B. V. Tronov and I. I. Zharikov.

Rate of Oxidation of Hydroxy Acids of the Fatty Series by Potassium Permanganate.

In the present work, the following ten hydroxy acids and one keto acid were taken for investigation:

$\mathrm{CH_2OH.COOH}$; $\mathrm{CH_3.CHOH.COOH}$; $\mathrm{CH_3.CH_2.CHOH.COOH}$;
$\mathrm{CH_3.CHOH.CH_2.COOH}$; $\mathrm{(CH_3)_2.C(OH).COOH}$; $\mathrm{COOH.CH_2.CHOH.COOH}$;
$\mathrm{COOH.CHOH.CHOH.COOH}$ (d); $\mathrm{COOH.CHOH.CHOH.COOH}$ (d + l);
$\mathrm{COOH.CHOH.CHOH.CHOH.CHOH.COOH}$ (mucic);
$\mathrm{COOH.CH_2.C(OH).CH_2.COOH}$; $\mathrm{CH_3.CO.CH_2.CH_2.COOH}$.

$\phantom{\mathrm{COOH.CH_2.C(OH)}}\vert$
$\phantom{\mathrm{COOH.CH_2.}}\mathrm{COOH}$

Before the experiments, the solid compounds were purified by crystallization; in the liquids, the actual acid content was determined by titration.

The substance under study and the oxidizing agent were taken in equimolecular ratios. The concentration of both was most often 0.03 mole per liter. When the reaction rate was too high (for the most part in acid solution), the concentration was reduced to 0.003 m/l. Some compounds were oxidized at two concentrations. In addition to experiments with free acids, i.e., in an acid medium, the investigation was also carried out in neutral solutions (the acids were neutralized with caustic soda) and in an alkaline medium with the addition of one more molecule of $\mathrm{NaOH}$*). The temperature was 17–19°. To measure the reaction rate, samples of the solutions were titrated with hyposulfite.

The results of the experiments are expressed in Tables 1–45, where the percentages of active oxygen consumed over definite intervals of time are given.

Oxidation in an Acid Medium.

1. Glycolic acid. Conc. 0.003 m/l.

*) Mucic acid was studied only in neutral and alkaline solution, since in the free state it is almost insoluble in water.

2. Lactic acid. Conc. 0.003.

3. α-Oxybutyric acid. Conc. 0.002.

4. β-Oxybutyric acid. Conc. 0.003.

5. Malic acid. Conc. 0.003.

6. d-Tartaric acid. Conc. 0.003.

7. Tartaric acid. Conc. 0.003.

8. Citric acid. Conc. 0.003.

9. Glycolic acid. Conc. 0.03.

10. Lactic acid. Conc. 0.03.

11. α—Hydroxybutyric acid. Conc. 0.03.

12. β—Hydroxybutyric acid. Conc. 0.03.

13. γ—Hydroxybutyric acid. Conc. 0.03.

14. Malic acid. Conc. 0.03.

15. Tartaric acid. Conc. 0.03.

16. Grape acid. Conc. 0.03.

17. Citric acid. Conc. 0.03.

18. Levulinic acid. Conc. 0.03.

Oxidation in neutral solution.

19. Glycolic acid. Conc. 0.03.

20. Lactic acid. Conc. 0.03.

21. α—Oxyisobutyric acid. Conc. 0.03.

21. α-Oxybutyric acid. Conc. 0.03.

22. β-Oxybutyric acid. Conc. 0.03.

23. α-Oxyisobutyric acid. Conc. 0.03.

24. Malic acid. Conc. 0.03.

25. Tartaric acid. Conc. 0.03.

26. Racemic acid. Conc. 0.03.

27. Mucic acid. Conc. 0.03.

28. Mucic acid. Conc. 0.003.

29. Citric acid. Conc. 0.03.

30. Citric acid. Conc. 0.003.

31. Levulinic acid. Conc. 0.03.

Oxidation in alkaline solution.

32. Glycolic acid. Conc. 0.03.

33. Lactic acid. Conc. 0.03.

34. α-Oxybutyric acid. Conc. 0.03.

35. β-Oxybutyric acid. Conc. 0.03.

36. β-Oxybutyric acid. Conc. 0.015.

37. γ-Oxyisobutyric acid. Conc. 0.03.

38. Malic acid. Conc. 0.03.

39. Tartaric acid. Conc. 0.03.

40. Tartaric acid. Conc. 0.03.

41. Mucic acid. Conc. 0.03.

42. Mucic acid. Conc. 0.003.

43. Citric acid. Conc. 0.03.

44. Levulinic acid. Conc. 0.03.

45. Levulinic acid. Conc. 0.003.

The calculation of the rate constants did not give a satisfactory result. Therefore the comparison of the oxidizability of the compounds studied was made according to the time intervals during which an equal percentage of oxidizing agent is consumed*). The values of the mean relative reaction rate calculated in this way are given in Table 46. The rate of oxidation of levulinic acid in neutral medium is taken as unity. Table 47 contains the numbers for the relative oxidizability of various acids under identical conditions; here the rate of oxidation of the simplest hydroxy acid—glycolic acid—is everywhere taken as unity. In Table 48 the oxidizability values of each acid in acidic, alkaline, and neutral media are compared; in all cases the unit is the smallest value. Table 49 gives the ratios of the reaction rates at different concentrations.

46. Table of mean relative reaction rates.

*) In this, only the course of the reaction up to 45% is taken into account, since beyond this for most acids the oxidation becomes very slow, which smooths out the difference.

47. Relative oxidizability under identical conditions

48. Oxidizability of hydroxy acids as a function of the reaction of the medium

49. Influence of the concentration of the reacting components.

CONCLUSIONS

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

1) The course of the oxidation by permanganate of the acids investigated cannot be expressed by the equation of a bimolecular reaction. For the most part, oxidation accelerated as the process proceeded. This is generally the case with permanganate*), but here it is manifested especially clearly. The acceleration is most sharply expressed in experiments in an acid medium and with polybasic acids.

Thus tartaric acid oxidized \(KMnO_4\) in 20 min by 1.77%; in 30 min—8.50%; in 40 min—59.65%. Citric acid: 15 min—2.82%; 30 min—12.32%; 32 min—19.89%; 36 min—47.71%; 38 min—62.66%. It is possible that here the electrolytic dissociation constant also has significance (perhaps, the concentration of hydrogen ions in the solution); the last two acids are precisely the most highly dissociated; the weak acids—\(\beta\)-hydroxybutyric and levulinic—show a more uniform course of reaction.

The dependence of the rate on the concentration of the reacting components is also rather complex, as is evident at least from the large difference in the ratio of rates at concentrations of 0.03 and 0.003. This difference ranged from 2.8 (citric acid in neutral solution) to 53 and 154 (levulinic and mucic acids in alkaline medium).

2) Almost all the hydroxy acids taken by us are oxidized more rapidly in an acid medium. The minimum rate for some (mostly monobasic monohydroxy acids) is observed in reaction in a neutral medium, for others—in alkaline solution. The keto acid—levulinic acid—is oxidized extremely rapidly in the presence of alkali, which may be due to tautomeric rearrangement:

\[ \underset{O}{\mathrm{CH_3.C.CH_2.CH_2COONa}} \;+\;\mathrm{NaOH} \;\rightleftarrows\; \underset{ONa}{\mathrm{CH_3.C{-}CH.CH_2COONa}} \;+\;\mathrm{H_2O} \]

In an acid medium this acid, on the contrary, reacts very slowly (even more slowly than hydroxyisobutyric acid).

3) Of the hydroxy acids, \(\alpha\)-hydroxyisobutyric acid (with a tertiary radical) is oxidized slowest of all; \(\beta\)-hydroxybutyric acid is considerably faster.

For the others the difference is small: in acid solution the ratios range from 1 to 5.6; in neutral solution—from 1 to 3.2; in alkaline solution—from \(-0.14\) to 2.4 (except also for mucic acid).

In alkaline solution, polybasic acids were in general oxidized (except for mucic acid) more slowly than monobasic acids.

4) Optical isomerism has no effect on the rate of oxidation (tartaric and racemic acids).

5) Comparing, on the basis of the few available examples, polyhydric alcohol acids with polyhydric alcohols**), we see that the addition of two carboxyl groups (to the carbon chain of the alcohol—ethylene glycol and tartaric acid) does not have a great effect on oxidizability; replacing two alcohol groups by carboxyl groups (erythritol and tartaric acid) greatly slows the reaction with \(KMnO_4\).

*) Catalytic influence of the oxidation products.

**) B. V. Tronov and M. P. Sinyavin, “Rate of oxidation of polyhydric alcohols by potassium permanganate.”