ON THE TYPE OF REACTIVITY OF THE CARBONYL GROUP IN ALDEHYDES AND KETONES

B. V. TRONOV, N. A. KHITRINA

One of the most important tasks of organic chemists at the present time is the detailed elucidation of the regularities of the mutual influence of atoms in the molecules of organic compounds. To solve this problem it is necessary to determine the reactivity of various, first of all the most important, functional groups, and to trace the change in the activity of these groups as a function of the composition and structure of the whole molecule. Modern electronic chemistry distinguishes three types of reactivity of atoms and atomic groups: electron-donor, or nucleophilic, electron-acceptor, or electrophilic, and atomic-radical [1]. The type of reactivity is determined by studying the action on a given compound of appropriately selected reagents. In doing so, it is best to take reactions that are the simplest and proceed in one phase. Such reactions include processes of complex formation with typical electron donors or electron acceptors [2].

On the question of the type of reactivity of the carbonyl group of aldehydes and ketones, contradictory data are found in the literature. Certain facts appear to indicate that electron-acceptor activity predominates in these compounds. Thus aldehydes and ketones are capable of adding hydrocyanic acid. It is believed that this reaction begins with the addition of the cyanide anion to the positively charged carbonyl carbon. Aldehydes readily react with ammonia, in which the unshared electron pair of the nitrogen atom is very active. Typical reagents for aldehydes and ketones are hydroxylamine, phenylhydrazine, semicarbazide, and other similar compounds, all of which may be regarded as derivatives of ammonia. Let us add also that the carbonyl group is not capable of adding halogen molecules, in which the electron-acceptor character is more strongly expressed.

However, the interaction of aldehydes and ketones with the reagents listed here (hydrocyanic acid, ammonia, and its derivatives) has a complex character and proceeds in at least two stages: addition not only of the cyanide ion but also of hydrogen, addition of the amino group and hydrogen, and then also elimination of water. Processes of complex formation, the simplest in mechanism, on the contrary, point rather to the electron-donor properties of the carbonyl. Many complexes of aldehydes and ketones with strong acids are known, for example with nitric acid, hydrogen halides, trichloroacetic acid and its salts [3], and with mineral halides such as boron fluoride, calcium chloride, and aluminum chloride [4]. Alkalis belong to definitely electron-donor reagents, and for acetone a compound with caustic potash has been described, but experiments carried out by us did not confirm the formation of such compounds.

The indicated contradiction compelled us to study the interaction of aldehydes and ketones with many organic compounds known for their ability to form complexes. In the present work are given—

... mainly the results of experiments with substances for which one type of reactivity is possible or sharply predominates, namely, with acids and amines. Several nitro compounds were also used, for which the regularities of complex formation have been studied rather fully.

Experimental Part

For the investigation, 5 representatives of aldehydes and 8 ketones were taken. Only in one system, namely with acetaldehyde and picric acid, was the complex formed isolated. It separated in the form of yellow crystals. The analysis was carried out by decomposing the substance with an aqueous solution of caustic soda, followed by precipitation of potassium picrate with potassium chloride. The results of the analyses:

1. Sample 0.26 g. Picrate obtained, 0.22 g. Picric-acid content, 0.189 g, i.e., 72.7%.

2. Sample 1.2930 g. Picrate, 1.09 g; acid, 0.935 g, i.e., 72.4%.

3. Sample 1.4916 g. Picrate, 1.054 g. Acid, 70.6%. For the composition: two molecules of aldehyde per one molecule of acid, 72.2% was calculated.

Most of the systems were investigated by methods of physicochemical analysis; solid substances—by the melting diagram and sometimes by an electrochemical method.

Thermal analysis gave a positive result only for four systems (Table 1; Fig. 1).

Fig. 1. Thermal analysis of systems:
I. Benzalacetone—trichloroacetic acid. II. Camphor—trichloroacetic acid.

Table 1

A negative result (eutectics) was given by the following systems: benzophenone with benzoic acid, salicylic acid, m-dinitrobenzene, α-nitronaphthalene, p-nitrotoluene, picric acid; benzylacetone with p-nitrotoluene and picric acid; dibenzylacetone with salicylic acid and p-nitrotoluene; cinnamylideneacetophenone with benzoic acid and p-nitrotoluene.

Electrochemical investigations were carried out by a method based on measuring the current strength in a closed circuit, in which the electrodes were metallic sodium and a platinum crucible. The latter simultaneously served as the vessel for the mixtures being studied. Benzene was used for the most part as the solvent, to which dioxane was sometimes added. In a few cases diethyl ether was used. As the measuring instrument we used a millivoltmeter (a galvanometer calibrated in millivolts) with an internal resistance of 2167 ohms and a scale of 30.3 mV.

The results of the measurements are presented in the form of summary Table 2, where the molecular composition of the systems is given at which the maximum deflection of the instrument pointer was observed. As previous investigations [5], [6] have shown, the maximum deflection corresponds exactly or approximately to the composition of the complex arising in the system under study.

We also give the curves obtained for some of the systems we studied (Fig. 2).

Fig. 2. Electrochemical investigation of systems.
I Acetaldehyde — acetic acid. II Acetaldehyde — trichloroacetic acid.

Table 2

Summary table of systems studied by electrochemical methods

Molecular percentage of the component corresponding to the maximum deflection of the galvanometer needle

Discussion of Results

The data obtained in our investigations confirm the conclusions that could be drawn on the basis of the literature material on complex formation by aldehydes and ketones. These compounds, in almost all the systems studied, gave complexes with carboxylic acids; among the acids, the strongest—trichloroacetic acid—proved to be the most active. Complex compounds of it with saturated and fatty-aromatic ketones were already known. In our work, by means of the melting diagram, the formation of a compound of trichloroacetic acid with benzylideneacetone was demonstrated. With acetic, isobutyric, and isovaleric acids, the aldehydes and ketones we took also enter into reaction, but show lower activity. The absence, so far, of signs of complex formation with benzoic acid remains unexplained. Salicylic acid, on the contrary, is a fairly strong complex-forming agent. In systems with aldehydes and ketones, picric acid also gave positive results. With it a maximum was obtained on the melting diagram with cinnamylideneacetophenone, and a crystalline compound was isolated with acetaldehyde. In both systems picric acid could either form a hydrogen bond with the carbonyl oxygen or act through the nitrogen of the nitro group, but not through the oxygen of the latter. This is evident from the results of experiments with meta-dinitrobenzene and para-nitrotoluene. The nitro group has a mixed type of reactivity; in dinitrobenzene the electron-acceptor function is manifested more strongly, while in nitrotoluene the electron-donor function is manifested.

No indisputable signs of complex formation with amines were observed in our experiments. Thus, as was stated above, aldehydes and ketones can react with substances exhibiting predominantly electron-donor properties; however, in complex-formation processes they are undoubtedly more active toward reagents of the electron-acceptor type. The composition of the complex compounds found by us by means of the electrochemical method is very diverse. Therefore we do not yet touch upon the question of the details of their structure.

Conclusions

1. The complex formation of 5 aldehydes and 7 ketones with carboxylic acids, amines, and nitro compounds was studied. The investigation was carried out by the method of direct isolation with analysis of the complexes formed, by the method of thermal analysis, and by the electrochemical method of B. V. Tronov and L. P. Kulev.

2. A compound of acetaldehyde with picric acid was isolated and analyzed.

3. Sixteen systems were studied by thermal analysis. Positive results were obtained in 4 systems.

4. Seventy systems were studied by the electrochemical method, and 54 complexes belonging to 44 systems were found.

5. As a result of comparison of the data of the present work, it has been shown that the carbonyl group of aldehydes and ketones has a mixed type of reactivity, but with a strong predominance of electron-donor properties.

LITERATURE

1. The State of the Theory of Chemical Structure in Organic Chemistry. All-Union Conference, 1951. Publishing House of the Academy of Sciences of the USSR, Moscow, 1952.

2. Tronov B. V. and Strelnikova N. D. Izvestiya TPI, 71, 63, Tomsk, 1952.

3. Kochetkov N. N. ZhRKhO, 35, 660, 1903.

4. Menshutkin B. N. ZhRKhO, 42, 1928, 1910.

5. Tronov B. V. and Kulev L. P. Izvestiya TPI, 64, 3, Tomsk, 1948.

6. Tronov B. V. and Bortovoi I. M. ZhOKh, 24, 1750, 1954.

Bull. TPI, vol. 102.