Reports of the Academy of Sciences of the USSR
1958. Volume 120, No. 2

CHEMISTRY

P. S. PELKIS and R. G. DUBENKO

STUDY OF THE EFFECT OF SOLVENT ON THE POSITION OF TAUTOMERIC EQUILIBRIUM IN A SERIES OF ARYL THIOCARBAZONES

(Presented by Academician B. A. Kazanskii, 27 XII 1957)

The effect of solvent on the position of tautomeric equilibrium is generally known. However, many questions in the theory of this phenomenon have until recently remained unclear. In recent years much has been done in this direction by M. I. Kabachnik and co-workers \((^{1-6})\).

In generalized form these questions have been considered by A. N. Nesmeyanov and M. I. Kabachnik \((^{7-10})\).

M. I. Kabachnik \((^{1})\) applied the modern theory of acid–base equilibrium, developed by Brønsted \((^{11})\) and N. A. Izmailov \((^{12})\), to the calculation of tautomeric equilibrium. Kabachnik derived a formula expressing the dependence between the constants of tautomeric equilibrium in two solvents: \(S_1\) and \(S_2\)

\[ pK_{\mathrm{TS}_2}=pK_{\mathrm{TS}_1}+\mathrm{const}. \tag{1} \]

If \(pK_{\mathrm{TS}}\) in two solvents is plotted along the coordinate axes, then for all keto–enols the points on the graph should lie on one straight line with a slope tangent equal to 1. Upon verification it was confirmed that the derived relation is valid for many keto–enols \((^{10})\). Recently M. I. Kabachnik and co-workers showed that relation (1) is valid for tautomeric equilibrium in a series of dialkyl thiophosphates \((^{6})\). It is of interest to verify whether relation (1) is applicable to tautomeric equilibrium in the series of aryl thiocarbazones.

One of us \((^{13})\) showed that 1,5-diphenylthiocarbazone (dithizone) in solvents is a mixture of the tautomers thione (a) and thiol (b):

\[ \begin{array}{cc} \mathrm{C_6H_5N{=}N} & \mathrm{C_6H_5N{=}N} \\ \quad\backslash & \quad\backslash \\ \quad C{=}S & \quad C{-}SH \\ \quad/ & \quad\| \\ \mathrm{C_6H_5NHNH} & \mathrm{C_6H_5NH{-}N} \end{array} \]

\[ \begin{array}{cc} a & b \end{array} \]

The ratio of thione and thiol forms in the tautomeric mixture depends on the nature of the solvent and of the substituent in the aromatic rings of the thiocarbazone. It was shown that the short-wave maximum \((\lambda_{\max 1})\) on the absorption-spectrum curve, 450 mµ, belongs to the thiol form, while the absorption maximum 620 mµ \((\lambda_{\max 2})\) belongs to the thione form of dithizone. Accordingly, on the absorption curves of substituted 1,5-diphenylthiocarbazone, the maxima in the short-wave region belong to the thiol forms, and the maxima in the long-wave region to the thione forms.

From the equations \(d_{\lambda \max_1}=lc_1\varepsilon_1\) and \(d_{\lambda \max_2}=lc_2\varepsilon_2\) it follows that the ratio

\[ \frac{d_{\lambda \max_2}}{d_{\lambda \max_1}} = \frac{c_2\varepsilon_2}{c_1\varepsilon_1}, \]

whence the constant of tautomeric equilibrium is

\[ K_{\mathrm{T}}=\frac{c_2}{c_1} = \frac{d_{\lambda \max_2}}{d_{\lambda \max_1}} \frac{\varepsilon_1}{\varepsilon_2} \]

\[ pK_{\mathrm{T}} = \lg \frac{d_{\lambda \max_2}}{d_{\lambda \max_1}} + \lg \frac{\varepsilon_1}{\varepsilon_2}; \tag{2} \]

where \(d_{\lambda\max_2}\) and \(d_{\lambda\max_1}\) are the optical densities (extinctions), \(c\) is the concentration, \(\varepsilon_1\) and \(\varepsilon_2\) are the molar extinction coefficients of the thiol and, respectively, thione, and \(l\) is the layer thickness.

As the experimental data show, the tautomeric equilibrium constant of dithizone and its substituted derivatives in solvents is approximately equal to the ratio \(\dfrac{d_{\lambda\max_2}}{d_{\lambda\max_1}}\). The ratio \(\dfrac{\varepsilon_1}{\varepsilon_2}\) is practically independent of the solvent.

Table 1

Table 2

Note. \(a\): above the line \(\lambda_{\max_1}\), below the line \(d_{\lambda\max_1}\); \(b\): above the line \(\lambda_{\max_2}\), below the line \(d_{\lambda\max_2}\).

solvent and is approximately equal to unity and, consequently, the value of the second term of equation (2) may be neglected.

For the calculation of the molar extinction coefficients \(\varepsilon_1\) we took as 100% for the thiol form of the thiocarbazones the absorption intensity at the maxima of the corresponding S-methyl derivatives of dithizone, synthesized by us \((^{16})\), of the general formula

\[ \begin{array}{c} \mathrm{RN{=}N}\\[-2mm] \quad \backslash\\[-2mm] \quad \mathrm{C(SCH_3)} \\[-2mm] /\\[-2mm] \mathrm{RNHN} \end{array} \]

\(\varepsilon_2\) was calculated by the formula:

\[ \varepsilon_2=\frac{d_{\lambda\ \max_2}}{l(a-a_1)}, \]

where \(a\) is the weighed amount of the corresponding thiocarbazone in g-mol/l, and \(a_1\) is the weight fraction of the thiol form in the weighed amount \((a)\).

Fig. 1. Relationship between \(pK_{TS_2}\) and \(pK_{TS_1}\) for five thiocarbazones.
\(1\) — 1,5-di-(phenyl)-thiocarbazone, \(2\) — 1,5-di-(o-tolyl)-thiocarbazone, \(3\) — 1,5-di-(o-phenoxyphenyl)-thiocarbazone, \(4\) — 1,5-di-(o-methoxyphenyl)-thiocarbazone, \(5\) — 1,5-di-(2,4-dibromophenyl)-thiocarbazone.
\(I\) — \(S_1\)=benzene, \(S_2\)=hexane; \(II\) — \(S_1\)=carbon tetrachloride, \(S_2\)=hexane; \(III\) — \(S_1\)=carbon tetrachloride, \(S_2\)=benzene; \(IV\) — \(S_1\)=benzene, \(S_2\)=chloroform.

Table 1 gives the data for determining the constants of tautomeric equilibrium of three thiocarbazones in different solvents. In Table 2, for the thiocarbazones studied,

\[ \begin{array}{c} \mathrm{RN{=}N}\\[-2mm] \quad \backslash\\[-2mm] \quad \mathrm{C{=}S}\\[-2mm] /\\[-2mm] \mathrm{RNHNH} \end{array} \]

the absorption maxima \(\lambda_{\max_1}\) and \(\lambda_{\max_2}\), the corresponding optical densities \(d_{\lambda\max_1}\), \(d_{\lambda\max_2}\), and their ratio

\[ \frac{d_{\lambda\max_2}}{d_{\lambda\max_1}}=K_T \]

are presented.

As can be seen from the data of Table 2 and Fig. 1, the linear character of the dependence and the angle of inclination of the straight lines of \(45^\circ\) are maintained quite satisfactorily.

It may therefore be considered that the relation derived by M. I. Kabachnik, which extends the regularities of acid–base protolytic equilibrium to tautomeric keto–enol equilibrium, is confirmed in the series of arylthiocarbazones for thione–thiol tautomeric equilibrium.

Arylthiocarbazones (preparations Nos. 1–4) were synthesized by the formazyl method according to literature data \((^{14,15})\). The absorption spectra of the thiocarbazones and their S-methyl derivatives were measured in the appropriate solvents at a concentration of \(6.6\cdot10^{-5}\) g-mol/l on an SF-4 spectrophotometer.

Institute of Organic Chemistry
Academy of Sciences of the Ukrainian SSR

Received
27. IX 1955

CITED LITERATURE

\(^{1}\) M. I. Kabachnik, DAN, 83, 407 (1952).
\(^{2}\) M. I. Kabachnik, DAN, 83, 859 (1952).
\(^{3}\) M. I. Kabachnik, Izv. AN SSSR, OKhN, 1955, 98.
\(^{4}\) M. I. Kabachnik, T. A. Mastryukova, Izv. AN SSSR OKhN, 1953, 163.
\(^{5}\) M. I. Kabachnik, S. T. Ioffe, DAN, 91, 833 (1953).
\(^{6}\) M. I. Kabachnik, S. T. Ioffe, T. A. Mastryukova, ZhOKh, No. 2 (1955).
\(^{7}\) M. I. Kabachnik, Usp. khim., 25, 137 (1956).
\(^{8}\) A. N. Nesmeyanov, M. I. Kabachnik, in: Problems of Chemical Kinetics, Catalysis, and Reactivity, Publishing House of the Academy of Sciences of the USSR, 1955, pp. 644–698.
\(^{9}\) A. N. Nesmeyanov, M. I. Kabachnik, ZhOKh, 25, 41 (1955).
\(^{10}\) M. I. Kabachnik, in: Problems of the Mechanism of Organic Reactions, Kiev, 1954, pp. 126–156.
\(^{11}\) J. N. Brönsted, Z. phys. Chem., A, 169, 32 (1934).
\(^{12}\) N. A. Izmailov, ZhFKh, 24, 321 (1950).
\(^{13}\) P. S. Pel’kis, DAN, 88, 999 (1953).
\(^{14}\) D. S. Tarbell et al., J. Am. Chem. Soc., 70, 1385 (1948).
\(^{15}\) P. S. Pel’kis, L. S. Pupko, Ukr. khim. zhurn., 17, 93 (1951).
\(^{16}\) P. S. Pel’kis, R. G. Dubenko, DAN, 110, 798 (1956). P. S. Pel’kis, R. G. Dubenko, L. S. Pupko, ZhOKh, 27, 2134 (1957).