B. V. Tronov and L. P. Kulev

Comparison of the Activity of Alcohols and Carboxylic Acids under Various Conditions

The comparison of the activity of carboxylic acids with one another, and also of acids with alcohols, was carried out by the method we developed, indicated in previous works*). The experiments were performed by us both with individual substances and in the presence of various solvents, for example in benzene and ether solutions.

We investigated the following series of acids: 1. Formic acid, 2. Acetic acid, 3. Propionic, 4. Butyric, 5. Valeric.

Interesting results were obtained in comparing these acids in a mixture with pyridine; therefore the largest number of experiments was carried out for them precisely under these conditions. Under the same conditions some alcohols were also tested, namely: methyl alcohol, n-butyl alcohol, and phenol.

Since all the acids gave the usual pattern of change in the electromotive force of reaction with sodium, i.e., on passing from the lower representatives of the series to the higher ones the EMF correspondingly decreases, only propionic acid was investigated most fully in benzene solution. Various concentrations of the acid per 5 cubic cm of benzene solution gave the following picture.

That is, the curve of change of EMF has this form (Fig. No. 1). At constant concentration (2.59 cu. cm of acid) the EMF, as a function of time, is expressed as follows: (Fig. No. 2). The same acid in ether solution (Fig. 3).

Thus, the maximum electromotive force for propionic acid without solvent is expressed as 870 millivolts. At identical concentrations, i.e. 2.59 cu. cm of acid + 2.41 cu. cm of solvent, the EMF in benzene solution is greater than in ether solution (20 and 12 mV).

*) B. V. Tronov and L. P. Kulev, “Determination of the mobility of hydrogen in organic compounds by the electromotive force of reaction with sodium.”

The addition of pyridine to the acid sharply increases the electromotive force, which reaches a maximum at definite ratios of amine and carboxylic acid; moreover, the maximum EMF in this case is considerably greater than for the pure acid. The following table gives a picture of the relation between the EMF and various concentrations of propionic acid in a mixture with pyridine, or this dependence may be expressed by the following curve (Fig. No. 4).

Consequently, the maximum EMF arises at a content of 70% propionic acid relative to pyridine and increases the electromotive force of the pure acid threefold.

Butyric acid, as was to be expected, gives in all cases a smaller electromotive force, but just as for propionic acid, in mixture with pyridine the maximum EMF corresponds to a 70% acid content in this mixture and far exceeds the limit attainable with the pure acid:

In a mixture with pyridine:

Or these ratios for a mixture of butyric acid with pyridine are expressed by the curve (Fig. No. 5).

For isovalerianic acid the maximum electromotive force likewise corresponds to 70% acid relative to pyridine:

For formic and acetic acids it was necessary to confine ourselves to a minimal number of determinations—owing, on the one hand, to the insolubility of the salts formed in the reaction (for acetic acid), and, on the other, to the liberation of a large amount of heat (for formic acid), preventing the establishment of the true maximum of the electromotive force under conditions analogous to those for the other acids.

For formic acid, an EMF exceeding the EMF of the pure acid is established already at 10% acid in the mixture with pyridine:

For acetic acid, the limiting EMF is likewise at the same concentration:

For comparison with carboxylic acids, we also carried out experiments with certain alcohols, likewise in mixtures with corresponding amounts of pyridine. For methyl alcohol, for example, the EMF in the presence of pyridine is expressed in the following table:

For butyl alcohol:

Thus, for alcohols the change in electromotive force in the presence of pyridine is of a different order than for the corresponding carboxylic acids. Evidently, alcohols behave in mixtures with pyridines in the same way as in the presence of other solvents, for example in benzene. Phenol, having a weakly acidic character, is an exception:

Here a peculiar regularity is observed. The maximum EMF corresponds to 10% phenol.

In order to ascertain the influence of amines on the electromotive force of the reaction of sodium with various organic compounds, we carried out exploratory experiments with nitrobenzene in mixtures with various amines:

Pure nitrobenzene gives an EMF of 1.4 mV.

The influence of amines on the EMF of the reaction of sodium with water is of interest. Pure piperidine gives no EMF at all; upon addition of 10% (molecular) water an EMF of 3.2 arises. Pure pyridine gives an EMF of 4.6 mV; with 10% water—70 mV, with 20%—1100 mV. A system consisting of 6 cu. cm pyridine + 1 cu. cm piperidine + 0.1 cu. cm $\mathrm{H_2O}$ gives 45 mV. With the same amount of water, at ratios of 4.5 cu. cm pyridine + 0.5 cu. cm piperidine, the EMF is 30 mV.

The data obtained by us make it possible to draw the following conclusions:

1. Comparison of alcohols and monobasic carboxylic acids of the saturated series.

The hydrogen of the carboxyl group in carboxylic acids is less active, and reacts with sodium with more difficulty, than the hydrogen of an alcoholic hydroxyl.

1. The influence of pyridine on the EMF of acids.

a) The acids studied by us give a maximum electromotive force in mixtures with pyridine at a ratio of 1 mole of pyridine + 2 moles of acid.

b) This maximum considerably exceeds even the EMF of the reaction of the free acid with sodium.

c) The magnitude of the EMF maximum in pyridine changes with increasing molecular weight of the acid in the same direction as the EMF given by the free acid or in benzene solution.

1. The influence of pyridine on the EMF of alcohols.

Here the large EMF of solutions of phenol in pyridine at low phenol concentrations is of interest.

Recently the view has been finding ever greater acceptance in the literature that electrolytic dissociation is based above all on complex formation¹. It is not the dissolved substance itself that dissociates into ions, but the product of its combination with the solvent. Some authors begin—

attention to the fact that not all complexes are capable of dissociation, but only those having a definite structure. Thus, M. A. Rabinovich notes that trichloroacetic acid with dimethylpyrone gives a conducting system, whereas in \( \mathrm{H_2SO_4} \) it practically does not lead to this, although there is reason to assume the formation of a compound here also. He explains this fact by saying that, apparently, the chemical interaction of acetic acids with \( \mathrm{H_2SO_4} \) is not the same as with dimethylpyrone\(^2\).

As a result of our experiments, the latter proposition finds full confirmation. The activity of hydrogen in organic hydroxyl compounds with respect to metals, determined from the emf of the reaction with sodium, is a good measure of the acidity of the given compound. And it turns out that the relative acidity of alcohols and carboxylic acids differs depending on whether we take them in their individual form, or mix them with water, benzene, pyridine.

In aqueous solution, carboxylic acids are much more active than alcohols. In individual form they prove to have lower acidity. The same is true in benzene. Propionic acid, for example, is under these conditions a considerably weaker acid than propyl alcohol.

Obviously, this must be explained by the different structure of the complexes. The alcohol forms associated molecules \((\mathrm{ROH})_n\) (where \(n\) for our alcohols is close to 3), which must be constructed according to the type:

\[ \begin{array}{ccccc} \mathrm{R{-}O{-}H} &&&& \mathrm{R{-}O{-}H}\\ \quad \vdots && \text{or} && \quad |\\ \mathrm{H{-}O{-}R} & \text{etc.} &&& \mathrm{H}\quad \mathrm{O{-}R} \end{array} \]

In such complexes the hydrogen is rather active.

In carboxylic acids we have fairly strong dimers, for example, for acetic acid \(((\mathrm{CH_3COOH})_2\), which have been established not only in benzene solution, but also in the vapor (i.e., at a temperature of about \(120^\circ\)). Water, evidently, destroys these dimers, giving \( \mathrm{R.COOH}\cdot n\mathrm{H_2O} \), which is why considerable acidity appears in aqueous solution. But the dimer itself \((\mathrm{RCOOH})_2\), apparently, is constructed quite differently than in alcohols. Most likely a structure of the type:

\[ \begin{array}{ccccc} & \mathrm{R{-}C{=}O} && \mathrm{OH} &\\ & \diagdown && \diagup &\\ \mathrm{O} && \cdots && \mathrm{C{-}R}\\ & \diagdown && \diagdown\!\!\diagdown &\\ & \mathrm{H}\cdots\mathrm{O} && \end{array} \]

Here there is no mobile hydrogen at the oxonium oxygen such as is present in alcohols.

With pyridine the acid forms a quite definite salt. The greatest

\(^2\) Zh. R. Kh. O., 60, 637 (1929).

electromotive force for the composition $\mathrm{C_5H_5N \cdot 2\,RCOOH}$, which indicates the participation of the complete dimer in the reaction. The probable structure may be considered to be

\[ \begin{array}{c} \mathrm{R}\\[-2mm] |\\[-1mm] \mathrm{C} \end{array} \quad \begin{array}{c} \mathrm{O{=}C{-}R}\\[-1mm] \backslash\\[-1mm] \mathrm{O}\\[-1mm] /\\[-1mm] \mathrm{H} \end{array} \qquad \begin{array}{c} \\[-6mm] \mathrm{C{=}O} \end{array} \qquad \begin{array}{c} \mathrm{O}\\[-1mm] |\\[-1mm] \text{pyridine ring}\\[-1mm] \mathrm{N}\\[-1mm] |\\[-1mm] \mathrm{H} \end{array} \]

Hydrogen has appeared at the ammoniacal nitrogen, and the capacity of this hydrogen for replacement by metals has increased. Nothing can yet be said about the structure of the pyridine complex with phenol. It must be thought that this addition product is quite peculiar. It is possible that the carbons of the benzene ring take part in its formation.

In general, we see that the acidity of a hydroxyl compound is determined not only by the nature of this compound itself, but to a very great extent is also determined by the nature of the other constituent part of the solution. If, for example, one compares water, propyl alcohol (as a representative of the alcohols of the limiting series), propionic acid, and phenol, then under different conditions different series are obtained according to the magnitude of the EMF.

Aqueous medium: $\mathrm{C_2H_5COOH > C_6H_5OH > HOH > C_3H_7OH}$

Pure compounds: $\mathrm{HOH > C_3H_7OH > C_2H_5COOH}$ ($\mathrm{C_6H_5OH}$ is excluded, as a solid substance at the temperature of the experiments).

Benzene solution: $\mathrm{C_3H_7OH > C_2H_5COOH > C_6H_5OH}$ ($\mathrm{H_2O}$ is excluded because of its poor solubility).

Weak solution in pyridine: $\mathrm{C_6H_5OH > H_2O > C_2H_5COOH > C_3H_7OH}$.

It is necessary, therefore, to speak not of one compound being a stronger acid and another a weaker one, but of such-and-such a compound having stronger acidic properties under such-and-such conditions.

1. V. Plotnikov and Izbekov, Zt. anord. Chemie. 71, 328 (1911);
   M. Rabinovich and E. Frengarten, Zt. Phys. Chem. 147, 345 (1930);
   H. Lasko, “Roczniki Chemie” (1932) 12, 35—47;
   M. I. Usanovich, Zh. O. Kh. 2, 443 (1932);
   Walden and Birr, Zt. Ph. Ch. (1933). ↩