STUDY OF THE COMPLEXATION REACTION OF NITRILES WITH OTHER ORGANIC COMPOUNDS

B. V. TRONOV and N. D. STRELNIKOVA

Complexation of nitriles with various inorganic as well as organic compounds has so far been studied very little. The few references available in the literature provide almost no basis for judging the type of reactivity of the nitrile group. It may act as an electron donor—through the unshared pair of nitrogen electrons—and as an electron acceptor—through the positive charge of the carbon atom [1]. In addition, for many nitriles a tautomeric transition to the imine form is possible, the hydrogen of which should possess considerable activity:

\[ \mathrm{CH_3 - C \equiv N \rightleftarrows CH_2 = C = NH.} \]

Literature Data

Molecular compounds of nitriles have been studied only in the mixed field with inorganic complex-formers. In numerous complex salts obtained, nitriles perform the function of addends, occupying, depending on their composition and structure, one or two coordination positions [6].

Widely known are tin salts \([\mathrm{Sn(CH_3CN)_2}] \mathrm{Cl_4}\), \([\mathrm{Sn(C_2H_5CN)_2}]\mathrm{Cl_4}\), salts of monovalent copper \([\mathrm{Cu(CN_3CN)_2}]\mathrm{Cl}\) and \([\mathrm{Cu(C_6H_5CN)_2}]\mathrm{Cl}\). Especially well studied by L. A. Chugaev [6] are simple and complex platinum complex salts containing nitriles in the inner coordination sphere: \([\mathrm{Pt(CH_3CN)_4}][\mathrm{PtCl_4}]\), \(\mathrm{Pt(CH_3CN)_4}[\mathrm{O\,C_6H_2(NO_2)_3}]_2\), \([\mathrm{Pt(CH_3CN)_2}]\mathrm{Cl_2}\), \([\mathrm{Pt(C_4H_9CN)_2Cl_2}]\), \([\mathrm{Pt(C_6H_5CN)_2}]\mathrm{Br_2}\), \([\mathrm{Pt(C_6H_5CN)_2}]\mathrm{Cl_4}\). Also known are compounds whose structure is still difficult to explain, for example, \(2\mathrm{C_2H_5CN}\cdot \mathrm{H_3[CoCN)_6]}\), \(\mathrm{C_6H_5CN}\cdot \mathrm{SbCl_5}\), and many others.

No information on purely organic complex compounds of nitriles was found in the literature reviewed. There is only an indication [5] that nitriles in ternary complex organic compounds are capable of replacing aldehydes and ketones.

Experimental Part

In the present work 13 binary systems were studied, in which one of the components was benzonitrile and isovaleronitrile.

\[ \mathrm{C_6H_5{-}C \equiv N;\ (CH_3)_2CH{-}CH_2{-}C \equiv N \rightleftarrows (CH_3)_2CH{-}CH = C = NH} \]

and the second component was one of the following organic compounds: isovaleric and benzoic acids, phenol, normal butyl alcohol, isoamyl alcohol, triisoamyl borate, acetone, nitromethane, o-nitrotoluene, pyridine, benzamide.

The binary systems chosen were studied by the electrochemical method of B. V. Tronov and L. P. Kulev [2], i.e., for them the current strength in a closed circuit was determined, the source of energy being the reaction of organic substances with the metal of the electrode. The electrodes were sodium and platinum. The experiments were carried out in toluene solution at a temperature of 20–22°C.

The preparation, isolation, and verification of the purity of the initial organic compounds were carried out by various generally accepted methods already described in our preceding works in this same field.

The quality of benzonitrile and isovaleronitrile was also checked by the method of B. V. Tronov and L. P. Kulev. Triisoamyl borate was synthesized from boric anhydride and isoamyl alcohol [4]. Purification was carried out chiefly by recrystallization; for the acids, their boiling or melting points were determined [3].

Results of the study of the e.m.f. of binary systems by the method of B. V. Tronov and L. P. Kulev

1. Benzonitrile—phenol. A well-defined maximum at the molecular ratio $\mathrm{C_6H_5CN}\cdot 2\,\mathrm{C_6H_5OH}$; e.m.f. of the maximum about 1000 mV.

2. Benzonitrile—isovaleric acid. A sharp maximum at the ratio $\mathrm{C_6H_5CN}\cdot \mathrm{C_4H_9COOH}$.

3. Benzonitrile—benzoic acid. A small break in the curve, indicating almost complete absence of chemical interaction between the substances studied.

4. Benzonitrile—isoamyl alcohol. A diffuse maximum in the region of $2\,\mathrm{C_6H_5CN}\cdot \mathrm{C_5H_{11}OH}$.

5. Benzonitrile—acetone. A maximum with an e.m.f. value of 182 mV in the region $\mathrm{C_6H_5CN}\cdot \mathrm{CH_3COCH_3}$.

6. Benzonitrile—isovaleronitrile. A sharply delimited maximum at a component ratio of 1:2 with an e.m.f. value of 37.2 mV.

7. Benzonitrile—nitromethane. A maximum in the region of a compound of composition $\mathrm{C_6H_5CN}\cdot 2\,\mathrm{CH_3CNO_2}$.

8. Benzonitrile—benzamide. A small (e.m.f. 2.84 mV), abruptly beginning maximum 1:1.

9. Isovaleronitrile—phenol. A well-defined maximum with an e.m.f. of 1080 mV, stable for a long time, revealing the presence of the chemical compound $\mathrm{C_4H_9CN}\cdot \mathrm{C_6H_5OH}$.

10. Isovaleronitrile—normal butyl alcohol. Maximum e.m.f. value of 9 mV in the region close to isovaleronitrile: $2\,\mathrm{C_4H_9CN}\cdot \mathrm{C_4H_9OH}$.

11. Isovaleronitrile—pyridine. A break at 50% $\mathrm{C_4H_9CN}$ and 50% $\mathrm{C_5H_5N}$, followed by a linearly decreasing e.m.f. value from isovaleronitrile to pyridine (e.m.f. = 0).

12. Isovaleronitrile—isoamyl ester of boric acid. A maximum in the region of 80% isovaleronitrile.

13. Isovaleronitrile—orthonitrotoluene. A 1:1 compound, detected from the maximum at 10.64 mV. A somewhat complicated course of the e.m.f. curve in the region of orthonitrotoluene.

Discussion of the experimental results

The results of the experiments with benzonitrile show that, with almost all compounds having active hydrogen, this nitrile gives an increase in activity with respect to metals. The composition of the most active systems is varied. With isovaleric acid, isoamyl alcohol, acetone,

with benzamide this composition is equal or close to equimolecular. In combination with phenol, nitromethane, and isovaleronitrile, the maximum of the observed emf occurred for a molecular compound containing two molecules of the other substance per one molecule of benzonitrile.

It is of interest that benzoic acid did not give a positive result; the system in which it was the second component exhibited an electromotive force dependent on the presence of the benzonitrile itself. Phenol, on the contrary, gave a very large increase in emf. This circumstance gives some grounds for thinking that in nitriles an electron-acceptor type of complex formation predominates. Carboxylic acids manifest electron-acceptor activity mainly, as always, on account of their carboxyl hydrogen; therefore, for the molecular compound of benzonitrile found in this work, and for nitriles in general with carboxylic acids, the following structure is most probable:

\[ \mathrm{C_6H_5{-}C{\equiv}N \ldots HO.CO.C_4H_9} \]

Phenol, as a complex-forming agent of mixed character, may enter into a complex-formation reaction with benzonitrile with the formation of a bond between the carbon of benzonitrile and the oxygen of phenol; moreover, as is evident from the experimental data (benzonitrile—phenol 1:2), phenol reacts as its dimerically associated molecule.

For the molecular compound of benzonitrile with phenol 1:2, another structure is also possible, analogous to the acid one. But if this complex were built analogously to the acid molecular compound, one would have to expect the absence of a noticeable increase in emf, since even benzoic acid, with stronger acidic properties, did not give such an increase. (Isovaleric acid, of all those studied by this method, proved to be the most active.)

Isovaleronitrile also showed very high activity in combination with phenol, but at a different molecular ratio—not 1:2, but 1:1. By the same considerations as for benzonitrile, for this molecular compound the most probable is a compound with a complex bond between the carbon of the nitrile and the oxygen of phenol.

Isovaleronitrile is capable of reacting in complex-forming reactions also in imine form—this is indicated by the results of experiments with nitrotoluene and benzonitrile. However, the ability of isovaleronitrile for such complex formation is rather weak; in the experiment with pyridine, for example, only a break in the curve is observed.

It is of interest that in the experiment with boric isoamyl ether the emf maximum was observed only with a large excess of nitrile.

Conclusions

1. Thirteen binary systems were studied, in which one of the components was a nitrile—benzonitrile or isovaleronitrile—and the second component was a representative of various classes of organic compounds with different types of reactivity. The study was carried out by the electrochemical method of B. V. Tronov and L. P. Kulev, using sodium and platinum electrodes.

2. In almost all systems an increase in electromotive force was observed, which indicates the presence of complex formation in nitriles with organic substances.

3. From comparison of the data obtained it is evident that nitriles have a mixed type of reactivity.

References

1. Tronov B. V. and Strelnikova N. D. Complexes of nitro compounds with other organic substances. Izv. TPI, vol. 71, pp. 63–108, 1952.

2. Tronov B. V. and Kugaev L. P. A new ebulliometric method for studying organic compounds and their mixtures. Izv. TPI, vol. 64, pp. 3–87, 1948.

3. Strelnikova N. D. Study of the complex-formation reactions of amides with phenols and carboxylic acids. Izv. TPI, vol. 77, pp. 154–163, 1953.

4. Tronov B. V. and Strelnikova N. D. On the complex formation of esters of boric acid with various organic compounds. Izv. TPI, vol. 77, pp. 147–153, 1953.

5. Tronov B. V., Dyakonova L. N., Zonova E. A. Complexes of aromatic polynitro compounds with unsaturated compounds and certain salts. Journal of the Russian Physico-Chemical Society, 59, 333–345, 1927.

6. Pfeiffer P. Organische Molekülverbindungen, Stuttgart, 1922.