RATE OF OXIDATION OF AROMATIC AMINES AND COMPOUNDS OF THE PYRIDINE GROUP BY POTASSIUM PERMANGANATE

B. V. Tronov, L. N. Dyakonova, and G. N. Khodalevich

The technical mixture obtained from coal tar, known as “pyridine bases,” in fact also contains compounds of the pyridine group and amines of the benzene series. Many methods have been proposed for the isolation and determination of the individual components of this mixture, but all of them prove to be either rather laborious or insufficiently accurate. With a view to using for this purpose the difference in the oxidizability of various compounds, we decided to determine the rate of oxidation of amines of both groups by potassium permanganate. The experiments were generally carried out under the same conditions as in previous studies on oxidation. The reaction was conducted either simply in aqueous solution or in the presence of additional alkali \((\mathrm{NaOH})\) or sulfuric acid, the acid being taken in an amount equimolecular to the amine and in half that amount. Thus the medium created was: 1) weakly alkaline, 2) strongly alkaline, 3) acidic, 4) almost neutral (sometimes weakly acidic). The concentration of the principal reacting components—the amine and the oxidizing agent—was \(0.03\) mol per liter. Oxidation was carried out at a temperature of \(17\text{–}22^\circ\). At definite intervals of time, the percentage of remaining active oxygen was determined by titration.

The results of the experiments are given in Tables (Nos. 1–48), where the time intervals and the amounts of active oxygen that had reacted are given in percent.

Oxidation in aqueous solution.

1. \(\mathrm{C_5H_5N}\), conc. \(0.03\) mol/l.

2. \(\alpha\text{-}\mathrm{C_5H_4NCH_3}\), conc. \(0.03\) mol/l.

3. \(\beta\)—\(C_5H_4NCH_3\), conc. \(0,03\) mol/L.

4 sim. \(C_5H_2N(CH_3)_3\), conc. \(0,03\) mol/L.

5. \(C_5H_5N\), conc. \(0,03\) mol/L.

6. \(C_6H_5NH_2\), conc. \(0,03\) mol/L.

7. \(C_6H_5NHCH_3\), conc. \(0,03\) mol/L.

8. \(C_6H_5N(CH_3)_2\), conc. \(0,03\) mol/L.

9. \(C_6H_5N(C_2H_5)_2\), conc. \(0,03\) mol/L.

10. \(O\)—\(C_6H_4CH_3NH_2\), conc. \(0,03\) mol/L.

11. \(M\)—\(C_6H_4CH_3NH_2\), conc. \(0,03\) mol/L.

Oxidation in an Acid Medium (in the Presence of 1 M \( \mathrm{H_2SO_4} \))

12. \( \mathrm{C_5H_5N} \), conc. 0.03 m/l.

13. \(\alpha\)-\( \mathrm{C_5H_4NCH_3} \), conc. 0.03 m/l.

14. \(\beta\)-\( \mathrm{C_5H_4NCH_3} \), conc. 0.03 m/l.

15. (sym.) \( \mathrm{C_5H_2N(CH_3)_3} \), conc. 0.03 m/l.

16. \( \mathrm{C_9H_7N} \), conc. 0.03 m/l.

17. \( \mathrm{C_6H_5NH_2} \), conc. 0.03 m/l.

18. \( \mathrm{C_6H_5NHCH_3} \), conc. 0.03 m/l.

19. \( \mathrm{C_6H_5N(CH_3)_2} \), conc. 0.03 m/l.

20. \( \mathrm{C_6H_5N(C_2H_5)_2} \), conc. 0.03 m/l.

21. O-\( \mathrm{C_6H_4(CH_3)NH_2} \), conc. 0.03 m/l.

22. M-\( \mathrm{C_6H_4(CH_3)NH_2} \), conc. 0.03 m/l.

Oxidation in a neutral medium ($1/2$ M. $H_2SO_4$)

23. $C_5H_5N$, conc. $0,03$ mol/l.

24. $\alpha$—$C_5H_4NCH_3$, conc. $0,03$ mol/l.

25. $\beta$—$C_5H_4NCH_3$, conc. $0,03$ mol/l.

26. Symm. $C_6H_2N(CH_3)_3$, conc. $0,03$ mol/l.

27. $C_9H_9N$, conc. $0,03$ mol/l.

28. $C_6H_5NH^2$, conc. $0,03$ mol/l.

29. $C_6H_5NH(CH_3)$, conc. $0,03$ mol/l.

30. $C_6H_5N(CH_3)_2$, conc. $0,03$ mol/l.

31. $C_6H_5N(C_2H_5)_2$, conc. $0,03$ mol/l.

32. o—$C_6H_4(CH_3)NH_2$, conc. $0,03$ mol/l.

33. m—$C_6H_4(CH_3)NH_2$, conc. $0,03$ mol/l.

Oxidation in alkaline medium (in the presence of NaOH—1 M).

34. \(C_5H_5N\), conc. \(0.03\) mol/l.

35. \(\alpha\text{-}C_5H_4NCH_3\), conc. \(0.03\) mol/l.

36. \(\beta\text{-}C_5H_4NCH_3\), conc. \(0.03\) mol/l.

37. Sym. \(C_5H_2N(CH_3)_3\), conc. \(0.03\) mol/l.

38. \(C_9H_7N\), conc. \(0.03\) mol/l.

39. \(C_6H_5NH_2\), conc. \(0.03\) mol/l.

40. \(C_6H_5NHCH_3\), conc. \(0.03\) mol/l.

41. \(C_6H_5N(CH_3)_2\), conc. \(0.03\) mol/l.

42. \(C_6H_5N(C_2H_5)_2\), conc. \(0.03\) mol/l.

43. o—C₆H₄(CH₃)NH₂, conc. 0.03 mol/l.

44. m—C₆H₄(CH₃)NH₂, conc. 0.03 mol/l.

Since the calculation of the rate constants did not give good results, the oxidizability of the compounds taken had to be compared according to the time intervals during which the same percentage of active oxygen was consumed. In Table 45, the rate of oxidation of permanganate by aniline in a complete solution without the addition of acids or alkalis is taken as unity. In calculating the mean values of the relative reaction rate, for the majority of experiments the interval from 0–50% was taken; in some cases only up to 30% or 40%.

Table 45.

CONCLUSIONS

COMPARISON OF AROMATIC AMINES WITH COMPOUNDS OF THE PYRIDINE GROUP

Table 46.

a) Comparison of o-toluidine with α-picoline:

b) Comparison of m-toluidine with β-picoline

c) Comparison of aniline with pyridine

The numbers in Table 46 show the ratios of the oxidation rates of amines of the benzene series to the oxidation rates of compounds of the pyridine group under the same conditions. Aniline is compared with pyridine, o-toluidine with α-picoline, and m-toluidine with β-picoline. From the numbers in the table it is evident that these two groups of amines differ sharply from each other in oxidizability in all media, especially in strongly or weakly alkaline solution. The difference is particularly great for aniline itself compared with pyridine; side chains on the benzene or pyridine ring somewhat smooth it out. Quinoline occupies an intermediate position.

2. EFFECT OF RADICALS

Table 47

a) Oxidizability of amines of the benzene series relative to aniline (in each medium)

b) Oxidizability of compounds of the pyridine group relative to pyridine (in each medium)

In Table 47 the methyl and ethyl derivatives of aniline are compared with aniline, and methylpyridines with pyridine. We see that methyl in all positions—both on nitrogen and on carbon—almost always accelerates the oxidation of aniline. It is interesting that even dimethylaniline is oxidized somewhat more readily than aniline. This shows (as, incidentally, do many other reactions of dimethylaniline) that, although in the oxidation of aniline the hydrogen atoms at nitrogen enter first of all:

\[ \mathrm{C_6H_5NH_2 + KMnO_4 \rightarrow C_6H_5N = O \rightarrow C_6H_5NO_2} \]

\[ \mathrm{C_6H_5NH_2 + CrO_3 \rightarrow C_6H_5N \rightarrow C_6H_5N = C_6H_4 = NH.} \]

Nevertheless the entire benzene ring, under the influence of the adjacent nitrogen, is also greatly loosened (in an acidic medium the acceleration of reactions is smaller, and sometimes even retardation is observed—as with pyridine).

This, in any case, applies to trivalent nitrogen. In neutral and especially in acidic medium nitrogen is pentavalent; dimethylaniline is oxidized faster than aniline even under these conditions, but the loosening influence of the pentavalent nitrogen atom cannot yet be considered proven, since hydrogen appears at the nitrogen. For a final solution of the question it is necessary to study tetrasubstituted phenylammonium salts, for example:

\[ \mathrm{C_6H_5N(CH_3)_3ONO_2} \]

and the like.

The introduction of two ethyls at nitrogen somewhat retards oxidation. In compounds of the pyridine group, methyl sharply increases oxidizability in alkaline medium. With three methyl groups the reaction rate increases still more (on the average by $31 \tfrac{1}{2}$ times) for alkaline solutions. In the form of salts, pyridine and its homologues almost equal one another. All this is readily explained by the fact that simply in an aqueous medium or in the presence of alkali the pyridine ring is very stable and the side chains are oxidized, whereas the addition of acid sharply decreases the stability of the ring.

In Table 48, for all amines the rate of oxidation in aqueous solution without the addition of acids or alkalis, i.e. in the weakly alkaline medium created by the amine itself, is taken as unity. From the table it is seen that, among the compounds we have taken, pyridine gives the greatest difference in oxidation rate depending on the reaction of the medium. The addition of acid accelerates its oxidation by 54–69 times. In the picolines a difference of 7–11 times is observed. In amines of the benzene series, the addition of $\mathrm{NaOH}$ or $\mathrm{H_2SO_4}$ has still less effect on the reaction rate; the smallest difference (less than twofold) was noted for aniline. In this respect the aromatic amines studied by us differ greatly from the amines of the fatty series, which in al—

in alkaline medium are oxidized many times (sometimes millions of times or more) faster than in the form of salts. These regularities may be used to determine the structure of amines. In any case, if we have some amine in pure form, then from the rate of its oxidation by potassium permanganate under various conditions it is possible to decide with sufficient certainty whether the given compound belongs to the amines of the fatty series, to aromatic amines with nitrogen attached to the benzene ring, or to pyridine bases.

3. EFFECT OF THE MEDIUM.

Comparison of oxidizability in all media relative to an aqueous solution of the same amine.

Table 48.