ON THE QUESTION OF THE MECHANISM OF THE OXIDATION OF HARD COAL AND OF THE CAUSES OF ITS SPONTANEOUS IGNITION

Article 7.

B. V. Tronov and M. Yu. Grigor'ev.

The rate of oxidation of certain complex phenols by potassium permanganate in an alkaline medium.

In the laboratories of organic chemistry of the Tomsk Industrial Institute and Tomsk State University, work has been conducted for several years¹ on determining the rate of oxidation of the most diverse organic compounds. The results of these studies show that, of all the classes of organic substances studied, the highest rate of oxidation—in particular, oxidation by potassium permanganate, which proved the most convenient for these experiments—was exhibited by phenols.² They are followed, at some distance, by unsaturated compounds of the fatty series²˒³ and aromatic amines⁴.

It is also known that humic substances, which in the opinion of many scholars⁵˒⁶ constitute the most readily oxidized part of fossil coals, contain a rather large number of phenolic groups in their molecules. For example, in the structural formulas of humic acids proposed by Fuchs⁷˒⁸˒⁹˒¹⁰˒¹¹ we find more phenolic hydroxyls than groups with double bonds, to which many authors, beginning with Ferd. Fischer¹², ascribe the decisive role in the process of coal oxidation.

A comparison of the facts indicated allowed one of us¹³ to express the opinion that the oxidation of coal by air begins precisely with hydroxylated benzene rings.

The proposed scheme explains without contradiction many phenomena observed during the oxidation of coals. Especially important, and a major confirmation of the phenolic theory, is the fact that it makes comprehensible the evolution, when air acts on coals even at ordinary temperature, not only of carbon dioxide but also of carbon monoxide.

However, in the basic premises of the new theory of coal oxidation there were certain gaps. Filling one of them was the subject of the present work.

In earlier works on this question the rate of the oxidation reaction was determined only for the simplest phenols, with one benzene ring. Meanwhile, in humic substances…

probable, although not universally recognized,^[14)] polynuclear systems. Therefore it was of interest to extend the study to phenols with several benzene rings.

We took the following phenols: α-naphthol, β-naphthol, phenolphthalein, alizarin, and aminoalizarin.

α-naphthol

β-naphthol

phenolphthalein

Alizarin

Aminoalizarin

For comparison, ordinary phenol \(C_6H_5OH\) was also taken, since the previous work on the rate of oxidation of phenols had been carried out under different conditions. Termination of the oxidative process was now achieved not by means of potassium iodide and hydrochloric or sulfuric acid, but by adding an excess of oxalic acid. The point is that phenols, as was found,^[15)] react not only with permanganate but also with iodine. The new method is also not free from shortcomings. In the subsequent titration with permanganate, both the oxalic acid and the remaining phe-

nol. However, we nevertheless decided to apply this method, since a more exact determination would have made the work exceedingly complicated.

The poor solubility of complex phenols in water made it necessary to carry out the oxidation only in an alkaline medium. Experiments with finely ground substances suspended in water did not give good results. Considerable fluctuations in the reaction rate are observed.

The working procedure was as follows. Into an Erlenmeyer flask are poured 10 ml of a decimolar solution of potassium permanganate; to this are added in advance measured 10 ml of an alkaline (KOH concentration = 0.2%) solution of the phenol under investigation (0.005 gram-mol per liter, and for phenolphthalein 0.02 gram-mol). The mixture is shaken vigorously and, after a definite time has elapsed, is poured into another beaker containing 200 ml of water heated to 70°, 25 ml of a decimolar solution of oxalic acid, and 30 ml of H₂SO₄ (1:4). The excess oxalic acid was titrated back with the same permanganate solution.

Table

Oxidation of phenols by KMnO₄ in an alkaline medium.

The table presented gives, for each phenol: the duration of the experiments, the amount of oxygen consumed for oxidation in milligrams, and the same amount in percent. In doing so (since permanganate was taken in excess), not all the active oxygen of the oxidizing agent was taken as 100%, but only that fraction of it which reacted at an appreciable rate, i.e., before the formation of products very resistant to further oxidation. In practice this amount was determined by carrying the experiments to 24 hours. For all the phenols taken, these experiments gave the same result as oxidation for 30 min.

The results obtained show quite clearly that complex phenols are oxidized by permanganate, in general, no less readily than simpler phenols. Naphthols react considerably faster than phenol. In alizarin the rate of oxidation was somewhat reduced. Evidently this must be attributed to the influence of the two carbonyl groups, since salicylaldehyde was oxidized more slowly than phenol. ²) Aminoalizarin is oxidized noticeably more readily than alizarin, which also agrees with earlier data (the amino group strongly loosens the benzene ring). Phenolphthalein was also oxidized very readily (but here, in alkaline medium, the quinonoid group that forms may also react).

The experiments were carried out only in alkaline medium and only with compounds having two or three rings, but from the results of oxidation of simple phenols it is evident that alkali and acid have little effect on the reaction rate for them. There is no reason to think that complex phenols would behave differently. There is likewise no reason to expect that further complication of the ring system to four, five, etc., would have a substantial influence on oxidizability. Therefore the conclusion drawn earlier concerning the special activity, with respect to oxidizing agents, of the phenolic groups of coal should be considered fully justified.

LITERATURE

1. B. V. Tronov.—“Trudy T. G. U.” 94, 3–37 (1938) (review article).
2. B. V. Tronov, A. I. Kravchenko and E. V. Yudenicheva.—Work submitted for publication.
3. B. V. Tronov and M. T. Yalovaya.—“Trudy T. G. U.” 94, (1938).
4. B. V. Tronov, L. N. Dyakonova and M. T. Khodalevich.—Work submitted for publication.
5. W. Francis, R. W. Wheeler. Fuel XI, 356–359 (1932)
6. V. S. Krym.—“Chemistry of Solid Fuel.”
7. W. Fuchs, W. Stengel, F. Bangert.—Br.—Ch. 9, 181 (1928).
8. W. Fuchs, W. Stengel.—Br.—Ch. 10, 303 (1929)
9. W. Fuchs.—Br.—Ch. 11, 332 (1930).
10. W. Fuchs.—Zeit. angew. Chem. 44, 116 (1931).
11. V. S. Krym.—“Chemistry of Solid Fuel,” 2nd ed., p. 140.
12. Ferd. Fischer. Zeit.—angew. Chem 1899, pp. 764–7, 787–790.
13. B. V. Tronov.—“On the mechanism of the oxidation of coal by atmospheric oxygen” (in press).
14. R. B. Randall, M. Benger, C. M. Groocock.—Proc. Roy. Soc. A 165, 432–452 (1938); Chem. Abstr. 1938, No. 14, p. 5373.
15. B. V. Tronov and V. Z. Grigoryeva.—“Iodine numbers of phenols” (submitted for publication.)