Influence of Impurities on the Electromotive Force and Electrical Conductivity of Nitrobenzene

L. P. KULEV and B. A. GRANDILEVSKII

We have established [2] that the electromotive force arising when metallic sodium is immersed in nitrobenzene can serve as a criterion of the purity of this preparation. Samples of nitrobenzene obtained by nitrating benzene under different conditions differ sharply from one another in the magnitude of the EMF, even in cases where they were subjected to the same purification and were distilled within the same narrow temperature limits. It is known that, in the nitration reaction, an increase in temperature is reflected, similarly to dilution of nitric acid, in an intensification of oxidation as compared with the nitrating effect; therefore phenol and, chiefly, products of its nitration and oxidation may always be present in nitrobenzene in greater or lesser amounts. Most of them are removed already when crude nitrobenzene is treated with a solution of soda and with water, but, as we were able to establish, even after distillation of C₆H₅NO₂, traces, for example, of para-nitrophenol remain as a constant impurity and very appreciably increase the EMF of nitrobenzene.

In order to determine which compounds, and to what extent, influence the magnitude of the electromotive force of nitrobenzene, we carried out a series of experiments under the following conditions: finished nitrobenzene (“Goslaborreaktiv,” ChDA) was thoroughly treated with a solution of caustic soda and with water, dried over calcium chloride, and distilled (b.p. 209°). In all experiments the same preparation of nitrobenzene was used; the criterion of its purity was the constant value of the EMF, i.e., 0.8 millivolt (on the scale of a galvanometer with \(R = 2167\Omega\)). The substances tested, partly synthesized by us and partly commercial, were purified by repeated recrystallization or distillation and had constant physical constants.

For determination of the EMF, freshly prepared solutions of these substances (\(1/50\) mole/l) were placed in the platinum crucible of the apparatus (5 ml each); the electromotive force arising in the reaction with metallic sodium was determined with the aid of a sensitive galvanometer (type C V, LFI, \(R = 2167\)). The electrical conductivity of the same substances was measured by the usual method, i.e., by the Wheatstone bridge method for alternating current (\(l/A = 1.404\)).

Electromotive force of nitrobenzene solutions of o-, m-, and p-nitrophenols.

From Table 1 it is seen that, at the moment sodium is immersed in the solution, at comparatively high concentrations (about 0.3%), the greatest EMF value corresponds to para-nitrophenol, but in all other cases metanitrophenol stands first. If the figures obtained are compared with the electromotive force of the purest preparation of nitrobenzene (experiment 19), it turns out that, upon dilution, the solutions of orthonitrophenol approach the EMF of the solvent, i.e. of C₆H₅NO₂ itself, earliest (limit about 0.01%), then para (0.001%), and finally meta (about 0.0001%). Since all three substances were subjected to especially careful purification and, moreover,

Table 1

Numerous experiments with preparations of various factory brands gave analogous results each time; the difference in the activity of these isomers should be attributed to their structural difference.

The nitroso group exerts a considerably stronger influence on the mobility of the hydrogen of the phenolic hydroxyl than the nitro group; therefore even hundred-thousandths of a percent of para-nitrosophenol can still be detected in nitrobenzene solutions by the reaction with sodium.

Table 2

As in the case of meta- and para-nitrophenols, a gradual increase in the EMF is also characteristic here, i.e. its maximum occurs not at the first moment of immersion of sodium in the solution, but after a rather long interval of time, and only when the content of para-nitrosophenol in nitrobenzene is on the order of ten-thousandths of a percent (experiment 5) does this order change. It is interesting that for the less active ortho-nitrophenol, at any concentrations no similar increase in the EMF is observed; there, as for pure nitrobenzene, its maximum is reached at once and differs little from the maximum of the solvent. The negative influence of the nitro group situated next to the hydroxyl becomes still more evident if the data obtained are compared with the electromotive force of unsubstituted phenol.

Table 3

The minimum amount of phenol detectable by EMF corresponds to hundred-thousandths of a percent (experiment 4), i.e. it is lower than the limit established for metanitrophenol, although the latter at high concentrations (cf., for example, the EMF maximum after 10 min. in experiment No. 1, Table 1) is considerably more active. As for the other substituted phenols indicated above, with the exception of \(o\text{-}C_6H_4\begin{matrix} /NO_2 \\ \backslash OH \end{matrix}\), here too the EMF maximum is not attained at the first moment of the reaction, and this circumstance apparently is not connected with the acidity of these compounds, since all nitro-substituted \(C_6H_5OH\) generally are stronger acids than phenol itself. The same is also observed for polynitro-substituted phenols, in particular for the dinitrophenols 2,4,1 and 2,6,1; picric acid, however, gives a pattern of EMF change similar to that of orthonitrophenol, although in aqueous solutions it is undoubtedly a stronger acid than all the compounds we tested.

Table 4

Thus, picric acid gives a considerable electromotive force even at high dilution (0.009%); however, subsequently its activity drops sharply (cf. experiments 1 and 2), and the determinable minimum amounts do not exceed hundred-thousandths of a percent, i.e., approximately the same as was obtained for phenol and para-nitrophenol. Dinitrophenols are considerably less active and differ from picric acid by a gradual increase in electromotive force; moreover, the EMF of $\mathrm{C_6H_3(NO_2)_2OH}$ $(2:6:1)$ is smaller than that of its isomer $\mathrm{C_6H_3(NO_2)_2OH}$ $(2:4:1)$; the difference is more noticeable at low concentrations of the substances (cf., for example, experiments 9 and 14), which does not agree with the acidity of these compounds (the electrolytic dissociation constant for dinitrophenol $(2:6:1)$ is $1.7\cdot 10^{-4}$, for dinitrophenol $(2:4:1)$ it is $8\cdot 10^{-5}$ at $25^\circ$). We have already noted [2] that the acidity of organic substances is not always proportional to the EMF values, since, depending on the nature of the solvent, the molecule of the dissolved substance may have different structures and possess a different degree of activity of the hydrogen capable of being substituted by a metal.

It was thus established that in organic solvents that exclude the dissociation of substances, for example in benzene, strong carboxylic acids give an insignificant electromotive force with sodium and are far inferior in “acidity” even to alcohols of the fatty series, which in aqueous solutions, as is known, are compounds of neutral character. The nitrophenols we tested also give an insignificant electromotive force in benzene solution, and among them picric acid proved to be especially little active.

Table 3

NOTE: concentration of substances—$1/100$ M in 5 ml of nitrobenzene.

Hence it may be concluded that compounds with a phenolic hydroxyl, like carboxylic acids, exist in benzene solutions in the form of rather inert complexes in which mobile hydrogen is absent. The activity of these same substances in nitrobenzene may be due to the salt-like character of the molecular compounds, in whose formation the solvent itself, possessing, as is known [3], weakly basic properties, also takes part. By analogy with the known complex compounds of nitrobenzene with strong mineral acids, nitrophenols can give a complex, for example, of the following structure:

\[ \begin{array}{cc} \left[\mathrm{C_6H_5N\begin{matrix}//O\\ \backslash OH\end{matrix}}\right]\mathrm{OC_6H_5}; & \left[\mathrm{C_6H_5N\begin{matrix}//O\\ \backslash OH\end{matrix}}\right]\mathrm{OC_6H_4NO_2}; \\ \text{with phenol} & \text{with mononitrophenols} \end{array} \]

\[ \left[ \mathrm{C_6H_5\ N} \begin{array}{c} \overset{O}{\|} \\ \backslash \mathrm{OH} \end{array} \right]\mathrm{OC_6H_2(NO_2)_3}. \]

with trinitrophenol.

The stability of such complexes and their activity toward metallic sodium must depend on the number and character of the arrangement of the nitro groups in the benzene ring of the phenol. The greater the number of nitro groups, the more strongly charged is the electronegative ionoid component, and the more mobile should be the hydrogen located at the nitrogen of the electropositive part of the complex. Dinitrophenols and picric acid, therefore, give a large electromotive force with sodium, whereas the activity of mononitrophenols is smaller and depends on the position of the nitro group. Since the EMF of orthonitrophenol is less than that of the meta and para compounds, it must be assumed that the proximity of the hydroxyl and nitro groups affects the mobility of hydrogen as a result of intramolecular compensation of the charges of these substituents, and this hinders the formation of a salt-like complex. Dinitrophenols and picric acid, in which one or two nitro groups are also present next to the hydroxyl, behave differently, since the total number of negative charges (precisely owing to the excess of nitro groups) is greater in them and therefore the capacity for complex formation with nitrobenzene is not diminished. In this case the position of the nitro groups in the ring is also important; for example, the activity of isomeric dinitrophenols is different because the charge of the electronegative ionoid component in the complex of nitrobenzene with \(\mathrm{C_6H_3(NO_2)_2OH}\) (2:6:1) is smaller, since both nitro groups are situated in immediate proximity to the hydroxyl. In picric acid this influence is manifested more weakly, owing to the presence of a third nitro group, which strengthens the electronegative character of the molecule as a whole.

Thus, from the electromotive force of the reaction with sodium one can not only determine traces of impurities of certain substances in nitrobenzene preparations and establish the complex-forming capacity of these substances, but also judge the role of functional groups in the construction of complexes. The values of the specific electrical conductivity are considerably less characteristic; this was measured for the same compounds after immersion of sodium in nitrobenzene solutions (at \(18^\circ\)).

The gradual increase in electrical conductivity depends on the accumulation in the solution of products of reaction with sodium; however, here too the numbers obtained apparently refer not to the individual phenols, but to their complexes with the solvent. If the electrical conductivity were due to the presence of individual substances in the solutions, it would be in direct dependence on the mobility of the hydrogen of the hydroxyl group of the phenol, i.e., on its acidity; meanwhile, already for the isomeric nitrophenols a different regularity is observed. The constants of electrolytic dissociation of ortho- and paranitrophenols are identical (\(5.6 \cdot 10^{-8}\)), but in electrical conductivity in nitrobenzene they differ sharply from one another, and the para isomer proves to be many times more active than the ortho (a decrease in the resistance of the solution with \(\mathrm{NO_2C_6H_4OH}\) is not detected by our instrument even at comparatively high concentrations). The electrical conductivity of metanitrophenol is less than that of the para compound, which also corresponds to the behavior of these substances in aqueous solutions (the constant of electrolytic dissociation of \(m\text{-}\mathrm{NO_2C_6H_4OH}\) is \(3.9 \cdot 10^{-9}\)); but already after a five-minute stay of sodium in the nitrobenzene solution, the activity of the former becomes appreciably greater, and then it increases almost twofold (experiments 6 and 10 of Table 6). In general, for monosubstituted nitrophenol, in the experiments on measuring electrical conduct—

Table 6

…ness with immersion of sodium in the solution, the same characteristic features are observed as were noted by us also when comparing these substances by the EMF method. It might be thought that the exceptionally low activity of ortho-nitrophenol is due to its reduced capacity for complex formation. With adjacent positions of the nitro group and the hydroxyl, the following structure of the molecule of this compound is possible, for example:

\[ \begin{array}{c} \text{intramolecularly bonded } o\text{-nitrophenol:}\\[4pt] \ce{C6H4(OH)(NO2)},\quad \ce{O-H\cdots O=N(=O)-} \end{array} \]

The complex-bound hydrogen of the phenolic hydroxyl must possess low mobility, which hinders its transition into the positively charged ionoid component of the salt-like complex with nitrobenzene. Less characteristic for judging the complex-forming capacity of a substance are the values of electrical conductivity obtained with phenol and para-nitrosophenol; they differ little from one another, whereas the presence of the nitroso group in the para position should undoubtedly enhance this capacity, as is also confirmed by comparison of the indicated substances by electromotive force. Moreover, determination of the electrical conductivity of solutions, even with preliminary immersion of sodium in them, does not make it possible to detect such traces of impurities as are readily recognized from the change in EMF. Suffice it to say that for certain compounds, for example for para-nitrosophenol, by the second method it is possible

determine a thousand times smaller amount of this substance in nitrobenzene than by the first method*) (cf. experiments 6, Table 2 and 3, Table 6).

The sensitivity of the EMF to impurities makes it possible to control the purity of certain organic preparations and to judge the effectiveness of one or another method of treating them. Thus, the decisive factor in obtaining the purest preparations of nitrobenzene, free from substances of phenolic character, is treatment with alkali solutions, and not repeated distillation of it. The experiments given below illustrate this proposition quite clearly.

Nitrobenzene with an impurity of paranitrosophenol (1/500 mole per liter)

Table 7

It should be noted that no changes in electrical conductivity (without immersing sodium in the solution) were observed in the indicated cases. The purity of nitrobenzene is affected also by drying it with the aid of one or another water-absorbing substance, especially, apparently, in those cases when the desiccant has an acidic character. M. A. Klochko [1] used phosphorus anhydride to remove traces of water; the nitrobenzene was dried over it before distillation. Examination by our method of nitrobenzene preparations (“reinst” Khb., with EMF = 0.8 mV) treated with P₂O₅ gave the following results.

Table 8

*) Without preliminary immersion of sodium in the solution, we were unable to observe any noticeable change in electrical conductivity even at considerably higher concentrations of p—NOC₆H₄OH.

Thus, even brief standing of nitrobenzene over phosphorus anhydride, followed by distillation of the liquid, noticeably affects the purity of the preparation; if one also takes into account that the author used \(C_6H_5NO_2\) with a greenish coloration, the quality of the nitro product must have been still lower (the EMF of such benzene should be 15–20 times greater than the standard value. See, for example, experiment 4, Table 8). True, the effect of the impurities on the electrical conductivity is not manifested here in any particularly noticeable way, but below we shall see that their presence, when nitrobenzene preparations are stored for a more or less prolonged time, already becomes appreciable. Experiments on determining the EMF of nitrobenzene with an admixture of \(P_2O_5\) confirm the assumption stated above concerning the nature of the active complexes that are formed. Phosphorus anhydride probably first reacts with the water dissolved in nitrobenzene; the acid formed then interacts with the solvent, giving an active salt-like complex:

\[ P_2O_5 + H_2O \longrightarrow 2HPO_3 \]

\[ 2C_6H_5N\!\left(\begin{array}{c} O\\[-2pt] O \end{array}\right) + 2HPO_3 \longrightarrow 2\left[C_6H_5N\!\left(\begin{array}{c} O\\[-2pt] OH \end{array}\right)\right]^+ PO_3^- \]

The identical character of such a compound with nitrophenol complexes is confirmed also by an analogous change in the electromotive force: it increases gradually, and its maximum is not reached at the first moment of the reaction. As with nitrophenols, a certain amount of the phosphorus-containing complex is evidently carried along with the vapors of nitrobenzene during its distillation and contributes to an increase in the EMF.

Impurities of an acidic character, especially those with a phenolic hydroxyl, promote an increase in the electromotive force of nitrobenzene when it is stored in the light. At the same time the color of samples of \(C_6H_5NO_2\) changes noticeably from light yellow to faint brown, and its electrical conductivity increases. Even preparations of nitrobenzene thoroughly purified of impurities undergo a noticeable change (in terms of the EMF); moreover, this process is accelerated by the influence of small quantities of air remaining above the layer of liquid in the vessel; however, in this case the initial color of \(C_6H_5NO_2\) and its conductivity do not change. The experiments were carried out with the purest preparations of nitrobenzene (with constant EMF), separate portions of which (50 ml each) were stored in diffused light in sealed glass ampoules.

From the change in electromotive force, therefore, one can judge the quality of nitrobenzene preparations, both freshly prepared and stored for a more or less prolonged time under various conditions, even when the external appearance and the usual physical constants (boiling point, electrical conductivity, etc.) do not make it possible to establish any noticeable change in the substance. The sensitivity of the EMF method is so great that it makes it possible to establish even a difference in the methods of preparation of so-called pure preparations, which, it would seem, do not differ from one another in any way. For example, in our practical work we had occasion to use both ready-made and synthetic substances; although both underwent the same, very thorough purification, in terms of EMF they proved far from equivalent and, consequently, had different degrees of purity. As an example we shall cite data from the investigation of preparations of phenol and para-nitrosophenol dissolved in nitrobenzene (1/50 mole per liter).

All that has been said leads to the conclusion that the method of investigating organic substances by the electromotive force of their reaction with metals is undoubtedly

Table 9

Table 10

must be recognized as one of the most convenient and sensitive methods, having considerable advantages over many other commonly used methods. The selection of appropriate solvents, with an already established complex-forming ability (heterocycles, carbonyl compounds, etc.), and perhaps also of other metals, as was done by us [3] in the study of carboxylic acids, should, one may think, make it possible to study more deeply the nature of organic compounds and to solve a number of practical questions connected with the production and analysis of these substances.

Conclusions

1. The presence of small quantities of nitrophenols in nitrobenzene contributes to a considerable increase in the EMF and makes it possible to detect these substances even at strong dilution of the solutions. The minimal quantities of substances of phenolic character detectable by the EMF method are characterized by the following limits:

For phenol—0.0003%; ortho-nitrophenol—0.05%; meta-nitrophenol—0.0003%; para-nitrophenol—0.001%; para-nitrosophenol—0.00009%; dinitrophenol 2,4,1—0.0003%; dinitrophenol 2,6,1—0.0003%; picric acid—0.00009%.

The electrical conductivity of nitrobenzene with such amounts of substances does not change.

1. The increase in EMF is due to the formation of complexes of phenols with nitrobenzene, active toward metallic sodium, which have a salt-like character. The activity of the complexes depends on the number and character of the arrangement of the nitro groups in the benzene ring; the least complex-forming ability is that of ortho-nitrophenol, in which, with the adjacent arrangement of substituents, intramolecular charge compensation takes place.

2. From the electromotive force it is possible to establish the influence of substances used in the treatment of nitrobenzene for the purpose of obtaining pure preparations of it. For example, phosphorus anhydride should be considered a less suitable desiccant for nitrobenzene than calcium chloride, since the increase in EMF indicates the tendency of $P_2O_5$ (or $HPO_3$) to give an active salt-like complex with $C_6H_5NO_2$, remaining in appreciable quantity even after distillation of the product.

3. During prolonged storage of nitrobenzene with an admixture of negligible amounts of compounds of phenolic character, especially in the light, an increase in electromotive force takes place, proportional to their complex-forming ability; the effect of light and air on the purest preparations of nitrobenzene is insignificant.

4. Investigation of nitrobenzene solutions of many substances by the EMF method makes it possible to establish the role of functional groups in the construction of complexes, to determine the degree of purity of preparations and their stability during storage under various conditions.

LITERATURE

1. Klochko M. A.—Izv. Ak. nauk SSSR, chemical series, issue 3, p. 664, 1937.

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

3. Kulev L. P.—On the method of analysis of carboxylic acids by the EMF of the reaction with metals. Izv. TII, vol. 59, issue III. 1942.

4. Tronov V. V. and Kulev L. P.—Determination of the activity of hydrogen in organic compounds by the EMF of the reaction with sodium. ZhOKh IV, p. 784. 1934.

5. Cherbuliez C. 1, 1491. 1923.