B. V. Tronov, P. P. Popov, B. F. Kolotovkina
and P. Kh. Stalash.

Rate of Oxidation of Aliphatic and Aromatic Hydrocarbons and Halogenated Hydrocarbons by Acetic–Chromic Anhydride

In one of our previous works¹, experiments on the oxidation of certain hydrocarbons by a solution of chromic anhydride were briefly described. This solution, in which $\mathrm{CrO_3}$ is present in the form of a mixed anhydride²

\[ \begin{array}{c} \mathrm{O}\\ \vert\\[-2mm] \mathrm{Cr}-\mathrm{O}-\mathrm{C}(=\mathrm{O})-\mathrm{CH_3}\\ \vert\\[-2mm] \mathrm{O}-\mathrm{C}-\mathrm{CH_3}\\ \vert\\[-2mm] \mathrm{O} \end{array} \]

is a very good means for studying the oxidizability of a number of organic compounds, since many hydrocarbons dissolve in it sufficiently well.

The present work had as its purpose to carry out the investigation more accurately and on a larger number of substances. Since the previously used 20% and 5% solutions of $\mathrm{CrO_3}$ proved to be too active, we have now, in the greater part of the experiments, reduced the concentration to 1%.

The reacting compounds were taken in equimolecular quantities. The temperature was $18$—$20^\circ$ or $0$—$3^\circ$.

The rate of the reaction was determined by titrating the unreacted oxidizing agent after definite intervals of time. The products were not studied. The substances studied were in most cases dissolved completely. Only saturated and alicyclic hydrocarbons dissolved only partially, so that the figures obtained for them are understated. In the calculations it was necessary to take into account the oxidation of the acetic anhydride itself. If mixed carelessly, it reacts with $\mathrm{CrO_3}$ so violently that spontaneous ignition sometimes occurs. A solution prepared with cooling can be stored safely, but nevertheless it changes at a very appreciable rate.

We followed the reaction in a saturated solution (48.18%—CrO\(_3\) and 51.82% acetic anhydride), at 5% CrO\(_3\), and at 1%

Table I

Oxidation of CrO\(_3\) in acetic anhydride.

In all, we studied the oxidation of 25 compounds; among them were 19 hydrocarbons, 5 halogenated hydrocarbons, and one heterocyclic compound—thiophene.

The results of the experiments are expressed in Table II. (See p. 3.) The comparison of the oxidizability of the compounds studied was made, as in several previous works, according to the intervals of time during which an identical % of active oxygen is consumed.

The figures obtained, taken after 5%, are presented in Table III (see p. 4). The oxidizability of toluene is taken here as unity. The smallest values were found for benzene, but in its case the measurements could not be sufficiently accurate, since oxidation of the solvent has too strong an effect.

CONCLUSIONS.

1) Comparison of the oxidizability of saturated hydrocarbons with different chain lengths.

Table IV

Saturated hydrocarbons in general oxidize the more rapidly, the longer the chain. The presence of a tertiary carbon atom increases oxidizability (which is also confirmed by the examples of methylcyclohexane and cyclohexane).

However, in this class of hydrocarbons the accuracy of comparison is very low, since some of them were taken as obtained from petroleum and, obviously, contained more readily oxidized impurities. Na-

SUMMARY TABLE II

Table III

Comparative rates of oxidation of various compounds

for example, petroleum hexane gave a considerably higher reaction rate than synthetic hexane.

2) Saturated, alicyclic, and benzene hydrocarbons.

Table V

Comparison of hexane and cyclohexane shows that closure of the ring noticeably increases the oxidizability of the saturated chain. This cannot be explained, moreover, by an increase in the number of the more active secondary links: in hexane there are four \(CH_2\) groups, in cyclohexane 6, while the latter is oxidized 10 times faster (if the beginning of the reaction is taken).

Benzene is, with respect to the oxidizing agent, considerably more stable and thus behaves quite differently than in certain other reactions (for example, nitration).

The difficult oxidizability of benzene makes it understandable, among other things, why not all volatile hydrocarbons are suitable in technology for internal-combustion engines. It is known that benzene has too high a flash point and therefore in pure form cannot replace gasoline. The extremely difficult oxidizability of benzene is possibly also connected with its strong action on the human organism\(^{1}\). Benzene is 4—10 times more poisonous than gasoline, and is also more poisonous than its nearest homologues—toluene and xylene. It chiefly affects the nervous system. With the low chemical activity of benzene, one may think that it acts as a solvent for fats and lipoids and differs from other hydrocarbons in that, owing to its difficult oxidizability, it is carried by the blood almost unchanged and then is probably extracted from it by nervous tissue.

(It may also be of significance, perhaps, that benzene is considerably more soluble in water than saturated hydrocarbons and, one must suppose, more soluble than toluene and xylene\(^{2}\), and consequently can enter the blood in larger quantity.)

3) Homologues of benzene.

An interesting result is obtained by comparing benzene hydrocarbons with one, two, and three \(CH_3\) groups, if the oxidizability is calculated per one methyl.

Table VI

\(^{1}\) Technical Encyclopedia, vol. II, pp. 394—6.
\(^{2}\) Jaeger. C. 1933, III, 1154

From the table it is evident that the accumulation of methyl groups almost always increases the oxidizability of hydrocarbons, but this increase is not proportional to the number of methyl groups. A comparison of the results of experiments with toluene, xylene, and mesitylene shows that methyls in the meta-position retard one another’s oxidation.

As for comparison with benzene itself, it is clear that its homologues are oxidized many times more rapidly. They are also oxidized more readily than aliphatic hydrocarbons, i.e., the benzene ring exerts a definite loosening effect on the side chain.

Table VII indicates the significance of secondary and tertiary linkages in benzene hydrocarbons.

Table VII

The oxidizability here is calculated per one hydrogen atom of the linkage that reacts first. The figures show that the hydrogen of a secondary linkage is split off 2–3 times, and that of a tertiary linkage 5–7 times, more rapidly than in a \(\mathrm{CH_3}\) group.

4) Mutual influence of benzene rings.

Table VIII

With two directly connected benzene rings, the stability toward the oxidizing agent employed (as also in other reactions) decreases, and in naphthalene it decreases somewhat more strongly than in diphenyl.

5) Influence of the halide.

Table IX

The introduction of chlorine into the molecule increases oxidizability: in the case of benzene, approximately twofold; in the case of cyclohexane, at first only slightly, and then very strongly.

With regard to halogen derivatives of saturated hydrocarbons, it is difficult to draw any conclusion as yet, since there is no material for comparison with hydrocarbons having the same carbon skeleton.

6) The heterocyclic compound—thiophene—is oxidized thousands of times more readily than benzene.

1. B. V. Tronov, P. P. Popov, and M. A. Nikol’skii, Zh. R. Kh. O., vol. 33, p. 56. ↩

2. V. 36, 2216 (1903). ↩