S. TRONOV and L. P. KULEV.

DETERMINATION OF THE ACTIVITY OF UNSATURATED ORGANIC COMPOUNDS FROM THE ELECTROMOTIVE FORCE OF THE REACTION WITH SODIUM

In organic chemistry a number of cases are known of the addition of metals, especially sodium, to unsaturated organic compounds. Some unsaturated hydrocarbons add sodium directly to the double bond¹, e.g.,

\[ (\mathrm{C}_6\mathrm{H}_5)_2\mathrm{C}=\mathrm{CHC}_6\mathrm{H}_5 + 2\mathrm{Na} \rightarrow (\mathrm{C}_6\mathrm{H}_5)_2\mathrm{CNa} — \mathrm{CHNa}\cdot\mathrm{C}_6\mathrm{H}_5. \]

The action of metals on the double bond of carbon with oxygen, e.g. in ketones, often leads to the formation of pinacols, and sometimes metal ketyls with trivalent carbon are obtained:

\[ (\mathrm{C}_6\mathrm{H}_5)_2\mathrm{C}=\mathrm{O} + \mathrm{Na} \rightarrow (\mathrm{C}_6\mathrm{H}_5)_2\mathrm{C} — \mathrm{ONa}. \]

Finally, a whole series of compounds of a high degree of unsaturation, without visibly adding the metal, undergo polymerization under its influence. Thus, nitriles with sodium are converted into cyclic trimers:

\[ 3\mathrm{C}_6\mathrm{H}_5\mathrm{C}\equiv \mathrm{N}\ (+\mathrm{Na}) \rightarrow \begin{array}{c} \mathrm{C}—\mathrm{C}_6\mathrm{H}_5\\[-2pt] /\!\!\!\!\!\backslash\\[-6pt] \mathrm{N}\quad \mathrm{N}\\[-2pt] | \quad |\\[-2pt] \mathrm{H}_5\mathrm{C}_6—\mathrm{C}\quad \mathrm{C}—\mathrm{C}_6\mathrm{H}_5\\[-2pt] \backslash\!\!\!\!\!/\\[-6pt] \mathrm{N} \end{array} \]

In synthetic-rubber factories sodium is used as a catalyst that very effectively condenses butadiene:

\[ n\ \mathrm{CH}_2=\mathrm{CH}—\mathrm{CH}=\mathrm{CH}_2\ (+\mathrm{Na}) \rightarrow (—\mathrm{CH}_2—\mathrm{CH}=\mathrm{CH}—\mathrm{CH}_2—)^n \]

It must be supposed that in these cases as well there occurs addition of the metal, even if only temporary, either to the unsaturated bond (and in divinyl to the conjugated system) as a whole or, at least, to one of the atoms between which the double or triple bond is located.

In all such processes the transition of the metal into an ion is possible,

i.e., donation of its electron to the molecule of the other reacting component:

\[ (\mathrm{C_6H_5})_2\mathrm{C}=\mathrm{O}+\mathrm{Na} \;\longrightarrow\; [(\mathrm{C_6H_5})_2\mathrm{C}-\mathrm{O}]^{-}\mathrm{Na}^{+} \]

It is possible, therefore, for an electromotive force to arise, always accompanying the transfer of electrons.

For this reason, while engaged in determining the activity of hydrogen in organic compounds from the electromotive force of the reaction with sodium, i.e., while studying reactions of replacement of hydrogen by sodium, we decided to try to apply this method also to the reaction of sodium addition. Having carried out several experiments with substances that could add this metal¹, we became convinced that, here too, a certain electromotive force is indeed observed. For example, nitrobenzene gave about 1 mv. With the unsaturated hydrocarbon indene, which can also replace one of its hydrogens by sodium and readily polymerizes (being of special interest in this respect as one of the coal-tar rubber-forming substances), nothing was obtained under our conditions.

Since with some substances a favorable result was nevertheless observed, in the present work we examined a large series of compounds containing double bonds between C and C, C and O, N and O, or the triple bond CN. Representatives of unsaturated hydrocarbons (indene), aldehydes, ketones, esters, nitriles, nitro compounds, and heterocyclic amines of the pyridine type were taken. With many compounds the appearance of an electromotive force was noticeable, but for the most part it was very small.

In order to make the method more suitable for determining the degree of unsaturation of unsaturated compounds, we began to look for ways of increasing their activity toward sodium. In doing so we decided to make use of the following circumstance.

The metal adds to the more electronegative of the atoms between which the double or triple bond is situated. If, for example, one adopts for benzophenone the electronic formula according to Lowry’s theory of the polarity of double bonds:

\[ \begin{array}{c} \mathrm{C_6H_5:C:C_6H_5}\\ \ddot{\phantom{C}}\!\!:\mathrm{O}:\!\!\ddot{\phantom{C}} \end{array} \]

then the positive sodium nucleus must go to one of the free electron pairs of the oxygen. The electron of the metal in this case either passes directly into another part of the molecule, or (as must be the case in our apparatus) goes into the external circuit, giving a current. If the excess positive charge of the carbonyl carbon is first neutralized in some way, this action should be transmitted to the oxygen, and the free electron pairs of the latter should become more active. Consequently, then the positive—

the sodium nucleus will be attracted with greater force, and its electron will more readily be driven outward.

Neutralization of the positive carbon charge and, consequently, activation of oxygen with respect to the metal can be accomplished, for example, by the complex addition of some negative ion. As such we chose the iodine ion, since it is especially capable of complex formation (for example, it gives the complex ion \(I'_3\)) and iodide salts dissolve fairly well in certain organic solvents. NaI was taken, which proved appreciably soluble in many alcohols, ketones, amines, nitro compounds, etc. The solutions could even be diluted with benzene, as we did in earlier work, so that the reaction would proceed less violently and so that different substances could be compared at equimolecular concentrations.

For comparison, tests were also carried out with alcohols and acids already studied previously. These substances react with sodium differently, but sodium iodide, of course, could here also influence the activity. Acids, moreover, could also react through their carbonyl group, i.e. analogously to ketones.

The experimental conditions were, in general, analogous to those used in the preceding investigations. The compound under examination, alone or after dissolving NaI in it, was diluted with benzene to 6 cubic cm. The concentration had to be varied in view of the very great difference in activity among certain compounds, but in such cases experiments were performed at both concentrations with some substance of intermediate activity. The concentration of NaI was also varied. As a rule, for the same unsaturated compound the determinations were made with different amounts of sodium iodide.

The initial temperature of the experiments was the ordinary one \((16^\circ—20^\circ)\). Almost no considerable self-heating of the liquid was observed. In each determination not only the maximum electromotive force was noted, but a series of readings was made at definite intervals of time.

The experiments gave the following results.

Unsaturated hydrocarbons

$^{1)}$ For comparison, experiments were also set up at identical concentrations, marked in the corresponding column by an underline (1/50 M). In the remaining tables these concentrations are also underlined.

Mesityl oxide and cyclohexanone, tested at the same concentrations as the other ketones, stand out sharply in this series by the magnitude of the EMF in experiments with NaI.

ESTERS AND KETO ESTERS.

For simple ethers, e.g., anisole, only a small increase in EMF was observed:

No change in the magnitude of the EMF in the presence of NaI was given by nitrates (\(\mathrm{CH_3ONO_2}\) and \(\mathrm{C_2H_5ONO_2}\)—at identical concentrations):

And noticeably greater—nitrites:

Significantly less active were the nitro compounds:

Conversely, nitriles, especially acetonitrile, gave a large EMF¹. At the same time, a sharp difference in the magnitude of the EMF was observed for $\mathrm{C_6H_5CN}$, although the solubility of NaI in the latter is less than in $\mathrm{CH_3CN}$.

A L C O H O L S

Thus, the alcohols most active in interaction with sodium alone gave a smaller increase in EMF in the presence of sodium iodide.

The same was also observed for the majority of carboxylic acids:

CONCLUSIONS.

In the following table are given the maximum values of the electromotive force of the reactions with sodium without addition of NaI and with NaI. The figures in the last column show how many times the EMF increases upon addition of sodium iodide.

Note. The substances listed in the table cannot always be compared with one another in absolute activity, since the experiments were carried out at different concentrations (see the preceding tables).

From the figures in this table and from other data obtained by us, the following conclusions may be drawn:

1. Almost all the unsaturated compounds we examined (except indene, whose activity under these conditions is evidently too small) give, on contact with sodium, a noticeable electromotive force, although in compounds having no mobile hydrogen this electromotive force is usually small.

2. In the presence of sodium iodide, the electromotive force of unsaturated compounds that cannot pass into the enol (or an analogous) form sometimes increases manyfold, and sometimes remains almost unchanged. The first group includes ketones (benzophenone), nitriles (benzonitrile), and heterocycles of the pyridine group; the second includes some complex esters (e.g., nitrates); nitrobenzene and isoamyl nitrite occupy an intermediate position. The rather small increase in EMF for benzoic ethyl ester is of interest, indicating the lower activity of the ester carbonyl as compared with the ketone carbonyl. The same was previously established in the study of ternary complexes of sodium iodide with unsaturated compounds and polynitro compounds of the benzene series.^1^ A small

^1^ B. V. Tronov, L. N. D’yakonova-Shults, and E. A. Zonova, “Complexes of aromatic polynitro compounds with unsaturated compounds and certain salts,” Zh. R. Kh. O., 59, 333–345 (1927).

an increase in EMF was also given by \( \mathrm{C_6H_5COCN} \), but for it it is difficult to indicate in advance the distribution of the electrons; in the formula usually written according to Lowry’s theory, two electropositive carbons must stand side by side, and this may lead to some redistribution of the electrons.

1. In compounds with a hydroxyl group, the EMF upon addition of NaI sometimes changes hardly at all (acids and some alcohols, such as allyl alcohol and cyclohexanol), and sometimes increases appreciably, but less than, for example, in ketones incapable of enolization. Here, probably, complexes with NaI are also formed, but, as must be supposed from the considerations set out above, the tendency toward complex formation in the absence of a double or triple bond is considerably diminished; in acids, moreover, the double bond is evidently only slightly active. The influence of the carbon double bond in the alcohol molecule is small. Pyrrole gave no increase in activity.

2. Compounds with the groups \( \mathrm{C=O} \), \( \mathrm{C\equiv N} \), \( \mathrm{NO_2} \), and with hydrogen atoms on the atom adjacent to the unsaturated group, may react either in the carbonyl, nitrile, etc., form, or in the form with hydrogen on oxygen or nitrogen. In such compounds we sometimes find a very large increase in EMF (cyclohexanone, mesityl oxide), and sometimes almost none or none at all (acetonitrile, nitromethane, acetaldehyde). This may be due both to differences in the degree of enolization of the compounds and to the different activity of the tautomeric forms possible for them. It may be supposed, for example, that in cyclohexanone, where enolization (as is seen from some of its reactions¹) is rather considerable, the carbonyl group is distinguished by very great activity²), so that its influence predominates. In the molecule of mesityl oxide the activity of the carbonyl may be increased by the carbon double bond, and it is possible that here the conjugated system of bonds participates directly in the reaction.

There is a very great difference in the effect of NaI between acetic and benzoic aldehydes. \( \mathrm{CH_3CHO} \) can pass into two forms with hydroxyl: \( \mathrm{CH_2=CHOH} \) and \( \mathrm{CH_3-C-OH} \) (divalent C). For \( \mathrm{C_6H_5CHO} \) the first transition is excluded, and the second, apparently, is almost absent, as may be inferred at least from the absence of paraldehyde polymerization. \( \mathrm{CH_3NO_2} \) behaved in our experiments not as a nitro compound, but as an acid, i.e. in the form

\[ \mathrm{CH_2=N\left(\begin{array}{c} O\\ OH \end{array}\right)} \]

This was fully confirmed by special experiments with its mixture with nitrobenzene. The same increase in EMF was obtained as is given under these conditions by carboxylic acids¹).

¹) With \( \mathrm{PCl_5} \) there is obtained

\[ \mathrm{H_2C\left(\begin{array}{c} CH_2-CH\\ CH_2-CH_2 \end{array}\right)CCl} \]

²) This is confirmed by a series of works by P. I. Petrenko-Kritchenko.
¹) B. V. Tronov and L. P. Kulev, “Comparison of the activity of alcohols and carboxylic acids under different conditions.” Izvestiya SKhTI (1934).

The results of our work clearly show that the activity of certain unsaturated groups, especially $\mathrm{C{=}O}$, $\mathrm{C{\equiv}N}$, $\mathrm{C{=}N}$ (in heterocycles), with respect to sodium increases to a high degree upon addition of NaI. In this respect, compounds containing these groups differ sharply from substances with active hydrogen. It was also found that the activity, for example, of the carbonyl itself depends very strongly on its position in the molecule. The CO group is especially active precisely in ketones and aldehydes, but not in esters and acids. Likewise, the $\mathrm{NO_2}$ group in nitro compounds behaves quite differently than in nitric-acid esters.

All this permits one to expect that the proposed method—the determination of the electromotive force of the reaction with sodium in general, and in particular upon addition of NaI and, perhaps, of certain other admixtures—may find application for recognizing various unsaturated groups in organic substances and even for determining their position in the molecule. It is especially interesting to carry out tests with readily polymerizing hydrocarbons and halogen derivatives, such as butadiene, isoprene, dimethylbutadiene, chloroprene, styrene:

\[ \begin{array}{ccccc} \mathrm{CH_2} & \mathrm{CH_2} & \mathrm{CH_2} & \mathrm{CH_2} & \mathrm{C_6H_5} \\ \| & \| & \| & \| & | \\ \mathrm{CH} & \mathrm{C{-}CH_3} & \mathrm{C{-}CH_3} & \mathrm{CCl} & \mathrm{CH} \\ | & | & | & | & \| \\ \mathrm{CO} & \mathrm{CH} & \mathrm{C{-}CH_3} & \mathrm{CH} & \mathrm{CH_2} \\ \| & \| & \| & \| & \\ \mathrm{CH_2} & \mathrm{CH_2} & \mathrm{CH_2} & \mathrm{CH_2} & \end{array} \]

which serve as intermediate products in the synthesis of rubber and the preparation of plastics.

It is also possible (with a suitably developed procedure) to apply this method to the determination of resin-forming substances (coumarone, indene, the same butadiene, cyclopentadiene) in fractions of coal tar, primary tar, and cracked gasolines.

Laboratory of Organic Chemistry
Tomsk Industrial Institute.
10.X.1934.

B. W. Tionow and L. P. Kulew.

Determination of the Activity of Unsaturated Organic Compounds from the Electromotive Force of the Reaction with Sodium

The method previously applied by us for determining chemical activity from the electromotive force of the reaction with sodium also proved suitable for those compounds which have a double or triple bond. In some cases these compounds combine with the metal (e.g., aromatic ketones, some hydrocarbons), and sometimes they are polymerized under its action (such as nitriles or hydrocarbons of the butadiene series). Both reactions result in the appearance of an electromotive force, which in itself is very small, but which increases considerably upon the addition of certain admixtures (especially sodium iodide).

Ketones, nitriles, and also compounds of the pyridine group that do not pass into the form with mobile hydrogen increase, in the presence of NaJ, the electromotive force of the reaction with sodium by 50–250 times. Nitro compounds and esters of carboxylic acids occupy an intermediate position. On acids and alcohols NaJ has a comparatively insignificant influence. Enolizing compounds behave differently.

An electronic explanation of the observed phenomena is thereby given.

1. This was observed by us earlier as well (see Zh. O. Kh. IV, issue 2 (1934)). ↩↩↩