L. P. Kulev.

On the Nature of Complexes of Phenols with Amines

In one of our papers¹ we indicated that the acidity of hydroxyl compounds is determined not only by the nature of the compound itself, but also by the nature of another constituent part of the solution. Thus, for example, propyl alcohol in benzene solution proved to be more active in reactions with metals, i.e., a stronger acid than propionic acid tested under the same conditions, although in an aqueous medium the former does not possess acidic properties. Conversely, in pyridine, carboxylic acids proved to be many times more active than alcohols, which can be explained by the mobility of the hydrogen in the ammonium nitrogen of the salt formed:

\[ \begin{array}{c} \text{pyridine ring}\\[-2mm] \quad N{-}O{-}C(R)\cdots O{=}C(R)O\cdots H \end{array} \]

In this case the pyridine molecule is bound to a dimer of an acid of a structure different from that in alcohols.

Phenols, in their structure, stand closer to tertiary alcohols, but at the same time they also possess weak acidic properties. In an aqueous medium, carboxylic acids are considerably more active than phenols. However, when they are compared at equal concentrations in pyridine, the reverse is observed: thus, 10% \(C_6H_5OH\) gives an EMF of 825 millivolts, whereas 10% \(C_3H_6O_2\) gives only 70 mV.

Such a difference in the magnitude of the EMF occurs only if they are compared at definite concentrations (for example, 10% of both, as indicated above, relative to pyridine); at other concentrations (for example, at 70%) the maximum EMF values for acids are greater than for phenols (saturated monobasic acids up to butyric acid inclusive were compared with \(C_6H_5OH\)).

Alcohols of the fatty series, unlike acids and phenols, do not increase the EMF in the presence of pyridine, with the exception of tertiary alcohols, for example \((CH_3)_3COH\), for which a maximum exceeding the electromotive force of the alcohols themselves is observed¹.

Thus, phenols, by the character of the change in EMF in reaction with sodium, occupy an intermediate position between carboxylic acids on the one hand and tertiary fatty alcohols on the other. The presence of the benzene ring in the phenol molecule affects the character of their complexes with other compounds; therefore the maximum (or minimum) of the electromotive force in reaction with sodium corresponds to other ratios of the constituent parts of mixtures with pyridine and other substances than for carboxylic acids and alcohols.

In order to compare the activity of different phenols with one another, and also with certain acids and alcohols, experiments were set up under conditions analogous to those used for the latter, i.e., \(1/200\) M of the substance was mixed with the same quantity of another substance, for example an amine, and the mixture was diluted to 6 cc with benzene. The electromotive force arising in these reactions made it possible to determine the approximate composition of phenol complexes with amines (for example, for \(\mathrm{C_6H_5OH}\) with pyridine) and to outline their possible structure.

It is interesting to note that even at a very considerable electromotive force (up to 1000 millivolts), in experiments with phenols no heating was observed,\(^{2}\) i.e., the free energy of the reaction in this case was completely converted into electrical energy, which facilitated their comparison at ordinary temperature.

Experimental data.

Table No. 1

With pyridine containing up to 3% water:

With anhydrous pyridine:

Table No. 2

The electromotive force of phenol with anhydrous pyridine is, consequently, more than two and a half times smaller than with pyridine containing water. At the same time the EMF maximum shifts (25% in the first case and 30% in the second).

\(^{2}\) With alcohols and acids, especially with the latter, even at low concentrations of them in solutions (benzene), rather strong heating was often observed.

In alcohols of the fatty series, under the same conditions, the reverse picture was observed.

Table No. 3

In order to compare $\mathrm{C_6H_5OH}$ with other phenols, less soluble in benzene and pyridine (for example, naphthols), experiments were carried out at other concentrations, i.e., $1/600\,M$, with the corresponding amount of pyridine and with dilution of the mixture to 6 cc with benzene. In this case the maximum EMF corresponded to other ratios of phenol to amine.

Table No. 4

Thus, strong dilution with benzene shifts the EMF maximum of the mixture.

For a more detailed study of the dependence of the electromotive force on the concentration of the solution, a series of experiments was carried out in pyridine (without benzene) with a gradual increase in the amount of phenol.

Table No. 5

In this case a sharper maximum of the EMF is observed than in the experiments in benzene solution (experiment No. 10), as well as a sharp decrease in the electromotive force (experiment No. 7) between two neighboring points. It is interesting that this minimum also corresponds to 25% \(C_6H_5OH\) relative to all the pyridine taken in this experiment. The same experiments (Nos. 7 and 9), tested after 24 hours (kept in sealed test tubes), gave no change in the EMF, which indicates stable, rapidly forming complexes.

With other amines phenol gives a considerably smaller electromotive force than with pyridine; with aniline and dimethylaniline no EMF maximum is observed at all.

Table No. 6

It is interesting to note that, on standing, the mixture of phenol with aniline in benzene solution gives an electromotive force that increases severalfold. Thus, for example, experiment No. 2 (50:50), set up in a sealed test tube for 24 hours, on testing gave 15.7 mV, i.e. 3 times more than it gave in the freshly prepared mixture.

With dimethylaniline, phenol, at low concentrations of it in solution, gives an insignificant EMF (less than with aniline); at higher concentrations, it is of the same order as in the preceding case.

Table No. 7

A similar decrease in the electromotive force was established not only in the presence of aromatic amines; it is also observed in mixtures of phenol with quinoline, but to a lesser degree.

Table No. 8

In contrast to aromatic amines, here, as in the case of pyridine, a maximum EMF is observed, which, however, does not exceed the EMF of phenol itself. With tertiary fatty amines, e.g., triethylamine, on the contrary, a very considerable increase in EMF takes place even with negligible quantities of amine, which is characteristic both for other phenols and for carboxylic acids.

Table No. 9

Cresols in general approach \(C_6H_5OH\) in activity in mixtures with pyridine, and here too the maximum electromotive force corresponds to 20–30% of them in solution:

Table No. 10

An admixture of water to pyridine also activates cresols, as it does phenol itself. For example, metacresol with aqueous pyridine gives:

Table No. 11

With triethylamine, metacresol behaves in the same way as \(C_6H_5OH\); in this case the EMF maximum, corresponding also here to 90%, is somewhat greater than that of the former.

Table No. 12

Naphthols, tested at low concentrations \((1/600\,M)\), gave a considerable electromotive force. At the same time, β-naphthol in a mixture with pyridine proved more active than \(\alpha\)-\(\mathrm{C}_{10}\mathrm{H}_7\mathrm{OH}\) and, at the same concentration, almost not inferior in activity to \(\mathrm{C}_6\mathrm{H}_5\mathrm{OH}\).

Table No. 13

However, β-naphthol is not in all cases more active than \(\alpha\)-\(\mathrm{C}_{10}\mathrm{H}_7\mathrm{OH}\). Thus, for example, in a nitrobenzene solution (weighed portion 0.72 g in 5 cc \(\mathrm{C}_6\mathrm{H}_5\mathrm{NO}_2\)) the latter proved more active:

\[ \begin{aligned} \alpha\text{—}\mathrm{C}_{10}\mathrm{H}_7\mathrm{OH} &\ldots\ldots 1350\ \mathrm{mV}\\ \beta\text{—}\mathrm{C}_{10}\mathrm{H}_7\mathrm{OH} &\ldots\ldots 1200\ \mathrm{mV} \end{aligned} \]

In pyridine, however, and without dilution with benzene, the order indicated above is preserved:

\[ \begin{aligned} \alpha\text{—}\mathrm{C}_{10}\mathrm{H}_7\mathrm{OH} &\ldots\ldots 685\ \mathrm{mV}\\ \beta\text{—}\mathrm{C}_{10}\mathrm{H}_7\mathrm{OH} &\ldots\ldots 710\ \mathrm{mV} \end{aligned} \]

Conclusions

1. Of the phenols, \(\mathrm{C}_6\mathrm{H}_5\mathrm{OH}\) proved the most active toward metallic sodium in mixtures with amines.

2. In comparing the results obtained in experiments with phenol in benzene and pyridine solutions, at identical concentrations of \(\mathrm{C}_6\mathrm{H}_5\mathrm{OH}\), a sharp difference is observed in the magnitude of the EMF. Thus, in the first case a maximum is observed at 25–30%, whereas in the second, on the contrary, at the same ratios (with respect to pyridine) there is a sharp minimum in comparison with neighboring points:

Apparently, both in the one case and in the other, the minimum and maximum of the EMF correspond to a phenol–pyridine complex of the same composition, i.e. \(1\,M\ C_6H_5OH : 3\,M\ C_5H_5N\), which, depending on the medium, gives either an increase or a decrease in the electromotive force.

Since an increase in EMF was observed in the presence of benzene, a series of experiments was carried out with the addition of small amounts of it to a solution of phenol in pyridine, and the following interesting phenomena were observed:

That is, upon addition of benzene the minimum electromotive force corresponding to 25% \(C_6H_5OH\) in pyridine is increased. This makes it possible to understand why a complex of one and the same composition corresponds to a maximum EMF in benzene and to a minimum EMF without solvent.

The influence of other compounds on the EMF of this complex is of interest:

Thus, alcohols either almost do not change the EMF or greatly lower it. Water sharply increases the EMF of the mixture. Indene, like benzene, increases it to 450 mV. (In the individual state indene gives no EMF. However, phenol, which in benzene solution also gives no EMF \((0.47\) g in 6 cc \(C_6H_6)\), in a solution of indene gives 0.4 mV).

Nitro compounds have different effects on the electromotive force of the complex. Thus, upon addition of one drop:

\[ \begin{array}{rcl} C_6H_5NO_2 & \text{EMF} & \text{increases to } 600\ \text{mV} \\ o\text{-}C_6H_4NO_2CH_3 & \text{”} & \text{decreases to } 275\ \text{mV} \\ m\text{-}C_6H_4NO_2CH_3 & \text{”} & \text{increases to } 450\ \text{mV} \end{array} \]

In this case a definite dependence is observed in the change of the electromotive force on the position of the substituent groups, which, perhaps,

will make it possible to determine even the character of one or another compound, operating with negligible quantities of it¹.

Phenol in pyridine solution, in addition to a minimum of the EMF, also gives a maximum (see the same Table No. 5). Experiments with the addition of one drop of certain substances to this mixture gave the following results:

Here only nitrobenzene did not change the EMF of the mixture; the others, even water, decreased it, although under other conditions (for example, in pyridine alone, in alcohols, acids) $H_2O$ in general very strongly increases the electromotive force of the compound present with it in the mixture.

All this makes it possible to suppose that phenol with pyridine forms several complexes, the activity of which with respect to sodium is different. The addition of small amounts of other compounds affects the composition of the complex or, in any case, sharply changes its activity, the result being a decrease or increase in the electromotive force.

One of the complexes, observed both in benzene and in pyridine solutions, apparently has a more definite composition
($1\,M$ $C_6H_5OH : 3\,M$ $C_5H_5N$), and its structure may be expressed, for example, by the formula:

\[ \begin{array}{c} \mathrm{H}\\ |\\ \mathrm{O}\\ |\\ \text{phenolic ring}\;\cdots\;\text{pyridine ring}\;\cdots\;\text{pyridine ring}\\ \phantom{\text{phenolic ring}}\;\;\;\;\;\;\;\;\;\;\vdots\\ \phantom{\text{phenolic ring}}\;\;\;\;\;\;\;\;\;\;\text{pyridine ring} \end{array} \]

The activity of such a complex, formed by three molecules of pyridine and one molecule of phenol, linked through carbon atoms, depends on the solvent. If the addition of one drop of benzene sharply changes the electromotive force of the complex, it is clear that with strong dilution

with the same benzene, the maximum EMF does not correspond to the indicated composition of the complex (see Table No. 4).

1. An increase in the electromotive force also occurs in the case where the phenol contains impurities that are usually formed during its oxidation

(phenoxynone). For example, experiments with one and the same preparation of phenol, colorless and slightly colored, gave the following results: that is, the magnitude of the EMF can serve as a criterion of the purity of the preparation.

1. With fatty amines, e.g. $(\mathrm{C_2H_5})_3\mathrm{N}$, phenols give a very considerable electromotive force at high concentrations of them in solution (for $\mathrm{C_6H_5OH}$ and $m$-$\mathrm{C_6H_4OHCH_3}$—90%). If with pyridine phenols form complexes of definite composition, one may think that with fatty amines (triethylamine and other strong amines) they give compounds of another type, most probably ammonium salts. For example,

\[ \begin{array}{c} \mathrm{C_2H_5}\\[-0.2em] \quad |\\[-0.2em] \mathrm{H_5C_2}\backslash \mathrm{N}\!-\!\mathrm{C_2H_5}\\ \quad / \ \backslash\\[-0.2em] \mathrm{H}\quad \mathrm{OC_6H_5} \end{array} \]

in which the hydrogen bound to nitrogen is readily replaced by metals.

1. Of the cresols in mixtures with pyridine, the most active proved to be $m$-$\mathrm{C_6H_4OHCH_3}$. In activity they are arranged in the following order:

\[ \text{meta} > \text{para} > \text{ortho}^{1}) \]

At the same time, their complexes with pyridine are probably built according to the same type as the complex of $\mathrm{C_6H_5OH}$ with the same amine (the maximum EMF corresponds to 20–30% of them in solution).

1. In contrast to heterocyclic amines (chiefly pyridine), aromatic amines ($\mathrm{C_6H_5NH_2}$ and $\mathrm{C_6H_5N(CH_3)_2}$) form complexes with phenols much more slowly and are by no means as active as the former.

Laboratory of Organic Chemistry
Tomsk Industrial Institute.

1) The same regularity was also observed by us earlier, when comparing these compounds in the individual state in benzene solution (B. V. Tronov and L. P. Kudev). Determination of the activity of hydrogen in organic compounds by the EMF of the reaction with sodium. Zh. O. Kh., vol. IV, pp. 197–202, 1934.

1. Specific reactions for nitro compounds are unknown (?). Emich, Microchemical Analysis, p. 251, 1932. ↩↩↩