Effect of Temperature on the Complexation of Aldehydes and Ketones

B. V. Tronov, N. A. Khitrina

In the study of complex-forming systems by various methods, cases of disagreement among the results are not uncommon. This may be due to several reasons.

First of all, the formation of a complex compound may noticeably change that property of the compounds under study which is taken as the basis of some particular method of physicochemical analysis, while having little effect on another property used in another method. Methods associated with the appearance in the complex of an easily observable new property absent in the individual components of the system prove especially sensitive. Colorimetry, for example, belongs here. Thus, colorless amines with colorless or almost colorless nitro compounds give very distinctly colored complexes. Meanwhile, the cryoscopic method for amine–nitro derivative systems gives negative results. Evidently, in these systems complex compounds do arise, but they are very unstable. The negligible amount of complex present in solution (perhaps on the order of thousandths of a percent) has practically no effect on the freezing point, but imparts a quite perceptible color to the mixture.

Good results are sometimes obtained in determining electrical conductivity, which increases manyfold as a result of complexation. The same may be said of the galvanometric method of B. V. Tronov and L. P. Kuleva [1]. Meanwhile, application of the refractometric method to the same systems gave a negative result. This was observed, for example, in the study of the systems ethyl acetate–phenol, nitrobenzene–phenol, nitrobenzene–isovaleric acid, nitrobenzene–ethyl acetate [2].

Apparently, in these systems the formation of a complex has little effect on the refractive index; meanwhile, with an electrochemical method, for example, for a mixture of nitrobenzene with isovaleric acid the deflection of the millivoltmeter needle was 700 times greater than for the pure acid.

In addition, different methods of investigation are sometimes applied under different conditions, which may also have a strong effect on the results of the study. Thus thermal analysis, which in general is considered hardly the most reliable of the physicochemical methods, usually requires heating, and complexes often decompose at high temperature. If, for example, at 100° we do not detect complexation, while by working with some other method at 20° we obtain a positive result, it by no means follows that one of these methods is incorrect. At 20° the presence of a complex compound that decomposes by 100° is quite possible.

Complexes found in solutions may very well have a composition different from that of compounds of the same components isolated in crystalline form. Sometimes a complex compound, very clearly detected in a liquid medium and by a number of indications not so very unstable, cannot at all be-

cannot be isolated, since the energy of formation of crystals of this compound is less than the energy of crystallization of its constituent parts. In some cases we obtained negative results in a topochemical investigation of such systems, where even thermal analysis (not to mention the more sensitive colorimetric analysis) definitely indicates complex formation. The quinone–phenanthrene and quinone–acenaphthene systems [3] gave maxima on the fusibility curves. Thus, on cooling such mixtures, crystals of complex compounds separated out. Meanwhile, quinone vapors were not at all absorbed by crystals of phenanthrene and acenaphthene. It must be supposed that here the decisive factor is the structure of the crystals of these hydrocarbons; evidently, quinone molecules cannot penetrate into the crystal lattice of phenanthrene or acenaphthene. The same was observed with anthracene as well, but with it a quinone complex was found only colorimetrically; thermal analysis was excluded, since it required heating to a temperature at which resinification occurred.

All that has been said leads to the necessity of applying several methods of investigation to each complex-forming system. Moreover, it is desirable, whenever possible, to test one and the same method under different conditions, in particular at different temperatures.

The following fact interested us. In studying certain complex-forming systems, we noticed that the special points of the composition–property diagrams, which usually indicate the composition of the complex compound, lie too close to the edge of the diagram. For example, in a colorimetric study of the piperidine–meta-dinitrobenzene system, the maximum of coloration corresponds to a molecular ratio of 1:9 [4]. The formation of complexes with a large excess of one of the components has often had to be observed when applying our electrochemical method. Thus, in mixtures of alcohols with boric acid esters, instead of the long-known complexes of the 1:1 type, we find “supercomplex” compounds of approximately 9:1 composition [5]. The same results were sometimes obtained in studying the complex formation of aldehydes and ketones with other organic substances.

These results, especially what was found for boric acid esters, suggest the possibility that in some systems complex molecules of a certain, rather simple composition—for example, 1:1 or 1:2—may form, to which molecules of one of the components then add in large numbers, forming “supercomplex” solvates. The composition of these solvates may be indefinite and depend strongly on external conditions, above all on temperature.

In the present work we decided to carry out an electrochemical investigation of a number of systems consisting, on the one hand, of aldehydes and ketones and, on the other, of phenols and carboxylic acids.

Experimental Part

For the work the following were taken: acetic, benzoic, and salicylic aldehydes, furfural, acetone, ethyl acetoacetate, phenol, two isomeric naphthols, and acetic acid. The investigation was carried out under the conditions described earlier [6], i.e., with sodium and platinum electrodes and with a millivoltmeter having an internal resistance of 2167 ohms and a scale of 30.3 mV.

The results of the measurements are given in the form of a summary table, where for each system the numbers characterizing the position of the maxima on the millivoltmeter-readings curve are given (we conventionally call these the values of the electromotive force).

Summary Table

Discussion of Results

In the study of complex-forming systems with phenols, it was found that in most cases the value of the electromotive force increases when the temperature is lowered. This occurs, one must suppose, because of an increase in the content of complexes in the solutions. An exception was observed only in experiments with salicylaldehyde and phenol. It is possible that here the content of the complex compound changed comparatively little upon heating from 8 to 26°, and this change is outweighed by the increase in the activity of the complex with respect to sodium.

The positions of the maxima on the curves in a number of cases remained unchanged. Sometimes, upon cooling, they shifted toward the phenols. It is interesting that in the acetone–phenol system at 20° the complex has the composition 2:1, whereas at 8° it evidently has the composition 1:1. Complexes with a large excess of one of the components were not found in the systems with phenols considered in the present work.

In studying systems with acetic acid, the formation of such “supercomplex” compounds was found for acetone, benzaldehyde, furfural, and, perhaps, a complex of the same type is given by salicylaldehyde. Here, in all cases, an increase in temperature led to a simpler molecular composition of the complex compounds; that is, loosely attached “extra” molecules of one of the components—namely, the aldehyde or ketone—were split off. The values of the electromotive force always increased with heating, and almost always increased strongly. This is difficult to explain by an increase in reactivity due solely to heating; evidently, the change in the composition of the complexes was also significant.

Conclusions

1. Eleven systems were studied by the electrochemical method; aldehydes and ketones were included in them as electron donors, and phenols and acetic acid as electron acceptors. The measurements were carried out at temperatures from 8 to 26°.

2. It was shown that in systems where, at low temperatures, complexes arise with a large excess of one of the constituent parts, the maxima on the composition–property diagram shift, as the temperature is raised, toward simpler molecular ratios. In other systems, the positions of the maxima sometimes also shifted, and sometimes remained unchanged.

3. A theoretical explanation of the regularities found is proposed.

Literature

1. Tronov B. V. and Kulev L. P. Izvestiya TPI, 64, 3, Tomsk, 1948.
2. Kulikova R. M. Trudy TGU, 126, 103, 1954.
3. Tronov B. V. and Sokolovich V. B. Izv. TPI, 83, 91, Tomsk, 1956.
4. Tronov B. V. and Strelnikova N. D. Izv. TPI, 71, 63, Tomsk, 1952.
5. Tronov B. V. and Strelnikova N. D. Izv. TPI, 77, 147, Tomsk, 1953.
6. Tronov B. V., Khitrina N. A. Article published in this collection.