PHYSICAL CHEMISTRY

O. A. IVANOV, N. V. FOK, and V. V. VOEVODSKII

REACTIONS OF METHYL RADICALS, OBTAINED BY THE POLANYI METHOD, WITH DEUTERIUM

(Presented by Academician N. N. Semenov on 26 VII 1957)

Recently a number of works have appeared in the literature devoted to the investigation of processes of thermal and photochemical decomposition of organic compounds which form, in the primary act of decomposition, the radical $\mathrm{CH_3}$, in the presence of molecular deuterium $^{(1–5)}$.

These investigations are carried out chiefly in order to determine the rate constant of the elementary substitution reaction of the methyl radical:

\[ \mathrm{CH_3 + D_2 = CH_3D + D.} \]

In the works cited, no methane containing more than one deuterium atom was found. We note only that Nesbitt, Gordon, and Smith $^{(5)}$, who carried out photolysis of a mixture of acetone with deuterium in the presence of small amounts of mercury, found, along with $\mathrm{CH_3D}$, insignificant amounts of $\mathrm{CH_2D_2}$.

On the other hand, Mardaleishvili, Pariiskii, Poltorak, and Voevodskii $^{(6)}$ discovered large amounts of deeply deuterated methane in studying the interaction with deuterium of methyl radicals obtained in the reaction of H atoms, drawn from an electric discharge, with olefins. The authors believe that these data may be regarded as confirmation of the hypothesis they had advanced earlier $^{(7)}$ concerning the existence of exchange between the hydrogen of free radicals and molecular deuterium. These experiments were arranged so as to exclude the participation of D atoms in the reaction, since in a number of experiments molecular deuterium was admixed at that point of the jet where the interaction of H atoms with unsaturated compounds could be considered to have ended.

The appearance of poly-substituted methanes could not have been connected with the low pressures at which the experiments with the electric discharge were carried out, since it was shown that, in the photolysis of acetone and methyl iodide, a change in pressure from tens to several tenths of a millimeter of mercury did not lead to the appearance of deeply exchanged methanes*.

The explanation of the indicated contradiction may lie in the different ways of obtaining methyl radicals. Therefore we set ourselves the task of studying the reaction with $\mathrm{D_2}$ of methyl radicals obtained by some other method.

We obtained methyl radicals by the Polanyi method $^{(8)}$ according to the reaction:

\[ \mathrm{CH_3J + Na = CH_3 + NaJ.} \]

The reaction was carried out in the presence of molecular deuterium, which was used as the carrier gas for Na vapors. The scheme of the apparatus is shown in Fig. 1. The reaction vessel, in which the jets of $\mathrm{CH_3J}$ and deuterium saturated with sodium vapor were combined, was a quartz cylinder connected through a nozzle with a boat containing sodium, and through—

* These results were obtained by A. S. Chernysheva.

through another nozzle into a flask containing CH₃I. The temperature of the boat with sodium during the experiments was maintained at 450°.

In those cases where it was desirable to exclude a volume reaction, the boat with sodium was not heated. The pressure of CH₃I in the reaction vessel was 0.5 mm Hg, the pressure of D₂ was 10 mm, and the stream rate was 8.5 cm³/min. The reaction products, together with the unreacted CH₃I, passed through a coil cooled with liquid nitrogen, where methyl iodide, as well as ethane and other condensable reaction products, were frozen out.

Fig. 1. Diagram of the apparatus. 1, 2, 3—flasks with CH₃I, D₂, and H₂, respectively; 4—trap for capturing mercury vapor from the manometer, 5—valves, 6—finger with liquid CH₃I, 7—boat with Na, 8—reaction vessel, 9—nozzle, 10—membrane manometer, 11, 12—furnaces, 13—finger with activated carbon, 14—trap, 15—coil, 16—trap with activated carbon, 17—comb for taking samples for analysis, 18—trap, 19—connection to the pump, 20—thermal manometer, 21—connection to the mercury manometer, 22—capillary, 23—connection to the atmosphere

Methane and part of the deuterium were captured in a trap with activated carbon, located after the coil and also cooled with liquid nitrogen. After the experiment was completed, the deuterium was pumped out of this trap, and the methane was sent for analysis. Mass-spectrometric analysis made it possible to judge the content of the variously deuterated methanes. The absolute amount of methane was not determined in most experiments.

In a number of experiments the inner surface of the reaction vessel was coated with sodium, deposited on the surface in the form of drops or as a mirror. The drops were obtained either by splashing pieces of sodium placed in the vessel by rapidly heating them, or by rapidly heating it in the boat while simultaneously passing a stream of deuterium. The mirror was obtained by gradually heating the sodium placed in the vessel to 300°.

The work used deuterium obtained electrolytically from D₂O and purified from impurities by diffusion through a palladium capillary. CH₃I was distilled in a rectification column (b.p. 41.5–42°, \(n^{19} = 1.532\)). The sodium used was chemically pure and was not subjected to additional purification.

The composition of the methanes obtained in a pure quartz vessel in the temperature range 20–480° was determined (Fig. 2). Under these conditions CH₄ and CH₃D are formed predominantly. The amount of polydeuterated methanes is small and only in a narrow temperature interval near 200° reaches 18–20%. The ratio CH₃D/CH₄ increases in the interval 20–100° from 0.6 to 2, and does not change with a further increase in temperature.

It was found that the deuterium content in the methanes formed changes substantially when metallic sodium is deposited from the stream onto the surface of the vessel. Further experiments showed that the relation of the percentage

the content of differently deuterated methanes with temperature depends on how the sodium was applied to the surface.

When the surface of the vessel is coated with a sodium mirror (Fig. 3, I), the percentage content of deeply deuterated methanes (\(\mathrm{CD_4}\), \(\mathrm{CD_3H}\), and \(\mathrm{CH_2D_2}\)) at room temperature is 5–8 times greater than in a clean quartz vessel. With increasing temperature the percentage of deeply deuterated methanes decreases. It seems to us, however, that the decrease is not connected with a reduction in the rate of formation of \(\mathrm{CD_4}\) and other heavier methanes. A direct experiment measuring the total concentration of methanes showed that, when the temperature was raised to \(70^\circ\), the yield of methanes in the presence of a mirror increased by approximately a factor of 10. The decrease in the percentage content of \(\mathrm{CD_4}\), \(\mathrm{CH_2D_2}\), and \(\mathrm{CD_3H}\) may therefore be explained by the presence of a heterogeneous reaction on the sodium surface, leading mainly to the formation of \(\mathrm{CH_4}\) and \(\mathrm{CH_3D}\).

Fig. 2. Dependence of the percentage content of methanes obtained in a clean reaction vessel on temperature:
\(a\)—\(\mathrm{CH_4}\), \(b\)—\(\mathrm{CH_3D}\), \(v\)—\(\mathrm{CH_2D_2}\), \(g\)—\(\mathrm{CH_3D}\), \(d\)—\(\mathrm{CD_4}\)

In the case of sodium droplets, the surface reaction is insignificant, and its share of the volume formation of methane amounts to only \(1/5\). In this case the methanes obtained from \(\mathrm{CH_3}\) radicals formed in the gas phase contain much more deuterium than in the case of a sodium mirror (Fig. 3, II). A maximum of deuteration near \(70\text{–}80^\circ\) is noteworthy. At higher temperatures \(\mathrm{CH_4}\) and \(\mathrm{CH_3D}\) again predominate.

Fig. 3. Dependence of the percentage content of methanes obtained in a vessel coated with a Na mirror (I) and with drops of Na (II) on temperature. The designations are the same as in Fig. 2.

It is important to note that light methane \(\mathrm{CH_4}\) does not exchange with \(\mathrm{D_2}\) either in the presence of a mirror or in the presence of sodium droplets.

From these data, and also from the fact that the content of \(\mathrm{CD_3H}\) is always less than that of \(\mathrm{CD_4}\) and \(\mathrm{CD_2H_2}\), it follows that, in our experiments, deeply exchanged methanes could not have appeared as a result of the successive replacement of H atoms by D in saturated methanes. Consequently, by whatever mechanism the formation of \(\mathrm{CH_2D_2}\), \(\mathrm{CH_3D}\), and \(\mathrm{CD_4}\) proceeded, this exchange must have taken place with the participation of methyl radicals.

The distribution of deuterated methanes in our experiments differs from the distribution in the experiments of Mardaleishvili, Pariiskii, Voevodskii, and Poltorak (\(^{6}\)), which apparently indicates a different mechanism of their formation.

The role of the Na surface in the exchange of \(\mathrm{CH_3}\) radicals with deuterium in our case is undoubted; however, at present there are insufficient data for constructing a detailed mechanism that would explain the character of deuteration of the methanes, as well as the maximum at a temperature of 70–80°.

Department of Chemical Kinetics
Moscow State University
named after M. V. Lomonosov

Institute of Chemical Physics
Academy of Sciences of the USSR

Received
19 VII 1957

CITED LITERATURE

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