Thermal Analysis and Colorimetric Study of Complex-Forming Systems Containing Quinone

V. B. Sokolovich

In the chemistry of complex organic compounds, quinone complexes deserve considerable attention, in particular the compounds of quinones with phenols, known under the general name “quinhydrones.”

Quinhydrones possess a number of interesting properties; many of them have practical applications, for example, in the preparation of dyes and pharmaceutical preparations, for dry seed treatment, in electrochemistry, etc.

The first representative of this group of complex compounds—quinhydrone—was obtained in 1844 [17]. According to the literature, by the present time more than 90 complex compounds of quinones have been synthesized [18; 19; 21; 23; 25; 28; 27].

Despite the fact that quinhydrones have been known for more than one hundred years, the question of their structure still remains insufficiently resolved.

Kekulé was the first, in 1866, to attempt to express the structure of quinhydrones; he assigned to quinhydrone, obtained from one molecule of quinone and one molecule of hydroquinone, the formula

\[ \mathrm{HO}\ \mathrm{C_6H_4} - \mathrm{O} - \mathrm{O} - \mathrm{C_6H_4}\ .\ \mathrm{OH} \]
\[ [20;5] \]

Fifteen years later this formula was rejected. However, for sixty years chemists still considered quinhydrones to be compounds of a valence type. It was proposed to express the structure of quinhydrones by formulas of various kinds. For example,

![structural formulas of quinhydrone forms]

[22]

Enol form

Ketone form [29]

Studies of subsequent years led scientists to the idea of the complex nature of quinhydrones. Concerning the structure of quinhydrones, numerous theories were advanced, but they quickly replaced one another, since they could not sufficiently explain the properties of quinhydrones.

The theory that enjoyed the greatest recognition in its time was that according to which the quinhydrone molecule was represented as being built from benzoid and quinoid components, connected with each other through the hydroxyl…

the benzenoid component and the carbonyl group of the quinoid component, owing to excess partial affinity [26].

$$ \begin{array}{ccccc} \mathrm{O{-}H} & \cdots & \cdots & \mathrm{O} \\ | & & & \| \\ \mathrm{C} & & & \mathrm{C} \\ /\!\!\! & \backslash & & / & \backslash \\ \text{benzene ring} & & & \text{quinone ring} \\ \backslash & / & & \backslash & / \\ \mathrm{C} & & & \mathrm{C} \\ | & & & \| \\ \mathrm{O{-}H} & \cdots & \cdots & \mathrm{O} \end{array} $$

A later theory also had some currency, according to which the quinoid and benzenoid components in the molecule of quinhydrone are joined through the oxygen of the carbonyl group of the quinoid component and an unsaturated carbon atom of the benzenoid component [20].

$$ \begin{array}{c} \mathrm{O} \\ \| \\ \mathrm{C} \\ / \ \backslash \\ \text{quinone ring} \\ \backslash \ / \\ \mathrm{C} \\ \| \\ \mathrm{O}\cdots\cdots \mathrm{C_6H_4(OH)_2} \end{array} $$

In recent years, the most widespread view has been that quinhydrones are complex compounds whose components are joined by a hydrogen bond.

The true founder of the doctrine of the hydrogen bond is the outstanding Russian chemist, Corresponding Member of the Academy of Sciences of the USSR M. A. Il’inskii, whose works are stubbornly passed over in silence in the foreign literature [9]. The discovery of the hydrogen bond is attributed exclusively to foreign scientists. The first to have allegedly pointed to the hydrogen bond were Moore and Winmill in 1912, and Pfeiffer introduced it into organic chemistry in 1913 [14, p. 280]. M. A. Il’inskii, however, as early as 1887, far ahead of contemporary science, revealed the type of phenomena explained by the hydrogen bond [15].

Soviet scientists are now devoting great attention to the study of the nature and properties of the hydrogen bond, which plays an important role in various physicochemical and biochemical phenomena. A major contribution to the solution of this problem has been made by the works of Batuev, Gragerov, Miklukhin, Sokolov, and others [1; 4; 6; 7; 10; 13].

In order to elucidate the nature and properties of the hydrogen bond, various objects, including quinhydrones, have been subjected to experimental study.

On the basis of investigations using X-ray structural analysis and the “labeled atoms” method, it was concluded that in benzoquinhydrone quinone and hydroquinone are joined to one another by hydrogen bonds, forming either isolated molecules or long chains.

\[ \begin{gathered} \text{(structural formula of a quinhydrone complex with two hydrogen bonds)}\\ \text{and a chain representation of hydrogen-bonded quinone--hydroquinone units} \end{gathered} \qquad [10,\ \text{p. }62] \]

To determine the position and mobility of hydrogen in the hydrogen bond of benzoquinhydrone, Gragerov and Miklukhin applied one of the newest methods of investigation in chemistry—the isotopic method, or the method of “tagged atoms” [2; 4; 6; 13].

From ordinary quinone and heavy hydroquinone, in which the hydrogen atoms in the nucleus had been replaced by deuterium, quinhydrone \( \mathrm{C_6H_4O_2 \cdot C_6D_4(OH)_2} \) was obtained [13].

The resulting heavy quinhydrone was separated either by distilling off the quinone in vacuum or by precipitation in an alcohol-water solution with hydrochloric semicarbazide [3].

An analogous experiment was carried out with benzoquinhydrone obtained from heavy quinone and ordinary hydroquinone. The results of all these experiments showed that, when quinhydrone is separated into quinone and hydroquinone, deuterium remains in the nucleus of the component with which it had been introduced into the molecule [7].

In the opinion of Miklukhin and Gragerov, the investigations they carried out refute the idea of decolorization of quinone and hydroquinone in the quinhydrone molecule, and also confirm the absence of “resonance” of equivalent structures and the absence of oscillations of hydrogen atoms in the hydrogen bond of the benzoquinhydrone molecule.

Miklukhin and Gragerov consider the most correct formula, one that makes it possible to explain a number of properties of benzoquinhydrone and to embrace from a single point of view all quinhydrone-like compounds, to be the formula proposed in 1942 by Weiss.

\[ \left[ \begin{array}{c} \mathrm{O}\\[-2pt] \hexagon\\[-2pt] \mathrm{O} \end{array} \right]^{-} \quad \left[ \begin{array}{c} \mathrm{O{-}H}\\[-2pt] \hexagon\\[-2pt] \mathrm{O{-}H} \end{array} \right]^{+} \]

The formula indicates that the nuclei of the components in the benzoquinhydrone molecule are held by electrostatic forces and are not made equivalent.

This formula, in the opinion of Miklukhin and Gragerov, also successfully reflects the hydrogen bond, since the charges are concentrated predominantly on the oxygen atoms and on the hydrogen atoms of the hydroxyl groups, which leads to the formation of a hydrogen bond [7].

However, in the literature of recent years it is still indicated that there are as yet no sufficiently clear ideas about the structure of quinhydrones [6; 16, p. 56].

Experimental Part

In connection with resolving the question of the structure of quinhydrones, we carried out a study of twenty binary systems composed of quinone (\(p\)-benzoquinone), on the one hand, and representatives of various classes of organic compounds, on the other, by methods of thermal and colorimetric analysis, with the aim of revealing certain regularities in the complex formation of quinone.

The organic substances used in the work were purified before use by recrystallization, sublimation, and distillation. The criterion of purity of the starting substances was their melting point, which was determined by the usual method in a capillary on a sulfuric-acid bath.

Quinone, salicylic acid, benzoic acid, dinitrobenzene, α-nitronaphthalene, m-nitrophenol, p-nitrophenol, o-nitrophenol, tribromophenol—1, 2, 4, 6, trichlorophenol—1, 2, 4, 6, dinitrophenol—1, 2, 4, thymol, phenanthrene, naphthalene, triphenylmethane, acenaphthene, diphenyl, dibenzyl sulfide, dibromocresol—1, 2, 4, anthracene, and azobenzene were used.

Thermal analysis was carried out by the method of B. N. Menshutkin [8; 11; 12]. The components of the system under investigation were taken in the ratios 10, 20, 30, etc., molecular percent of one component to 90, 80, 70, etc., molecular percent of the other. To refine the composition of the eutectic or of a compound with a maximum melting point, mixtures of intermediate composition were analyzed.

A glycerol bath served as the heating device; into it, almost up to the rim, was immersed a short “Pyrex” glass test tube fastened to a stand by means of a wire frame. The test tube was covered with a stopper, through an opening in which passed a calibrated thermometer that at the same time served as a stirrer.

Calculated weighed portions of the components of the system under study were placed in the test tube, mixed, and melted. Then, with constant stirring, the melt was cooled uniformly.

Every minute (in some experiments every two minutes) the temperature of the cooling system was recorded. Special note was made of the temperature at which the first crystals separated from the liquid phase, as well as of the crystallization of the eutectic, which was usually accompanied by temperature arrests.

The data obtained were used to construct a “composition—melting temperature” diagram. From the character of the fusibility diagram, a conclusion was drawn as to the presence or absence of chemical interaction in the system and as to the composition of the complex compounds formed.

Colorimetric analysis, proposed for the study of binary colored organic systems in 1911 by I. Ostromyslensky [30], was carried out as follows.

Calculated weighed portions of the components of the system (in molecular percentages of 1/800 mole) were placed in graduated test tubes. All the prepared mixtures of components were simultaneously dissolved in some organic solvent. The volume of the solution in each test tube was brought to 10 cm. A solution with a component ratio of 1:1 served as the standard. The color intensity of the standard solution was taken as equal to unity. Using a Duboscq colorimeter, the relative color intensity was determined for solutions containing all the other ratios of components (from 0% quinone and 100% of the second component to 100% quinone and 0% of the second component).

On the basis of the data obtained, a “composition—color intensity” diagram was constructed. The maximum on the diagram corresponded to the complex compound formed in the system. The composition of the complex was determined by projecting the point of maximum onto the abscissa axis.

Results of the study by the method of thermal analysis

A. Quinone—hydrocarbon systems

1. Quinone—naphthalene system

The color of the melt of the component mixtures is brown; the melt is transparent. The numerical material obtained is presented in Table 1 and shown graphically (Fig. 1).

Table 1

The melting diagram indicates that there is no chemical interaction between the components in the system. Only the formation of a mechanical mixture with one eutectic point takes place. The crystallization temperature of the eutectic, determined for all mixtures containing a comparatively large amount of eutectic, lies on the eutectic horizontal.

2. Quinone—diphenyl system

The color of the melt of the mixture of components is brown; the melt is transparent. The experimental data are given in Table 2 and presented in the graph (Fig. 2).

Table 2

The melting diagram indicates the absence of chemical interaction in the system. The formation of a mechanical mixture of the components with one eutectic point takes place. The crystallization temperature of the eutectic for all mixtures in which it was determined lies on the eutectic horizontal.

3. Quinone—acenaphthene system

The melt of the mixture of components is transparent, reddish-brown in color. The numerical data obtained for this system are presented in Table 3 and shown graphically (Fig. 3).

Fig. 1

Fig. 2

Table 3

The fusibility diagram indicates the presence of chemical interaction in the system. The composition of the complex formed is determined by the projection of the maximum point onto the abscissa axis and is expressed by the component ratio 2:1, i.e., \(2\mathrm{C}_6\mathrm{H}_4\mathrm{O}_2:\mathrm{C}_{12}\mathrm{H}_{10}\).

The crystallization temperature of the eutectics of the component mixtures lies on two eutectic horizontals. For mixtures of composition 1:9, 6:4, and 9:1, only the temperature of the onset of crystallization was determined.

4. Quinone—triphenylmethane system

The melt of the mixture of components is transparent and orange-brown in color. The numerical material obtained is given in Table 4 and represented graphically (Fig. 4).

According to the data of the fusibility diagram, chemical interaction between the components of the system is absent. A mechanical mixture with one eutectic is formed. The crystallization temperature of the eutectic, determined for mixtures of components containing a relatively large amount of eutectic, lies on the eutectic horizontal.

Table 4

Fig. 3

Fig. 4

5. Quinone—phenanthrene system

The melt of the mixture of components is thick, only slightly transparent, dark brown, almost black in color. The numerical material obtained for this system is presented in Table 5 and in the graph (Fig. 5).

Table 5

In the study of this system, only the temperature of formation of the first crystals was determined. It was not possible to determine the crystallization temperature of the eutectic, since upon further cooling of the melt it gradually darkened and thickened, but did not crystallize.

N. N. Efremov, speaking of the peculiarities of physicochemical analysis of organic substances, pointed out that in the thermal analysis of organic systems one may encounter cases of low-lying eutectics (below room temperature).

Fig. 5

If a low-lying eutectic is given by a system in which chemical interaction is absent, then the uncrystallized eutectic, even near the freezing temperature, remains an easily mobile liquid.

If the melt of the mixture of components is a viscous liquid which becomes still more viscous upon cooling, then the eutectic often cannot crystallize at all, or crystallizes only with great difficulty.

Thickening of the melt without crystallization of the eutectic serves as an indication of the formation of a chemical compound between the components of the system. Sometimes this compound can be isolated, but more often it cannot be isolated.

The numerical data obtained for this system, the melting diagram, and the fact of thickening of the melt without crystallization of the eutectic (under room-temperature conditions) indicate the presence of chemical interaction in the system.

The maximum of the melting diagram corresponds to two complex compounds formed in the system, of composition \(1:1\) and \(1:2\), i.e.,

\[ \mathrm{C_6H_4O_2:C_{14}H_{10}} \quad \text{and} \quad \mathrm{C_6H_4O_2:2C_{14}H_{10}}. \]

Thus, thermal analysis of binary organic systems composed of quinone and polynuclear hydrocarbons indicates that quinone is capable of forming complex compounds with polynuclear hydrocarbons.

In particular, complexes were found with phenanthrene, of composition \(\mathrm{C_6H_4O_2:C_{14}H_{10}}\) and \(\mathrm{C_6H_4O_2:2C_{14}H_{10}}\), and with acenaphthene, of composition \(\mathrm{2C_6H_4O_2:C_{12}H_{10}}\).

B. Quinone—Polyhalogen-Substituted Phenol Systems

1. Quinone—2,4,6-Trichlorophenol System

The melt of the mixture of components is reddish-brown in color and transparent. The numerical material obtained for this system is presented in Table 6 and in the graph (Fig. 6).

Table 6

The fusibility diagram indicates the absence of chemical interaction in the system. Formation of a mechanical mixture of the components with one eutectic takes place. The crystallization temperature of the eutectic for the mixtures for which it was determined by us lies on the eutectic horizontal.

2. Quinone—2,4,6-Tribromophenol System

The melt of the mixture of components is transparent and reddish-brown in color. The numerical data are shown in Table 7 and in the graph (Fig. 7).

Table 7

The fusibility diagram indicates the absence of chemical interaction in the system. Formation of a mechanical mixture with one eutectic takes place. The crystallization temperature of the eutectic for a number of mixtures of the components lies on the eutectic horizontal.

Figure 6

Figure 7

3. Quinone—2,4-dibromocresol system

The melt of the mixture of components is transparent, reddish-brown in color. The numerical material obtained in the study of this system is presented in Table 8 and in the graph (Fig. 8).

Table 8

The results of the study indicate the absence of chemical interaction in the system. The system is a mechanical mixture of the components with one eutectic. The crystallization temperature of the eutectic for a number of mixtures lies on the eutectic horizontal.

Thus, by the method of thermal analysis, complex compounds of quinone with polyhalogen-substituted phenols were not detected.

B. Quinone—nitrophenol systems

1. Quinone—o-nitrophenol system

The melt of the mixture of components is transparent, light-brown in color. The numerical material presented in Table 9 and in the graph (Fig. 9) indicates the absence of chemical interaction in the system. There is only the formation of a mechanical mixture of the components with one eutectic.

The crystallization temperature of the eutectic for a number of mixtures lies on the eutectic horizontal.

Table 9

Fig. 8

Fig. 9

2. Quinone—m-nitrophenol system

The melt of the mixture of components is transparent, brown in color. The results of the study are given in Table 10 and in the graph (Fig. 10).

The fusibility diagram indicates the absence of a chemical compound in the system. Formation of a mechanical mixture of the components with a single eutectic takes place. The crystallization temperature of the eutectic for a number of mixtures lies on the eutectic horizontal.

3. Quinone—p-nitrophenol system

The melt of the mixture of components is transparent, reddish-brown in color. The numerical material obtained is presented in Table 11 and in the graph (Fig. 11).

Table 10

Fig. 10

Fig. 11

Table 11

Analysis of the experimental data indicates that the system is a mechanical mixture of the components with one eutectic. The crystallization temperature of the eutectic for a number of mixtures lies on the eutectic horizontal.

4. The quinone—1,2,4-dinitrophenol system

The melt of the mixture of components is transparent, wine-red in color. The results of the study are given in Table 12 and represented graphically (Fig. 12).

Table 12

The melting diagram indicates the absence of chemical interaction in the system. A mechanical mixture of the components with one eutectic is formed. The crystallization temperature of the eutectic for a number of mixtures lies on the eutectic horizontal.

Thus, by the method of thermal analysis, complex compounds of quinone with mono- and dinitrophenols were not detected.

G. Quinone systems—sulfur-containing compounds

1. The quinone—dibenzyl sulfide system

On melting the mixture of the components of the system, the appearance of a bright-red coloration was observed at the moment of contact of the molten dibenzyl sulfide with the quinone crystals that had not yet melted. Then the color of the melt becomes reddish-brown; the melt is transparent.

The numerical data obtained in the study of the system are presented in Table 13 and represented graphically (Fig. 13).

Table 13

The melting diagram is characterized by two minima with a maximum between them.

The presence of the maximum, the above-described observations of the change in color, and also the presence of a liquid, gradually thickening eutectic indicate chemical interaction between the components of the system, accompanied by the formation of a complex compound.

Fig. 12

Fig. 13

The composition of the complex, determined by projecting the point of the maximum onto the composition axis, is expressed by the ratio of the components 1 : 2, i.e., the composition of the complex is \(C_6H_4O_2 : 2C_{14}H_{14}S\).

D. Quinone systems—compounds containing nitrogen

(except nitrophenols)

1. Quinone—dinitrobenzene system

The melt of the mixture of components is transparent, reddish-brown in color. The numerical material obtained for this system is presented in Table 14 and in the graph (Fig. 14).

Table 14

According to the melting diagram, there is no chemical interaction in the system. A mechanical mixture of the components with one eutectic is formed. The crystallization temperature of the eutectic, determined for all mixtures containing a relatively large amount of eutectic, lies on the eutectic horizontal.

2. The quinone—α-nitronaphthalene system

The melt of the mixture of components is reddish-brown in color, transparent. The results of the investigation are given in Table 15 and are shown graphically (Fig. 15).

Table 15

The melting diagram indicates that the system is a mechanical mixture of the components with one eutectic. Chemical interaction in the system is absent.

Fig. 14

Fig. 15

3. The quinone—azobenzene system

The melt of the mixture of components is slightly transparent, wine-red in color. The numerical material obtained for this system is presented in Table 16 and in the graph (Fig. 16).

Table 16

The system is a mechanical mixture of the components with one eutectic. The crystallization temperature of the eutectic lies, for a number of mixtures, on the eutectic horizontal.

Thus, according to the data of thermal analysis, dinitrobenzene, α-nitronaphthalene, and azobenzene do not form complex compounds with quinone.

E. Systems quinone—carboxylic acids and others

1. System quinone—benzoic acid

The melt of the mixture of components is reddish-brown in color and only slightly transparent. The numerical data obtained are presented in Table 17 and in the graph (Fig. 17).

Table 17

The fusibility diagram indicates the absence of chemical interaction in the system. Only the formation of a mechanical mixture of the components with one eutectic takes place. The crystallization temperature of the eutectic, determined for mixtures containing relatively much eutectic, lies on the eutectic horizontal.

Fig. 16

Fig. 17

2. System quinone—salicylic acid

The melt of the mixture of components is slightly transparent, dark brown, almost black in color. The numerical material obtained is presented in Table 18 and in the graph (Fig. 18).

Table 18

According to the experimental data, the system is a mechanical mixture of components with one eutectic point. The crystallization temperature of the eutectic for a series of mixtures lies on the eutectic horizontal.

3. System quinone—benzyl (ketone)

The color of the melt of the mixture of components is brownish-red. The numerical material obtained as a result of the study is presented in Table 19 and in the graph (Fig. 19).

The melting diagram indicates the absence of chemical interaction in the system. Only the formation of a mechanical mixture with one—

Table 19

Fig. 18

Fig. 19

with a eutectic. The crystallization temperature of the eutectic for a series of mixtures lies on the eutectic horizontal.

Results of the Investigation by the Method of Colorimetric Analysis

A. Quinone–Hydrocarbon Systems

1. Quinone–Anthracene System

Solvent—chloroform.

The color of the solution of the mixture of components is reddish-brown.

The results of the investigation are presented in Table 20 and in the graph (Fig. 20). The “composition–color-intensity” diagram has two maxima with a minimum between them, which indicates the presence of chemical interaction in the system. The maxima of the diagram correspond to component ratios of 1:1 and 2:1; consequently, the complex compounds formed have the compositions $\mathrm{C_6H_4O_2:C_{14}H_{10}}$ and $2\mathrm{C_6H_4O_2:C_{14}H_{10}}$.

Table 20

Fig. 20

Fig. 21

2. Quinone—phenanthrene system

The color of the solution of the mixture of components is light brown.
Solvent—toluene. The results of the investigation are presented in Table 21 and in the graph (Fig. 21).

Table 21

The “composition—color intensity” diagram indicates the formation in the system of two complex compounds, whose composition is determined by projections from the points of the maxima onto the composition axis.

The points of the maxima correspond to component ratios of 1 : 1 and 1 : 2; consequently, the composition of the complexes is expressed by the formulas

\[ \mathrm{C_6H_4O_2:C_{14}H_{10}} \quad\text{and}\quad \mathrm{C_6H_4O_2:2C_{14}H_{10}}. \]

3. Quinone—acenaphthene system

The color of the solution of the mixture of components is light orange. The solvent is toluene. The numerical material obtained for this system is presented in Table 22 and shown graphically (Fig. 22).

Table 22

On the “composition—color intensity” diagram there is a maximum corresponding to a component ratio of 2 : 1. Consequently, the complex formed in the system has the composition \(\mathrm{2C_6H_4O_2:C_{12}H_{10}}\).

Thus, the data of colorimetric analysis coincide with the results of thermal analysis, indicating the possibility of complex formation between quinone and hydrocarbons.

B. Quinone—phenol systems

1. Quinone—thymol system

The benzene solution of the mixture of components is pink in color. The sample with a component ratio of 2 : 1 is colored noticeably more brightly than the others. The numerical material of the results of the study is given in Table 23 and shown graphically (Fig. 23).

The “composition—color intensity” diagram has a maximum corresponding to a component ratio of 2 : 1, which indicates the presence in the system of a complex of composition \(\mathrm{2C_6H_4O_2:C_{10}H_{14}O}\).

Discussion of the results

The experimental data obtained on the presence or absence of chemical interaction in the systems are summarized in Table 24.

It was not possible to investigate the “quinone—anthracene” system by the method of thermal analysis, since under conditions of high temperature (\(T^\circ_{\text{m.p.}}\).

Table 23

Fig. 22

Vertical axis: color intensity.
Horizontal axis: molecular % quinone.
Left endpoint: \(C_{10}H_8\). Right endpoint: \(C_6H_4O_2\).
Maximum marked: \(2:1\).

Fig. 23

Vertical axis: color intensity.
Horizontal axis: molecular % quinone.
Left endpoint: \(C_{10}H_{10}O\). Right endpoint: \(C_6H_4O_2\).

anthracene 217°) a deep reaction proceeded between the components of the system, accompanied by the evolution of gaseous products. The “quinone—thymol” system also proved unsuitable for study by the visual method of thermal analysis, since the melt obtained was thick and viscous. By the colorimetric method only those systems were studied which gave characteristic colorations in solvents.

Thermal and colorimetric analyses gave concordant results indicating the formation of complexes of definite composition in systems with phenanthrene and acenaphthene. The data of colorimetric analysis indicate the presence of complexes with anthracene and phenol. The complex with dibenzyl sulfide is characterized by the data of thermal analysis.

Complex compounds with dibromocresol, trichlorophenol, and tribromophenol were not detected by the method of thermal analysis, evidently because of the insufficient sensitivity of thermal analysis for detecting unstable complexes. These complexes were later detected by us and synthesized by the topochemical method. They proved to be very unstable, rapidly decomposing in air.

Table 24

In all the remaining systems subjected to testing, no complex compounds were found.

Generalization and analysis of the experimental material obtained, taking into account the available literature data, allow us to express certain considerations regarding the structure of quinhydrones.

It is probable that quinones can form complex compounds of two structural types. In complexes of the first type, the oxygen of the carbonyl group reacts as an electron donor; in complexes of the second type, the carbon functions as an acceptor.

Phenols, in our opinion, can also give complexes of two kinds: first, through the formation of a hydrogen bond (with participation of the hydrogen of the hydroxyl group); second, through the oxygen of the hydroxyl group, which is an electron donor.

A comparison of the chemical properties of quinones with the properties of ketones of the limiting series shows that ketones are to a considerably greater extent than quinones inclined to form complexes with acids, i.e., they possess stronger donor properties than quinone.

Quinones, on the contrary, exhibit acceptor properties more strongly than ketones, which is confirmed, for example, by their strong oxidizing ability (quinones are reduced more readily than ketones). Since quinones are reduced more readily than ketones, consequently they attract electrons more readily, because during reduction an oxidizing agent attracts electrons from a reducing agent.

Taking this into account, it should be expected that in complex-formation reactions quinones will tend to exhibit acceptor properties.

A series of phenols—phenol, hydroquinone, resorcinol, naphthol, chlorophenol, and others—have very small dissociation constants (of the order of \(10^{-10}\)), i.e., the hydrogen in their molecules is firmly bound and should with difficulty participate in the formation of a hydrogen bond. However, complexes of quinone with the indicated compounds are known.

Polyhalogen-substituted phenols, mono- and dinitrophenols, have stronger acidic properties (dissociation constants of the order of \(10^{-8}\)—\(10^{-5}\)). In benzoic acid the hydrogen is still more active.

Consequently, if quinhydrones are formed by virtue of a hydrogen bond, one would have to expect the formation of complexes with carboxylic acids and nitrophenols. However, no complexes were detected either with acids or with nitrophenols, whereas polyhalogen-substituted phenols gave unstable quinhydrones that are readily destroyed in air.

Thus, the factual material and our considerations do not agree with the view that a hydrogen bond is present in all quinhydrone-like complexes.

We are inclined to assume that complex compounds of quinones with phenols are formed through the attraction of the phenol oxygen to the carbon atom of the quinone carbonyl group. In the interaction of quinone

phenoquinone      quinhydrone

(or chains)

with picric acid (a very strong acid with \(K = 0.16\)), in our opinion, formation of a complex of another type, by means of a hydrogen bond, is possible.

Conclusions

1. New complex compounds of quinone with polynuclear hydrocarbons and dibenzyl sulfide have been discovered.

2. Complex formation between quinone and carboxylic acids, quinone and nitro compounds, quinone and mono- and dinitrophenols is absent.

3. Quinones form complexes with organic compounds of two structural types: through a hydrogen bond and through a carbon–oxygen bond.

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