Research in the Chemistry of Carbazole Derivatives

95. Ionization of 3-Nitrocarbazole in Basic Media

I. P. Zherebtsov, V. P. Lopatinskii, S. P. Yudina,
T. V. Melnikova

(Presented by the scientific-methodological seminar of the organic departments of the Chemical-Technological Faculty)

Carbazole and its derivatives substituted in the nucleus are weak acids [1]. This property of carbazoles makes it possible to assume the occurrence of certain base-catalyzed reactions involving them through the formation of anions, whose concentration and properties largely determine the direction and kinetics of the transformations.

In this connection it is important to know the thermodynamic characteristics of the ionization of carbazoles in basic media; the present work is devoted to this problem. As a model characterizing the behavior of carbazoles, 3-nitrocarbazole (3-NC) was chosen.

3-NC is a fairly strong acid, the pKa of which is known [2]. The optical properties of the 3-NC anion and of its nonionized molecule make it possible to work in the visible region of the spectrum, using such simple instruments as photoelectric colorimeters [3]. The ionization of 3-NC was studied both in individual organic solvents and in their mixtures under the action of potassium hydroxide.

According to Brønsted [4], the strength of an acid in a solvent is determined by the protophilicity of the solvent and by its dielectric constant; moreover, the more protophilic the solvent, the more ionized the acid is in it. Since the measure of the solvent’s influence on the degree of ionization is the quantity \(\Delta pK\), which is defined by expression (5):

\[ \Delta pK = pK_i - pK_a, \tag{5} \]

where \(pK_i\) is the ionization constant of 3-NK in the given solvent,

\(pK_a\) is the ionization constant of 3-NK in water, known from the literature [2],

the problem was reduced to determining \(pK_i\) in the given medium.

Since the acidity values for the solvents (alcohols) used by us, reported in the literature [6], were determined by different methods and differ considerably from one another, which does not allow these values to be used in our work, to determine the relative acidities of the alcohols we used the dependence of the basicity function \(H_-\) [7], which is a measure of the tendency of the medium to remove a proton, on the amount of potassium hydroxide (Fig. 1). 3-NK was used as the indicator.

Table I

\(t = 24^\circ\mathrm{C}\)

* Values of the acidity function are given for \(C_{\mathrm{KOH}} = 0.0251\) mol/L and \(50^\circ\mathrm{C}\).

The experimental data presented in Fig. 1 and in Table I make it possible to arrange the alcohols according to their influence on the ability of the alkaline medium to abstract a proton in the series: ethylene glycol < methanol < ethanol < monomethyl ether of ethylene glycol < butanol < n-propyl alcohol < i-propyl alcohol. As can be seen from the data presented, the degree of ionization of 3-NK is determined by the protophilicity of the solvent at the same amount of potassium hydroxide. The small positive value of \(\Delta S^\circ\) indicates insignificant steric hindrance to ionization. Moreover, it is lowest in methanol and highest in isopropyl alcohol, which is probably associated with relatively small

sizes of the methanol molecule and to steric difficulties in the case of isopropyl alcohol.

Table 2

\(t = 24^\circ\mathrm{C}\)

* Values of the acidity function are given for \(C_{\mathrm{KOH}} = 0{,}0159\ \mathrm{mol/l}\).

In addition, the higher the protophilicity of the solvent, the stronger its bond with the proton, which causes an increase in ordering during solvation and a rise in \(\Delta S^\circ\) (Tables 1 and 2). On the other hand, the energy expenditure for ionization is thereby reduced, which leads to a decrease in \(\Delta H^\circ\).

The parallelism of the straight lines in Fig. 1 indicates that the behavior of 3-NK in all the solutions studied is the same; moreover, the tangent of the angle of inclination of these straight lines is equal to unity, which indicates that in all the solvents used one molecule of potassium hydroxide is consumed for the ionization of one molecule of 3-NK.

Vertical axis: \(H^-\). Horizontal axis: \(\lg C_{\text{conc.}}\). Curves: isopropyl alcohol; \(n\)-propyl alcohol; butyl alcohol; methyl cellosolve; ethyl alcohol; methyl alcohol; ethylene glycol.

Fig. I.

The results of the investigation of the acidities of mixed solvents, presented in Fig. 2 and in Table 2, show that the basicity of the mixture increases with an increase in the mole percent of a solvent more protophilic than methanol. As can be seen from Fig. 2, the more protophilic the added solvent, the steeper the slope of the straight line expressing the dependence of \(H^-\) on the mole percent of the protophilic solvent.

When comparing \(\Delta pK\) of 3-NC in the mixtures used and the values of the functions \(H^-\) at a constant amount of alkali, it is evident that the degree of ionization increases with increasing concentration of the protophilic component of the mixture (Table 2).

Thus, by changing the nature and composition of the solvent or the alkali content in it, it is possible to attain the required degree of ionization of 3-NC. The results obtained by us were used in studying the kinetics of the interaction of carbazole and its derivatives with olefin oxides [8].

Experimental Part

The ionization constants of 3-NC both in individual and in mixed solvents (\(pK_i\)) were determined graphically by the method described by Chattanathan and Kalidas [9]. The point of intersection of the straight line expressing the dependence of the logarithm of the ionization ratio (\(\lg I\))

Figure labels: vertical axis \(H\); horizontal axis, mol.% solvent. Curves: dimethyl sulfoxide; dimethylformamide; acetone; butanol; monomethyl ether; ethylene glycol.

Fig. 2.

from the logarithm of the concentration of potassium ethoxide for each solvent, with the abscissa axis, where \(\lg I = 0\), gives the pK of 3-NK in this solvent.

The ionization ratio is the ratio of the concentration of the 3-NK anion to the concentration of the nonionized 3-NK molecule:

\[ I=\frac{[A^-]}{[HA]}; \qquad [HA]=C_0-[A], \]

where \(C_0\) is the initial concentration of 3-NK.

The concentration of 3-NK ions was determined using the photometric method described in [3]. An FEKN-57 photoelectric colorimeter was used; the analytical wavelength was 453 nm,

\[ [A^-]=\frac{D}{\varepsilon \ell}, \]

where \(D\) is the optical density of the solution;
\(\ell\) is the cuvette length, cm;
\(\varepsilon\) is the molar absorption coefficient of the 3-NK ion.

Since complete ionization of 3-NK could not be achieved in the solutions studied, the molar absorption coefficient of the 3-NK ion for each solvent was determined graphically by the Benesi–Hildebrand method [10].

Literature

1. R. Stewart, J. P. B. Donnel. Canad. J. Chem., 42, 1681, 1964.

1. R. Stewart, J. P. D. Donnel. J. Am. Chem. Soc., 84, 493, 1962.

2. V. P. Lopatinskii, I. P. Zherebtsov, S. P. Yudina. Izv. TPI, vol. 257, 76, 1972.

3. N. A. Izmailov. Electrochemistry of Solutions. Moscow, “Khimiya,” 1966.

4. A. I. Shatenshtein. Isotopic Exchange and Substitution of Hydrogen in Organic Compounds. Moscow, Publishing House of the Academy of Sciences of the USSR, 1960.

5. Collection: Modern Problems of Physical Organic Chemistry. Moscow, “Mir,” 1967.

6. K. Bowden. Chem. Revs., 66, 119, 1966.

7. I. P. Zherebtsov, V. P. Lopatinskii, S. P. Yudina, Z. A. Michurina. Izv. TPI, present collection, 91, 1973.

8. N. Chattanathan, C. Kalidas. Austral. J. Chem., 24, No. 1, 83, 1971.

9. L. Andrews, R. Kifer. Molecular Complexes in Organic Chemistry. Moscow, “Mir,” 1967.