ELECTRICAL CONDUCTIVITY AND ELECTROMOTIVE FORCE OF NITROBENZENE SOLUTIONS OF NITRO COMPOUNDS

L. P. KULEV and B. A. GRANDILEVSKII

With the introduction of a nitro group into the molecule of an organic substance, its reactivity and its tendency to form addition products of the complex type are increased. Especially numerous examples of molecular compounds of this kind have been established in the aromatic series, where even for the simplest mononitro derivatives of benzene, stable crystalline substances of definite composition with sulfuric acid are known [1], while the presence of two or more nitro groups in one nucleus is favorable for the formation of intensely colored complexes with a variety of organic substances [2].

The method we developed [3] for investigating organic compounds by the electromotive force of their reaction with metals proved convenient for evaluating the complex-forming ability of many substances, including nitro-substituted compounds of the aromatic series, and made it possible to establish the presence of unstable, but very active toward metallic sodium, molecular compounds of these substances with carboxylic acids [4]. Among the nitro compounds we studied, nitrobenzene proved to be the best complex former; it promoted a very considerable increase in EMF not only in the presence of weak carboxylic acids, but also of phenols [5]. It is known that the nitro group exerts a definite influence on other substituents located in the same benzene ring. Thus, nitro-substituted phenols are stronger acids than phenol itself, and some of them—for example, picric acid—are, moreover, characterized by a clearly expressed tendency to form stable complex compounds even with aromatic hydrocarbons [6]. Since in the formation of such complexes by picric acid the oxygen of the nitro group takes part, the decisive factor for evaluating the complex-forming ability of trinitrophenol should not be the hydrogen of the hydroxyl group and the acidity of the compound dependent on it, but the number and the nature of the arrangement of the nitro groups.

The electromotive force of the reaction of acidic organic compounds with metals is, to be sure, primarily a measure of the mobility of the hydrogen of the hydroxyl group; but, as we have already indicated earlier [7], this mobility depends on the nature of the solvent, and even strong carboxylic acids may have negligible activity if they exist in the form of stable dimers in which there is no hydrogen at the oxonium oxygen (for example, in benzene solutions).

Carboxylic acids and phenols exhibit great activity toward metallic sodium in nitrobenzene solutions; however, the electromotive force arising in the reaction relates not to the individual substances, but to complex compounds, in whose formation the solvent itself undoubtedly also takes part. Therefore the EMF values do not always characterize the “acidity” of a compound in the usual sense—

understanding of this term. Leaving aside for the time being the question of the composition and structure of the complexes formed, we set ourselves the aim of comparing the activity of certain nitroso and nitro compounds, dissolved in equimolecular ratios in nitrobenzene, and of clarifying the possibility of applying methods based on electrical conductivity and electromotive force to the study of these substances and complex molecular compounds.

The experiments were carried out in an apparatus with a platinum crucible; the EMF was measured with the aid of a CV-type galvanometer (LFI, \(R = 2167\Omega\)). In parallel, the electrical conductivity of nitrobenzene solutions of all the indicated compounds was measured by the usual method under the same conditions, but over wider temperature limits (from 0 to 60°; the EMF was determined at \(t^\circ 17\)—18°), since for some substances it could be detected only upon heating the solution. The nitroso compounds were obtained by us synthetically and were carefully purified by recrystallization.

Table 1

Characteristics of the substances tested

Para-nitrosophenol was obtained from phenol (as an intermediate product in the preparation of para-quinone dioxime) and from para-nitrosodimethylaniline. Pure nitrobenzene preparations were used as solvent; they were treated with a solution of caustic soda, dried over calcium chloride, and distilled (\(t^\circ\) boiling point 209°). All samples of \(\mathrm{C_6H_5NO_2}\) were brought by purification to the standard EMF value (0.7—0.8 millivolt on a galvanometer with \(R = 2167\Omega\)). The experiments gave the following results (see Table 2).

The greatest electromotive force at the first moment of the reaction corresponds to para-nitrosophenol. It is interesting to note that a gradual increase in EMF is observed only for phenol and its derivatives, with the exception of ortho-nitrophenol, for which the EMF maximum is reached immediately upon immersion of sodium in the solution and then remains constant for a rather long period of time. Alkylated para-nitroanilines proved to be more active than \(p\)-\(\mathrm{C_6H_4}\!\begin{matrix}\mathrm{NO}\\[-2pt]\mathrm{NH_2}\end{matrix}\); the EMF of nitrosodiethylaniline is more than twice that for para-nitrosoaniline. Among the isomeric nitrosodiphenylamines, \(p\)-\(\mathrm{C_6H_5NC_6H_4NOH}\) stands out sharply; in activity it may be compared with phenols, but it gives a different order of change of EMF.

Table 2

The electromotive force of the same compounds in benzene solutions is many times smaller and can be detected only at comparatively high concentrations (Table 3).

Table 3

The electrical conductivity of the nitroso compounds was determined with a Wheatstone bridge for alternating current in a Kohlrausch vessel (capacity—1.404), in which a nitrobenzene solution of the substance under investigation was placed \([1/50\ \mathrm{M/l}]\). The results are presented in Table 4.

Thus, the electrical conductivity of nitrosobenzene, p-nitrosoanilide, \(N\)-nitrosodiphenylamine, and quinone can be determined only at \(t^\circ\) 30–40°; whereas the electromotive force for these substances is readily detected even at 0° without any noticeable in-

Table 4

of \(t^\circ\) until the end of the experiment. The electromotive force of equimolecular solutions of these substances does not exceed 8 mv (Table 2); however, \(p\)-nitrosodimethylaniline, for which the EMF is also close to this value, differs very considerably from them in electrical conductivity. If one compares \(p\text{-}C_6H_4\!\begin{matrix}/NO\\ \backslash N(CH_3)_2\end{matrix}\) and \(p\text{-}C_6H_4\!\begin{matrix}/NO\\ \backslash N(C_2H_5)_2\end{matrix}\), it turns out that the electrical conductivity for both substances, although it increases with increasing temperature, does not make it possible to establish which of them is the more active, since different temperature ranges correspond now to larger and now to smaller values of electrical conductivity (compare, for example, the electrical conductivity of these substances at 5, 10, 12, 22, and 60°). It is evident from the table that comparable values for all the substances investigated are obtained at 40°, and at this temperature \(C_6H_5NC_6H_4NOH\) proves to be the most active, while at 50 and 60° it is \(p\text{-}C_6H_4\!\begin{matrix}/NO\\ \backslash OH\end{matrix}\), to which para-nitrosodiethylaniline is very close (at 60°), although it contains no hydroxyl group. In general, it must be said that the electrical-conductivity values obtained by us are apparently less characteristic than the values of the electromotive force, since the latter agree better with the chemical character of the compounds investigated and depend to a lesser degree on temperature conditions, as is evident, for example, from the following comparison (Table 5).

It may be expected that, with increasing temperature, the components of the mixture under investigation undergo more or less considerable changes in composition; therefore the electrical-conductivity values established at comparatively high temperatures cannot always be referred to the original substances and, consequently, are a less accurate measure for comparing the activity of nitroso compounds than are the values of the electromotive force. The change in the dissolved substance upon heating is confirmed by determination of the EMF under the following conditions. If a nitrobenzene solution of para-nitrosophenol is first heated and then cooled to 40° or 18°, a considerable decrease in the electromotive force is observed at the first moment of the reaction.

However, in all the cases cited, the character of the change in EMF remains the same, i.e., the electromotive force increases gradually, and 5 minutes after the immersion of sodium in the solution it already almost does not differ from the initial value (at 18°). Since the indicated order of change in EMF was observed by us only for phenol and its derivatives

Table 5

Table 6

(See Table 2), i.e., in the case when a phenolic hydroxyl is present in the molecule, it must be supposed that heating to the indicated temperature limits is not associated with a profound change in the molecule of para-nitrophenol; the decrease in electromotive force at the first moment of the reaction may depend, for example, on a change in the composition of the complex

\[ p\text{-}\mathrm{C_6H_4}\begin{matrix}\mathrm{NO}\\[-2pt]\mathrm{OH}\end{matrix} \]

with nitrobenzene.

With the immersion of sodium in the solution, an increase in electrical conductivity must undoubtedly take place, especially in those cases where substances with a greater degree of dissociation accumulate in the mixture, for example, phenolates; therefore, for comparison, experiments were set up under the following conditions. A piece of metallic sodium was immersed in the vessel (capacity 1.404) for the same intervals of time as in the determination of the EMF, after which the metal was removed and, after the solution had been stirred, the electrical conductivity was measured. All the conditions (surface of the sodium immersed in the solution, amounts of reacting substances, etc.) corresponded exactly to our experiments on measuring the electromotive force (see Table 7).

With the exception of pure nitrobenzene and a nitrobenzene solution of quinone, in all the remaining cases an increase in electrical conductivity is observed. Of the nitrophenols measured, at the first moment of reaction with sodium, para-nitrophenol proved to be the most active, but already after 5 minutes the electrical conductivity of

\[ m\text{-}\mathrm{C_6H_4}\begin{matrix}\mathrm{NO_2}\\[-2pt]\mathrm{OH}\end{matrix}, \]

increases sharply; all the other compounds we tested readily yield to it. Meta- and para-nitrophenols behave analogously also under the conditions of determining the electro-

Table 7

Specific electrical conductivity of nitrobenzene solutions of the substances studied after immersion of metallic sodium.

of the driving force, i.e., the greatest jump in potential at the moment of immersion of sodium in the solution corresponds to \(p\text{-}\mathrm{C_6H_4}\begin{matrix}\mathrm{NO_2}\\[-2pt]\mathrm{OH}\end{matrix}\), and after some time to \(m\text{-}\mathrm{C_6H_4}\begin{matrix}\mathrm{NO_2}\\[-2pt]\mathrm{OH}\end{matrix}\) (see Table 2). If the values of the emf are compared with the values of the electrical conductivity for the same groups of substances, then in general a direct dependence is not infrequently observed between these constants. Sometimes, however, on the contrary, a considerable increase in emf is not connected with any noticeable change in electrical conductivity, as has been established, for example, for quinone. If it is taken into account that when sodium is immersed in a nitrobenzene solution of quinone the surface of the metal becomes covered with a blue coating that rapidly blackens, the increase in electromotive force may be connected with the formation of a complex directly at the interface of the two phases (metal–solution). Such a complex, it must be supposed, is constructed analogously to quinhydrone and should possess a deeper coloration than the latter, since with the introduction of sodium atoms a salt is formed in whose anion the oxygen is directly bound to only one ring and, moreover, is in the ionoid state, which contributes to an increase in the bathochromic effect, i.e., to a deepening of the color [8]. The role of the anion in such a complex is played by the benzene and quinoid rings, connected with one another by secondary valences, while the role of the cation is played by the electropositive sodium ions. The formation and structure of a molecular compound of this type may be represented by the following schemes:

\[ \begin{gathered} \text{quinone} + 2\mathrm{Na} \longrightarrow \text{disodium salt} \longrightarrow \left[ \text{quinone ring}\,\cdots\,\text{quinoid ring} \right]\mathrm{Na}^{+}\mathrm{Na}^{+} \end{gathered} \]

\[ \begin{gathered} \mathrm{CONa}\,\cdots\,\mathrm{CO}\\ \text{benzene ring}\,\cdots\,\text{quinone ring}\\ \mathrm{CONa} \end{gathered} \]

Since the light-yellow color of the nitrobenzene solution of quinone, when sodium is immersed, does not change over any intervals of the experiment, it may be concluded that the complex formed on the surface of the metal does not pass into solution and, consequently, does not influence the electrical conductivity. In order to compare the activity of molecular compounds of the indicated type with the activity of the individual components composing them, quinhydrone and naphthalene picrate were tested, as well as mixtures of the individual substances entering into the composition of these complexes.

Table 8

The electrical conductivity of a nitrobenzene solution of quinhydrone is less than the electrical conductivity of hydroquinone, while its maximum corresponds to that of the mixture of hydroquinone with quinone after immersion of sodium in the solutions under investigation. The increase in electrical conductivity in this case may be explained by the fact that the amount of \(C_6H_4(ONa)_2\) in solution gradually increases, since the hydrogen evolved in the reaction with sodium readily reduces quinone to hydroquinone.

It is seen from Table 8 (experiment 4) that the electrical conductivity of quinhydrone begins to increase appreciably only after a more or less prolonged action of sodium, i.e., under conditions in which partial decomposition of quinhydrone into its constituent components can occur and, consequently, accumulation of phenolate in the solution can take place. The electromotive force arises in the reaction of sodium with the same substances under other conditions and, apparently, depends only to a slight degree on the electrical conductivity of the solution. The EMF of quinone is greater than the EMF of hydroquinone, and also than that of the mixture of the latter with quinone and of ready-made quinhydrone, which indicates primarily a reaction of sodium with the quinone itself, which forms the above-mentioned complex covering the surface of the metal with a film insoluble in nitrobenzene. The electromotive force of the mixture of quinone with hydroquinone, as well as that of quinhydrone, is smaller, since in these systems quinone is more or less firmly bound with the second component and enters into reaction with sodium with greater difficulty.

The values of the electrical conductivity of nitrobenzene solutions of picric acid, naphthalene picrate, and the mixture of trinitrophenol with naphthalene are almost identical and change according to the time during which sodium remains in the solution. The EMF values for the same substances are more characteristic and more sharply emphasize the difference between the activity of picric acid and that of its complex with naphthalene (experiments 5, 7, and 8) with respect to sodium. In this connection it is interesting to note that, whereas the electrical conductivity of the nitrobenzene solution of naphthalene (experiment 6) does not change even upon immersion of sodium, the electromotive force increases almost twofold in comparison with the EMF of nitrobenzene (0.8 mV—1.2 mV). Since for picrate

for naphthalene, higher values of electrical conductivity and EMF are observed than for picric acid; it may be assumed that naphthalene also forms a complex with nitrobenzene, but one that is considerably less active and has no effect on the conductivity of the solution.

Consequently, the method of determining the EMF is more sensitive and makes it possible to establish the presence of molecular compounds in such mixtures for which other methods, at any ratios of the components, give no positive results. The considerable increase in electromotive force for nitrobenzene solutions of naphthalene makes it possible to study a series of complexes with other polynuclear hydrocarbons and to enter the interesting field of molecular compounds of nitro compounds with regard to elucidating their stability and the relation of structure to color.

Conclusions

1. The specific electrical conductivity of some of the nitrobenzene solutions of nitroso compounds examined by us can be determined only at elevated temperature. At low temperatures it is measurable for secondary and tertiary aromatic nitrosoamines and nitrosophenols, and, with immersion of metallic sodium into the solution, for all the substances studied without exception (except nitrobenzene itself).

2. The electromotive force arising in the reaction of nitroso compounds with sodium is almost independent of temperature and makes it easy to compare the activity of these substances under ordinary conditions. The activity of tertiary aromatic nitrosoamines depends on the nature of the radical at nitrogen (the EMF of nitrosodiethylaniline is greater than that of p-nitrosodimethylaniline); the activity of secondary ones depends on the position of the nitroso group (the smallest EMF corresponds to N-nitrosodiphenylamine).

3. The electromotive force of nitrosophenols is greater than that of the corresponding nitrophenols; for compounds with a phenolic hydroxyl, a gradual increase in EMF is characteristic, which makes it possible to detect them easily even in highly dilute nitrobenzene solutions.

4. The electrical conductivity of nitrobenzene changes hardly at all in the presence of quinone (even after immersion of sodium in the solution), whereas the electromotive force increases by a factor of 9–10; i.e., it has significant advantages over the former for evaluating the complex-forming ability of this substance. The same is also observed for solutions of naphthalene in nitrobenzene.

5. From the electromotive force of the reaction with sodium it is possible to establish the presence of complex compounds even in those cases where no appreciable change in electrical conductivity is observed, for example, for nitro-substituted benzenes with polynuclear aromatic hydrocarbons.

Literature

1. Guben I.—Methods of Organic Chemistry, III, 2, p. 323, 1935.

2. Kulev L. P. and Tronov B. V.—Determination of the activity of hydrogen in organic compounds from the EMF of the reaction with sodium. ZhOKh, vol. 4, p. 197, 1934.

3. Kulev L. P. and Ivanov B. V.—Determination of the purity and standardization of nitrobenzene. Collection of Works of TGPI, vol. 1, p. 160, 1939.

4. Tronov B. V. and Kulev L. P.—Complexes of nitroso compounds of the benzene series with acids. ZhOKh, vol. V, p. 401, 1935.

5. Tronov B. V. and Kulev L. P.—The electromotive force of reactions with metals as a method for studying organic compounds. Jubilee Collection of Works of TPI, p. 335, 1937.

6. Cherbuliez C. 1, 1491. 1923.

7. Ley H., e. Engelhardt K. Ph. Ch, 74, 30, 39. 1910.

8. Pfeiffer. Organische Molekülverbindungen, 226–236, 237–245 (1922). Bitto, Annalen, 269, 377. 1892. Hantzsch u. Kussel. Ber. 32, 3140.

9. Werner A. Ber. 42 (1909), 4324–28.