3,6-Dichlorocarbazole, m.p. 206°C, was prepared according to the known method [3].
The solvents were purified by methods accepted in the literature [4]. It should be noted that traces of water in the solvents significantly distort the spectral pattern. The working solutions were prepared by dissolving a weighed portion of the substance under investigation in a specified volume of solvent to obtain concentrations within the range in which Beer’s law is obeyed. The spectra were recorded on an SF-4 spectrophotometer at 20°C in a quartz cuvette with a thickness of 1 cm.
Conclusions
- A study of the ultraviolet absorption spectra of carbazole and 3,6-dichlorocarbazole has shown that the latter has greater proton-donor activity than carbazole.
LITERATURE
- E. A. Chernitskii, S. V. Konev. Doklady AN BSSR, 8, 4, 1964.
- A. K. Chandra, S. B. Lu. Trans. Faraday, Soc., 56, 632, 1960.
- V. P. Lopatinskii, I. P. Zherebtsov, S. F. Zelinskaya. Izv. TPI (in press).
- A. Weisberger et al. Organic Solvents. IL, 1958.