B. V. Tronov and A. A. Grigor'eva.

The Rate of Oxidation of Acids of the Benzene Series by Potassium Permanganate in an Alkaline Medium

From the Laboratory of Organic Chemistry of the Siberian Technological Institute and Tomsk University.

The following acids were taken:
1) Carbon acids with a carboxyl group in the benzene nucleus:

1) benzoic acid.
2) chlorobenzoic acid.
3) bromobenzoic acid.

4) nitrobenzoic acid.
5) nitrobenzoic acid.
6) nitrobenzoic acid.

7) methylbenzoic acid.
8) methylbenzoic acid.
9) methylbenzoic acid.

10) hydroxybenzoic acid.
11) hydroxybenzoic acid.
12) hydroxybenzoic acid.

13) aminobenzoic acid.
14) aminobenzoic acid.
15) aminobenzoic acid.

16) phthalic acid.
17) phthalic acid.
18) phthalic acid.

19) methoxybenzoic acid.
20) trihydroxybenzoic acid.

21) naphthoic acid.
22) naphthoic acid.

II. Acids with a carboxyl in the side chain:
23) $\mathrm{C_6H_5.CH_2.COOH}$; 24) $\mathrm{C_6H_5.CH_2.CH_2.COOH}$; 25) $\mathrm{C_6H_5.CHOH.COOH}$; 26) $(\mathrm{C_6H_5})_2\mathrm{COH.COOH}$; 27) $\mathrm{C_6H_5.CH{=}CH.COOH}$;
28) $o$-$\mathrm{NO_2.C_6H_4.CH{=}CH.COOH}$; 29) $m$-$\mathrm{NO_2.C_6H_4.CH{=}CH.COOH}$;
30) $p$-$\mathrm{NO_2.C_6H_4.CH{=}CH.COOH}$.

III. Acid with a furan ring: 31)

$$ \text{furan ring–}\mathrm{COOH} $$

IV. Sulfonic acids.

32) $\mathrm{C_6H_5.SO_2OH}$; 33) naphthalenesulfonic acid, $\mathrm{SO_2OH}$ in the nucleus; 34) naphthalenesulfonic acid, $\mathrm{SO_2H}$ in the nucleus; 35) aminobenzenesulfonic acid, $\mathrm{SO_2OH}$ and $\mathrm{NH_2}$ in the nucleus.

The acids (Kahlbaum preparations of the highest possible purity; not all were recrystallized, since there was too little of some of them) were taken in the form of sodium salts, to which an equimolecular amount of $\mathrm{NaOH}$ was further added. The oxidation was carried out for all the compounds studied in a $0.1\,n$ solution (with respect to $\mathrm{NaOH}$; titre 0.00325), and for some (the more difficultly oxidized) also in a $1\,n$ solution (titre $\mathrm{NaOH}=0.037024$). To the mixture there was added a $5\frac12\%$ solution of $\mathrm{KMnO_4}$ (titre 0.055085) in such an amount that 10 mol. of acid corresponded to 1 mol. of oxidizing agent. The reaction was conducted in aqueous solution at a temperature of $18^\circ$—$20^\circ$.

The course of the process was followed by titrating, at definite intervals of time, the oxidizing agent remaining.

For the majority of the acids two experiments each were set up. The reaction products were not investigated. According to the literature data, which are available by no means for all the substances we used, in aromatic acids with a carboxyl in the side chain and in toluic acids the action of oxidizing agents is directed to the side chains. In acids with $\mathrm{COOH}$ or $\mathrm{SO_2OH}$ in the nucleus, destruction of the benzene ring takes place, and sometimes coupling of the benzene nuclei of two molecules at the expense of removal of hydrogens.¹) In sulfanilic acid the $\mathrm{NH_2}$ group is oxidized with formation of azobenzenedisulfonic acid²).

The results of the experiments are expressed in the following tables, where the amount of $\mathrm{KMnO_4}$ that reacted in a definite time is indicated in percent. In the upper line the time is given everywhere, in the lower lines—the % of oxidizing agent consumed.

Experiments with $0.1\,n$ solution:

1. $\mathrm{C_6H_5.COOH}$.

¹) Thus $\mathrm{C_6H_5.COOH}$ with $\mathrm{MnO_2}$ and $\mathrm{H_2SO_4}$ gives $\mathrm{CO_2}$, $\mathrm{H.COOH}$, phthalic and terephthalic acids [Carius, Lieb. Ann. 148, 72 (1868)]. The latter can be obtained only after cleavage of two benzene rings.

²) Laar. J. pr. Ch. [2] 20, 264 (1879).

1. o—Cl · C₆H₄ · COOH.

1. p—Br · C₆H₄ · COOH.

1. o—NO₂ · C₆H₄ · COOH.

1. m—NO₂ · C₆H₄ · COOH.

1. p—NO₂ · C₆H₄ · COOH.

1. m o—CH₃ · C₆H₄ · COOH.

1. m—CH₃ . C₆H₄ . COOH.

1. p—CH₃ . C₆H₄ . COOH.

1. o—OH . C₆H₄ . COOH.

1. m—OH . C₆H₄ . COOH.

1. p—OH . C₆H₄ . COOH.

1. o—NH₂ . C₆H₄ . COOH.

1. m—NH₂ · C₆H₄ · COOH.

1. p—NH₂ · C₆H₄ · COOH.

1. o—C₆H₄ · (COOH)₂.

1. m—C₆H₄ (COOH)₂,

1. p—C₆H₄ (COOH)₂.

1. p—H₃C.O.C₆H₄.COOH.

1. (OH)₃ C₆H₂.COOH.

It was not possible to measure the rate of oxidation of gallic acid, since in alkaline solution it reacts rapidly even with atmospheric oxygen.

1. α—C₁₀H₇.COOH.

22. $\beta$—$\mathrm{C_{10}H_7\cdot COOH}$.

23. $\mathrm{C_6H_5\cdot CH_2\cdot COOH}$.

24. $\mathrm{C_6H_5\cdot CH_2CH_2\cdot COOH}$.

25. $\mathrm{C_6H_5\cdot CHOH\cdot COOH}$.

26. $(\mathrm{C_6H_5})_2\ \mathrm{COH\cdot COOH}$.

1. C₆H₅ · CH=CH · COOH.

1. o—NO₂ · C₆H₄ · CH=CH · COOH.

1. m—NO₂ · C₆H₄ · CH=CH · COOH.

1. p—NO₂ · C₆H₄ CH=CH · COOH.

1. Pyromucic acid.

1. \( \mathrm{C_6H_5.SO_2OH.} \)

1. \(\alpha\)—\( \mathrm{C_{10}H_7.SO_2OH.} \)

1. \(\beta\)—\( \mathrm{C_{10}H_7.SO_2OH.} \)

1. p—\( \mathrm{NH_2.C_6H_4.SO_2OH.} \)

Experiments in a 1 n-solution.

1. \( \mathrm{C_6H_5.COOH.} \)

37. o—Cl.C₆H₄.COOH.

38. p—Br. C₆H₄.COOH.

39. o—NO₂.C₆H₄.COOH.

40. p—NO₂.C₆H₄.COOH.

41. p—CH₃O · C₆H₄ · COOH.

42. (C₆H₅)₂COH · COOH.

The calculation of the rate constants, as was to be expected, gave favorable results only in a few cases and only at the beginning of the process, while Mn had not been reduced approximately to the hexavalent state. We give the constants for four compounds which interested us most and therefore were better studied: C₆H₅ · COOH, O—NO₂ · C₆H₄ · COOH, m—NO₂ · C₆H₄ · COOH, and p—NO₂ · C₆H₄ · COOH. The calculation was carried out according to the equation for a monomolecular reaction, since one of the reacting components (the oxidized substance) was taken in large excess (10 : 1). In the formula:

\[ K=\frac{2.303}{t}\cdot \lg \frac{a}{a-x} \]

\(t\) is expressed in hours, \(x\) is the amount of oxidizer that has reacted, in percent (thus \(a = 100\)).

Table 43. Experiment with C₆H₅ · COOH in 0.1 n solution:

Average: \(K = 1.54 \cdot 10^{-4}\)

Table 44. Experiment with o—NO₂ · C₆H₄ · COOH in 0.1 n solution:

Average: \(K = 3{,}09 \cdot 10^{-4}\)

Table 45. Experiment with \(m-\mathrm{NO_2}\cdot \mathrm{C_6H_4}\cdot \mathrm{COOH}\) in 0.1 n solution:

Average: \(K = 3{,}41 \cdot 10^{-4}\)

Table 46. Experiment with \(p-\mathrm{NO_2}\cdot \mathrm{C_6H_4}\cdot \mathrm{COOH}\) in 0.1 n solution.

Average: \(K = 14{,}2 \cdot 10^{-4}\)

Table 47. Experiment with \(\mathrm{C_6H_5}\cdot \mathrm{COOH}\) in 1 n solution.

Average: \(K = 12{,}3 \cdot 10^{-4}\) \(K_{\mathrm{n}} : K_{0,1\mathrm{n}} = 8{,}3\)

Table 48. Experiment with \(o-\mathrm{NO_2}\cdot \mathrm{C_6H_4}\cdot \mathrm{COOH}\) in 1 n solution.

Average: \(K = 20{,}4 \cdot 10^{-4}\); \(K_{\mathrm{n}} : K_{0,1\mathrm{n}} = 6{,}3\).

Comparison of the oxidizability of all the acids studied had to be carried out according to the time during which an equal percentage of active oxygen was consumed. In the present work, where for each substance there were many data, this method (using the curves: time—percentage) gives fairly accurate results, almost coinciding with what is derived from the rate constants.

Table 49. Relative oxidizability of acids of the benzene series,

Conclusions

The results of our experiments are still insufficient for establishing definite quantitative relationships. For the present, the following conclusions may be drawn from them:

1) With the exception of one case (ortho-chlorobenzoic acid in a more concentrated solution), all substituents accelerate the oxidation of benzoic acid. This applies both to ortho-para-directing atoms and groups (\(\mathrm{OH}\), \(\mathrm{NH_2}\), \(\mathrm{OCH_3}\), and, less definitely, haloids) and to meta-directing ones. Even the nitro group,¹ which strongly retards the reaction of further nitration, facilitates the action of \(\mathrm{KMnO_4}\) in the presence of alkali on \(\mathrm{C_6H_5COOH}\), and in all positions relative to the carboxyl.

2) With substituents \(\mathrm{Cl}\), \(\mathrm{Br}\), \(\mathrm{NO_2}\), \(\mathrm{COOH}\), \(\mathrm{OCH_3}\), an acceleration only from 2 to 100 times is observed (if one speaks of the more thoroughly studied reaction in \(0.1n\) solution). \(\mathrm{OH}\) and \(\mathrm{NH_2}\) accelerate the reaction hundreds of thousands of times. The great difference between anisic acid and \(p\)-oxybenzoic acid (the former is oxidized almost 500 times more slowly than the latter) makes it very probable that the hydroxyl or amino hydrogen participates in the reaction; either this \(\mathrm{H}\) itself is removed first, or the benzene ring is isomerized into the quinoid form.

3) As to the influence of the position of the substituent, no rule can yet be stated. In oxy- and aminobenzoic acids, the meta isomer apparently oxidizes more readily (but here the experiments are not accurate). \(\mathrm{NO_2}\) noticeably accelerates oxidation more when it is in the para position to the carboxyl. Finally, among the dicarboxylic acids, the para acid, on the contrary, is oxidized almost 10 times more slowly than its isomers.

4) Benzenesulfonic acids are oxidized tens of thousands of times faster than benzoic acid. Substituents that accelerate the reaction smooth out the difference (naphthoic and naphthalenesulfonic acids, \(p\)-aminobenzoic and sulfanilic).

5) Saturated side chains are oxidized much faster than the benzene ring. The position of the carboxyl—in the nucleus (toluic acids) or in the side chain (phenylacetic and phenylpropionic)—has little effect on oxidizability. Unsaturated chains (cinnamic acid and its substituted derivatives) are oxidized thousands of times more readily still.

6) The furan ring (in pyromucic acid), in resistance to oxidizing agents, is many times inferior to the benzene ring, but proves markedly stronger than an unsaturated open chain.

¹ In view of the unexpected nature of the result obtained with nitrobenzoic acids, the experiments were repeated with substances recrystallized once more, which produced no significant changes. The data given in the tables refer to the purest preparations.