ON METHODS FOR THE SYNTHESIS OF COMPLEX ORGANIC COMPOUNDS

B. V. TRONOV and V. B. SOKOLOVICH

Complex compounds, as a rule, differ from valence compounds by their lower stability. This is also reflected in the methods of obtaining and studying them. Sufficiently stable complexes—for example, a number of complexes of the quinhydrone type, many picrates, and others—can be prepared by ordinary methods, for example by precipitation from solutions, evaporation of solutions, or fusion of the constituent parts followed by crystallization from a suitable solvent.

However, in the study of a large number of complex-forming systems in organic chemistry, one has to be content with detecting complex formation by one or another method of physicochemical analysis; moreover, it is not always possible even to determine exactly the composition of the complexes formed.

We decided to use, for obtaining unstable complex compounds, the difference in vapor pressure of the volatile principal part of the complex over the pure substance and over the complex. In the chemistry of inorganic complex compounds—for example, crystal hydrates and crystalloammines of various salts—it is known that each crystal hydrate or crystalloammine has a definite vapor pressure of water or ammonia, naturally lower than the saturated vapor pressure of pure \(H_2O\) or \(NH_3\) at the same temperature. This leads to the fact that a crystal hydrate or crystalloammine can be obtained by absorption of water or ammonia vapor by the corresponding salt.

The same was observed by us in the study of complexes of quinone with various organic compounds.

When quinone and another substance were left standing in one desiccator or another closed vessel, the crystals of the second substance, if it was capable of interacting with quinone, often were transformed directly into crystals of the complex, usually, in our experiments, brightly colored. The change in the external appearance of the solid organic substance was accompanied by a gradual increase in its weight.

We used this method for the detection and synthesis of complex compounds, applying it to many systems whose composition, together with quinone, included hydrocarbons, phenols, acids, and others. In the majority of the systems studied, quinone was considerably more volatile than the other substance. In such cases there sometimes occurred a simple transformation of this substance into a complex with preservation of the general form of the crystals. As microphotographs showed, this was observed, for example, with pyrocatechol and resorcinol. In Fig. 1 are shown crystals of resorcinol before the reaction; in Fig. 2—crystals of resorcinol in the initial stage of the reaction. In other cases the crystals of the less volatile substance were first overgrown with small crystals of the complex (with pyrogallol and \(\alpha\)-naphthol). In Fig. 3 is shown a crystal of pyrogallol before the reaction; in Fig. 4—a crystal of pyrogallol in the initial-

stage of the reaction. In systems with meta-cresol and ortho-bromophenol, the formation of a complex compound on the surface of quinone crystals was observed. The synthesis of the complex was considered complete when the weight of the boat in which the reaction was taking place ceased to change.

Fig. 1

In order to improve the synthesis procedure (to accelerate the synthesis and to obtain pure products not containing an excess amount of quinone), an apparatus was assembled according to the scheme (Fig. 5).

Fig. 2

The main parts of the apparatus: 1—column filled with cotton wool for purifying the air from dust; 2—washing bottle with alkali for absorbing CO₂ from the air; 3—column with CaCl₂ for drying the air; 4—rheometer;

Fig. 3

Fig. 4

Fig. 5

Visible labels in Fig. 5: “air”; “to the pump”; 1, 2, 3, 4, 5, 6, 7, 11, 12, 13, 14, 15, 16.

5—a wash bottle with KJ and HCl for absorbing excess quinone; 6—a buffer bottle; 7—a chamber for quinone, surrounded by an asbestos jacket with an opening for thermometer 13; 8—a reaction chamber provided with a ground-glass lid with an outlet tube; 9—a reaction beaker with a mesh bottom, ground to the walls of the chamber; 10—a thermostat provided with stirrer 11 and thermometer 12; 14 and 16—three-way stopcocks; 15—clamp.

The principle of operation of the apparatus is that air, which is drawn by means of a water-jet pump at a definite rate through a system of absorption bottles, passing over the surface of heated quinone, entrains its vapors and carries them into the reaction beaker filled with the second solid component. The bottom of the beaker is a mesh, and the air with quinone vapors passes through the entire mass of the substance from bottom to top. After completion of the synthesis, pure air is passed through the reaction beaker in order to remove unreacted quinone. The synthesis was considered complete when the weight of the substance contained in the reaction beaker became constant.

The use of the apparatus described is limited, however, to the use of nonvolatile substances as second components.

Some compounds gave two complexes with quinone, differing in color. In such systems one color appeared first, then another. For example, on the surface of crystals of para-chlorophenol there first appeared yellowish-orange needles of composition 1 quinone—1 phenol, which then turned into dark brownish-red crystals of composition 1 quinone—2 phenols.

The composition of the complexes was determined from the increase in weight of the nonvolatile substance, from the quinone content (iodometrically), and from the halogen content (where halogen was present). Some of the complexes with halogen-substituted phenols proved so unstable that their crystals rapidly lost quinone in air.

Table 1 gives the results obtained with phenols.

For those phenols whose complexes with quinone have already been described in the literature, we obtained results coinciding with the data of other authors. In several cases new complex compounds were obtained which had not been possible not only to prepare, but even to detect by other methods, for example, in our experiments, by the method of thermal analysis.

We describe these complexes in more detail.

Quinone complex with trichlorophenol

The complex synthesized by our method consists of thin soft needle-like crystals of orange color. At a temperature of 43.5° the complex melts to a thick yellow liquid. It dissolves well in benzene, chloroform, carbon tetrachloride, alcohol, ether, and acetone. It decomposes in water. With alkali it gives a brown coloration.

The complex nature of the synthesized compound is confirmed by:

1) easy decomposition into components,
2) displacement of trichlorophenol by phenol, with the formation of phenoquinone, on standing in a desiccator with phenol.

The quinone was determined by a method based on reduction to hydroquinone with hydrogen iodide and titration of the liberated iodine with thiosulfate [2; 3]. Halogen was determined by Stepanov’s method. The results of analysis correspond to the formula $\mathrm{C_6H_4O_2 \cdot 4C_6H_2Cl_3OH}$.

Quinone content calculated 12.02%, found 11.98%.

Chlorine content calculated 47.45%, found 46.88%.

Table 1

Complex of quinone with tribromophenol

The complex has the form of small yellow needle-like crystals. It melts without decomposition at 85°. The crystals are readily decolorized in air, turning into tribromophenol. It dissolves readily in benzene, chloroform, carbon tetrachloride, alcohol, ether, and acetone. It is decomposed by water.

The complex nature of the compound is confirmed by decomposition into quinone and tribromophenol and by conversion into phenoquinone under the action of phenol vapors. Analysis indicates the composition $\mathrm{C_6H_4O_2 \cdot 4C_6H_2Br_3OH}$.

Quinone content: calculated 7.55%, found 7.55%.

Bromine content: calculated 67.02%, found 66.75%.

Complex of quinone with 1,2,4-dibromocresol

Orange-yellow soft needles with m.p. 39°. It dissolves with yellow coloration in the same solvents. It is decomposed by water. In air it readily decomposes into the components. With phenol vapors it is converted into phenoquinone.

Analysis led to the formula $\mathrm{C_6H_4O_2 \cdot 2CH_3C_6H_2Br_2OH}$.

Quinone content: calculated 16.89%, found 17.01%.

Bromine content: calculated 24.93%, found 25.57%.

Complex of Quinone with meta-Cresol

Garnet-red elongated plates with m.p. 39°. In air the complex sublimes without decomposition. It dissolves with a yellowish-green coloration in benzene, xylene, alcohol, ether, and acetone. In chloroform it exhibits strong green fluorescence. Determination of quinone led to the formula \(C_6H_4O_2 \cdot 2CH_3C_6H_4OH\). The quinone content calculated is 33.33%; found, 33.48%.

A negative result in our experiments was given by: diphenyl, naphthalene, anthracene, phenanthrene, acenaphthene, benzyl, benzoic and salicylic acids, dibenzyl sulfide, metadinitrobenzene, the three isomeric nitrophenols, 2,4-dinitrophenol, \(\alpha\)-nitronaphthalene, and azobenzene. Of these compounds, complex formation with quinone has been demonstrated for phenanthrene and acenaphthene by thermal and colorimetric methods, for dibenzyl sulfide by thermal analysis, and for anthracene by the colorimetric method [1]. The reactions of the crystals of these four compounds are probably hindered by features of the crystal structure.

Kinetic Investigation

For a number of the compounds studied by us, the rate of reaction with quinone vapors was determined. In these experiments only phenols that do not possess appreciable volatility at ordinary temperature were taken.

The reaction rate was determined from the increase in weight of the phenol. The experiments were continued up to 48 days at an average temperature of 13°, with fluctuations from 10 to 15°. For all the substances taken, the measurements were carried out under identical conditions.

The results are given in Table 2.

Table 2

As the table shows, among the seven phenols listed, \(\alpha\)-naphthol has the greatest reactivity toward quinone; then follow, in decreasing order of activity, \(\beta\)-naphthol, pyrocatechol, pyrogallol, resorcinol, hydroquinone, and phloroglucinol. In 25 days, during which \(\alpha\)-naphthol had time to react to 100%, the other phenols listed here reacted to 97%, 60%, 46%, 13%, 12.9%, and 1.8%.

For the remaining phenols, because of their greater volatility, it was not possible to make accurate determinations. However, approximate observations were nevertheless made. In p-cresol and the somewhat less active $C_6H_5OH$, conversion into the complex was completed in approximately 6 days. Next followed p-chlorophenol, dibromocresol, tribromophenol, and trichlorophenol (conversion time about 10 days).

It is interesting that no parallelism is observed between the reactivity of phenols toward quinone and their ability for electrolytic dissociation. The most active proved to be phenol and p-cresol, which possess weaker acidic properties. The dissociation constant of phenol is $K_{25}^{0}=1.3\cdot10^{-10}$. Trichlorophenol has $K_{25}^{0}=3.76\cdot10^{-8}$, but it enters into the complex-formation reaction with quinone with considerably more difficulty than phenol and is readily displaced from the complex by the latter. Nitrophenols did not react with quinone at all.

If these facts are compared, it must be considered possible that phenols react with quinone not by the hydroxyl hydrogen, but by the oxygen atom, i.e., they exhibit electron-donor activity, while quinone functions as an acceptor, probably through the carbon of the carbonyl group. However, in other cases—for example, in the interaction of quinone with picric acid—complex formation of another type is also possible, involving the hydrogen of the hydroxyl group and the carbonyl oxygen, since picric acid belongs to the strongest acids.

Let us also note that in our experiments the rate of complex formation was greatly influenced by external conditions specific to reactions between solid substances and gases. Complex formation was accelerated severalfold by fine grinding of the substance, especially with mechanical stirring during the process. Evacuation of air had a strong effect. The significance of crystal structure has already been discussed above. In a crystal we have not individual molecules, but a large aggregate of molecules connected with one another in a definite way, which must inevitably affect the chemical properties.

Conclusions

1. In the study of complex-forming systems in organic chemistry, a method based on the reaction of vapors of a volatile substance with crystals of a nonvolatile one is of great utility. This method makes it possible to synthesize even such complex compounds as cannot be obtained by other means, and sometimes cannot even be detected by physicochemical analysis.

2. The proposed method is especially convenient in the study of systems in which complex formation leads to a change in color.

3. A drawback of the method is that the possibility of complex formation here depends not only on the composition and structure of the substances taken, but also on the structure of the crystals. This sometimes leads to the result that substances capable of interacting with one another in solution do not, under the given conditions, enter into reaction.

Literature

1. Sokolovich V. B. Thermal analysis and colorimetric study of complex-forming systems containing quinone. GTTI Publishing House, p. 77, p. 164, 1953.

2. Valeur. Über die volumetrische Bestimmung der chinonderivate des Benzols. Chem. Zbl. II, p. 906, 1909.

3. Vaubel. Die physikalischen und chemischen Methoden der quantitativen Bestimmung organischer Verbindungen. Berlin, 102 Bd II, p. 24.