RATE OF OXIDATION OF ALCOHOLS OF THE SATURATED SERIES BY POTASSIUM PERMANGANATE IN AN ACID MEDIUM

B. V. TRONOV and V. I. LUTKOVA

In several previous works on the oxidation of alcohols, the rate of action of potassium permanganate on these compounds was determined in neutral aqueous solution [1] and in the presence of alkali [2], and also the rate of reactions with chromic anhydride in aqueous solution [3] and with dilute nitric acid [3]. At first monohydric alcohols were studied, chiefly those of the saturated series; later, polyhydric alcohols up to and including hexitols [4] were studied.

The most interesting result from the theoretical standpoint in the above-mentioned works was the establishment of the fact that neutral and alkaline permanganate solutions act more readily on secondary alcohols than on primary ones (this was also to be expected from all the literature data), whereas $\mathrm{CrO_3}$ reacts more rapidly with primary alcohols. This difference in the behavior of these oxidizing agents was explained [5] by a difference in the reaction mechanism. It was suggested that with $\mathrm{KMnO_4}$ the removal of hydrogen atoms from the carbon of the alcohol group occurs at once, whereas $\mathrm{CrO_3}$ first forms an ester. From this point of view it was of interest to see how permanganate acts on alcohols in an acid medium, for example, in the presence of sulfuric acid. Here permanganic acid should be liberated; it may even give manganese anhydride. Consequently, under these conditions esterification is also quite possible, and it proceeds more readily with a primary alcohol than with a secondary one.

In the present work we took 9 saturated monohydric alcohols with different positions of the hydroxyl in the molecule, namely:

1. $\mathrm{CH_3OH}$ . . . . . . . . . . . . . . methyl.
2. $\mathrm{CH_3—CH_2OH}$ . . . . . . . . . . . ethyl.
3. $\mathrm{CH_3—CH_2—CH_2OH}$ . . . . . primary propyl.
4. $\mathrm{CH_3—CHOH—CH_3}$ . . . . . isopropyl secondary.
5. $\mathrm{CH_3—CH_2CH_2—CH_2OH}$ . . . . primary butyl.
6. $\mathrm{CH_3—CH_2—CHOH—CH_3}$ . . . . secondary butyl.
7. $\mathrm{(CH_3)_3COH}$ . . . . . . . . . . tertiary butyl.
8. $\mathrm{(CH_3)_2CH—CH_2—CH_2OH}$ . . . primary isoamyl.
9. $\mathrm{(CH_3)_2COH—CH_2—CH_3}$ . . . tertiary isoamyl.

Of the polyhydric alcohols we subjected to oxidation:

1. $\mathrm{CH_2OH—CH_2OH}$ . . . . . . . . . . ethylene glycol.
2. $\mathrm{CH_2OH—CHOH—CH_2OH}$ . . . . . . . glycerol.
3. $\mathrm{CH_2OH(CHOH)_4CH_2OH}$ . . . . . . . sorbitol.

The rate of oxidation of these compounds by potassium permanganate with addition of \(H_2SO_4\) was determined; the concentration of all three reacting components was \(0.03\) gram-mole per liter. The oxidation was carried out at \(18^\circ\).

Methyl alcohol, glycol, and glycerol were also oxidized in a neutral solution of the same concentration. Titration of the remaining active oxygen, carried out at definite time intervals, gave the following results:

Table 1

Relative rate of oxidation of alcohols in neutral solution.

Comparison of the magnitudes of the reaction rate was made according to the time intervals during which the same percentage of active oxygen was consumed. In the table the rate of oxidation of methyl alcohol in an acid medium is taken as unity. The values of the relative reaction rate are given for time intervals corresponding to the consumption of 10, 20, 30, and 40% of active oxygen, and the average values for the whole interval from 0 to 40%.

Table 2

Relative rate of reaction

Conclusions

1. Comparing the magnitudes of the relative reaction rate obtained for primary, secondary, and tertiary alcohols, we see that with permanganate in an acid medium propyl alcohol reacts 11 times faster than isopropyl alcohol, and butyl alcohol 17 times faster than the corresponding isobutyl alcohol. If one takes into account the fact that during oxidation two hydrogens are removed from a primary alcohol, whereas only one is removed from a secondary alcohol, and reduces the given figures by half to 5.5 and 8, the greater activity under these conditions of the primary alcohol group nevertheless remains undoubted. (See Table 3.)

Thus, the experiment with permanganate in an acid medium gave very good confirmation of the proposed theory of oxidation. Tertiary alcohols were oxidized here also more slowly than all the others, although the difference proved to be smaller than when the reaction was carried out in alkaline [2] or neutral solutions.

Table 3

Dependence of the oxidation rate on the position of the hydroxyl in the molecule

1. The figures in Table 4 show that, for primary alcohols, the rate of oxidation increases somewhat with increasing chain length. The same was observed in earlier work.

Table 4

Influence of chain length

For secondary alcohols a slight decrease in the reaction rate is noted, for example, for secondary butyl alcohol as compared with isopropyl alcohol, but the experimental material is too insufficient for this to be regarded as a general rule.

1. Table 5 gives the ratio of the oxidation rates of glycol and ethyl alcohol, glycerol and propyl alcohol under different conditions.

Table 5

Ratio of reaction rates to the medium

As is evident from the figures given, for an acidic medium we have quite different ratios than for a neutral and an alkaline one. Obviously, this is explained by the same difference in the mechanism of the process. The more hydroxyls there are in the molecule, the more loosened are the C and H bonds, while the mobility of the hydroxyl hydrogens in polyhydric alcohols may be reduced. This is indicated, for example, by the results of determining the electromotive force of the reaction with sodium [5]. The hexahydric alcohol sorbitol is oxidized very rapidly. Probably, in its molecule the C and H bonds are so loosened that they can easily be broken under the action of permanganate under all conditions.

1. A comparison of the oxidizability of the same alcohol in neutral, alkaline, and acidic solutions leads to very interesting results. In Table 6, for each alcohol, the rate of oxidation by permanganate in neutral solution is taken as unity. We see that all primary monohydric alcohols (except for the anomalous methyl alcohol, which, strictly speaking, cannot be regarded as primary) react with permanganate in an acidic medium 3–4 times faster than in an alkaline one, and 40–70 times faster than in a neutral one.

Secondary alcohols in the presence of acid react about 5 times more slowly than in alkaline solution, and only slightly (2.5 times) faster than in neutral solution. Polyhydric alcohols, even the diprimary ethylene glycol, in the character of their oxidizability are closer to secondary alcohols.

Table 6.

Relative reaction rate in the medium

In general, from the magnitudes and ratios of the rates of reaction with permanganate under different conditions, we can sometimes judge the structure of an alcohol quite well. For monohydric alcohols, for example, it is possible to say quite definitely whether the hydroxyl is at a primary, secondary, or tertiary position. The possibility is not excluded of determining in this way also the form of the carbon skeleton (isoamyl alcohol deviates somewhat from alcohols with a straight chain), but this question has so far been touched upon too little by our investigations.

LITERATURE

1. B. V. Tronov, A. A. Lukanin, and I. I. Pavlinov.—The rate of oxidation of alcohols by potassium permanganate. Zh. RKhO, 59, pp. 1117–1193, 1927.

2. B. V. Tronov and A. I. Korshun.—The rate of oxidation of monohydric alcohols of the limiting series by potassium permanganate in an alkaline medium. Proceedings of TSU, 87, No. 2, pp. 100–104, 1935.

3. B. V. Tronov and A. A. Lukanin.—The rate of oxidation of alcohols and ethers by potassium permanganate and chromic anhydride under various conditions. Zh. RKhO, 59, pp. 1157–1172, 1927.

4. B. V. Tronov, V. F. Udolova, and M. I. Chizhova.—The rate of oxidation of alcohols by $CeO_3$ and $HNO_3$ in aqueous solutions. Zh. RKhO, 59, pp. 1149–1156, 1927.

5. B. V. Tronov and L. P. Kulev.—Determination of the activity of hydrogen in organic compounds from the electromotive force of the reaction with sodium. ZhOKh, 4, pp. 197–202, 1934.