ON THE MECHANISM OF OXIDATION OF HARD COAL AND THE CAUSES OF ITS SPONTANEOUS COMBUSTION

Article 8.

B. V. Tronov and V. Z. Grigor’eva.

Iodine Numbers of Phenols

As one of the methods for revealing the capacity of the coals under study for vigorous oxidation in air, it has been proposed to determine the amount of iodine or bromine absorbed by a given weighed portion of coal. The first investigator to draw attention to the great activity of coals toward halides, Ferd. Fischer¹), recommended using bromine or an acidified solution of KBr and KBrO₃ for this purpose, since he found that iodine acts on coals too weakly. Among subsequent authors, some², ³) adhered to Fischer’s method, while others⁴, ⁵) used iodine.

The ability of coals to react with halides was explained¹) by the presence of double bonds between carbon atoms, and the observed fairly good parallelism between the activity of coals toward halides and toward oxygen gave grounds to suppose¹) that the oxidation of coals in air likewise begins with unsaturated groups.

On the basis of a whole series of facts, one of us⁶) proposed another scheme for the oxidation of coals. According to this scheme, atmospheric oxygen acts first of all not on double bonds between carbon atoms, but on phenolic groups.

How, then, in that case is one to explain the fact that, nevertheless, coals more prone to oxidation usually also give higher bromine and iodine numbers, since iodine and bromine are classical reagents precisely for the double bond?

To answer this question it was necessary to see how phenols behave toward halides.

As regards bromine, it has long been known that, for example, carbolic acid readily reacts with three and even four molecules of it, giving tribromophenols and tetrabromophenols:

![structural scheme: phenol → tribromophenol → tetrabromophenol]

The reaction of phenols with iodine has been little studied. Orlov^7) acted on the corresponding phenol, dissolved in a saturated solution of borax, with a solution of iodine in potassium iodide. After 24 hours the solution was acidified with acetic acid, the excess iodine was titrated with hyposulfite, and the precipitate formed was subjected to further treatment. As a result of his investigations, E. I. Orlov found that one gram-molecule of phenol requires 3 atoms of iodine for iodination; one gram-molecule of β-naphthol likewise 3 atoms of iodine; α-naphthol, 4.19 atoms of \(I_2\); resorcinol, 3.135 atoms of \(I_2\); pyrogallol, about 5 atoms of \(I_2\); hydroquinone, 1.996 atoms of \(I_2\), etc. Of the iodination products he investigated most closely the product of phenol iodination, which he called pseudoiodosoiodobenzene, assigning to it, on the basis of the corresponding determinations, the empirical formula \((C_6H_4IOI)_3\).

We decided to test the action of iodine on phenols under the usual conditions for determining the iodine number of unsaturated compounds. Two of the most common methods were chosen: those of Hübl and Margosches. The following phenols were taken: phenol, α-naphthol, β-naphthol, pyrocatechin, resorcinol, hydroquinone. The iodine number was determined by the Hübl method as follows:^8,9)

A weighed sample of the phenol under investigation (0.05 g) was dissolved in 10 ml of chloroform. For resorcinol, hydroquinone, and pyrogallol it was necessary to use alcohol as the solvent, since they are poorly soluble in chloroform. To the solution were added 20 ml of Hübl’s reagent.

For α-naphthol, pyrocatechin, resorcinol, and pyrogallol, 40 ml of Hübl’s solution were taken. The mixture was shaken thoroughly and left for 24 hours. Then 10 ml of 10% KI and 200 ml of water were added, and the remaining iodine was titrated with hyposulfite. A blank experiment was run in parallel.

The iodine numbers were determined by Margosches’ method in the following way:^8,9)

In an Erlenmeyer flask, to 500 ml of a centimolar solution of phenol were added 10 ml of 0.2 N iodine solution in alcohol and 200 ml of water. After shaking, the contents of the flask were left for 5 minutes; then the remaining iodine was titrated with hyposulfite.

In Table 1 the iodine numbers are expressed as percentages relative to the phenol. Also indicated is the number of gram-molecules of iodine that reacted with one gram-molecule of phenol.

It was also of interest to determine the rate of reaction of phenols with iodine, especially in the Margosches method, since here the experiment lasts a comparatively very short time and it could be thought that the reaction is far from complete.

We determined the rate in aqueous-alcoholic solutions at a temperature of 16–20°. Of the phenols, the following were taken: phenol, α-naphthol, β-naphthol, resorcinol, hydroquinone, and pyrogallol.

Iodine Numbers of Phenols

Table 1

The working procedure was as follows:

In an Erlenmeyer flask, 10 ml of an alcoholic iodine solution (0.2 N) is mixed with 200 ml of water. To the resulting aqueous-alcoholic solution, with stirring, 10 ml of a centimolar aqueous solution of phenol is rapidly added. After definite intervals of time, the remaining iodine was titrated with a 0.2 N hyposulfite solution, the titer of which had been established against freshly sublimed iodine. At the beginning of the work a blank experiment is set up. From the difference between it and the experiment with phenol, the amount of iodine that entered into the reaction is calculated.

For each phenol we took four points: 2 sec., 60 sec., 300 sec. and 1800 sec. The data obtained are collected in Table 2, where the amounts of iodine are given in mg per 100 mg of phenol and in gram-molecules per 1 gram-molecule.

Table 2

Rate of Reaction of Phenols with Iodine

Conclusions.

From the results of our experiments the following conclusions may be drawn:

1) All the phenols that we took proved to be fairly active with respect to iodine, with which they react not only in the presence of alkali, but also under the conditions used for determining the iodine number of unsaturated compounds.

Thus, from the iodine number of some technical mixture or of some complex organic substance, one cannot judge with certainty the presence of unsaturated bonds. One must also take into account the possibility of the presence of phenols (or phenolic groups).

2) In terms of the magnitude of the iodine number and the rate of reaction with iodine, individual phenols differ sharply from one another.

\(C_6H_5OH\) reacts most slowly of all. β-naphthol and hydroquinone are more active. Next, in order of increasing reaction rate, come resorcinol, α-naphthol, and pyrogallol. Hydroquinone is characterized by the rapid attainment of the limiting iodine absorption. Evidently, it is oxidized to quinone, and the process ends there. For the other phenols the reaction, apparently, proceeds in a more complicated fashion. The products have not yet been investigated, but the results of preliminary experiments suggest that, with iodine in an aqueous-alcoholic solution, both oxidation and iodination occur. Thus, with resorcinol, a flocculent precipitate was isolated, soluble in alkalis, resembling halogen-substituted phenols. At the same time gas evolution was observed; paper moistened with a solution of palladium chloride darkened from this gas, as from carbon monoxide.

The different behavior of different phenols makes it impossible to determine phenols or phenolic groups qualitatively by means of iodine.

3) A comparison of the Hübl and Margoshes methods has not yet yielded definite results.

Almost all the phenols we studied gave higher iodine numbers by Margoshes than by Hübl, although the duration of the reaction was much shorter. For resorcinol, on the contrary, the iodine number by Hübl proved to be considerably higher.

4) Comparison of the data we obtained with the results of determining the oxidation rate of phenols \(^{10, 11)}\) by potassium permanganate indicates a very good correspondence in the activity of phenols toward iodine and toward oxidizing agents. This has been sufficiently proved for permanganate, but is apparently also true for free oxygen. At least, of the two naphthols, α-naphthol is iodinated more rapidly, and it also darkens more rapidly in air. Pyrogallol, used in gas analysis as a good absorber of oxygen, is oxidized by permanganate (under all conditions) and reacts with iodine more readily than all phenols and, in general, all the organic compounds we studied.

The work was carried out with funds from KHIUKI.

References

1) Ferd. Fischer. Zeit. angew. Chem. 1899, pp. 764–7, 787–90.
2) Journ. f. Jusbel 1906, 422.
3) Fischer’s Jaresber 39, 13 (1908).
4) E. P. Voitova. — “Chemistry of Solid Fuel,” VII, No. 3, pp. 249–262 (1936).
5) G. A. Zilberg, N. N. Saikovich, N. B. Denisova. — Proceedings of the Kuzbassugol Research Institute, 1935, “Hard Coals of Kuzbass,” pp. 157–179.
6) B. V. Tronov, “On the Mechanism of Oxidation of Hard Coal by Atmospheric Oxygen” (in press).
7) E. I. Orlov. Zh.R.Kh.O., 38, pp. 1204–1210 (1906).
8) Tyutyunnikov. — “Food Industry,” 1925, No. 4–5.
9) Demyanov — “Fats and Waxes.”
10) B. V. Tronov, A. I. Kravchenko, and E. V. Yudenicheva. — (Work submitted for publication).
11) B. V. Tronov and M. Yu. Grigor’ev — (work in press).