Chemistry

I. V. PATSEVICH, Academician A. V. TOPCHIEV, and V. Ya. SHTERN

INTERACTION OF ALKYL RADICALS WITH NITROGEN DIOXIDE

According to current views, the central elementary process in the vapor-phase nitration of alkanes is the interaction of the alkyl radical R with NO₂. It is assumed that it proceeds along two parallel paths, with the formation of nitro compounds and alkyl nitrites:

\[ \dot{R} + \mathrm{NO}_2 \begin{array}{c} \nearrow\ \mathrm{RNP}_2\\ \searrow\ \mathrm{ROEO} \end{array} \qquad \begin{array}{r} (1a)\\ (1b) \end{array} \]

The further transformation of alkyl nitrites is usually regarded as the source of the oxidation and destruction products of the paraffin chain that are actually obtained in nitration. It is assumed that the primary act of transformation of the alkyl nitrite is its decomposition according to the equation:

\[ \mathrm{HONO} \longrightarrow \dot{\mathrm{R}}\mathrm{O} + \mathrm{NO}. \tag{2} \]

Such decomposition occurs either thermally (¹), or, as Gray (²) suggested, at the moment of formation of the alkyl nitrite according to equation (1b). Gray explains such immediate decomposition by the fact that, upon formation of the R′CH₂—ONO bond, 57 kcal/mole of energy is released, whereas rupture of the neighboring R′CH₂O—NO bond requires only 37 kcal/mole. The alkoxy radical, in its subsequent transformations, gives such oxidation products as aldehydes, CO, CO₂. In addition, by interacting with NO, it can give a nitrite already of secondary origin:

\[ \dot{\mathrm{R}}\mathrm{O} + \mathrm{NO} \longrightarrow \mathrm{RONO}. \tag{3} \]

The ideas set forth concerning the mechanism of nitration are hypothetical and have never been tested by a direct experiment. In the present work the direct interaction of R with NO₂ was studied at low temperatures, when thermal transformations of intermediate and final products are excluded. This proved possible because NO₂, having an unpaired electron, is a radical-like molecule, and the interaction \(\dot{R} + \mathrm{NO}_2\) proceeds readily at low temperatures. The purpose of the work was to determine whether the two mentioned paths (1a) and (1b) actually take place, to obtain data on the further behavior of RONO, and, from the dependence of the ratio of the amounts of RNO₂ and RONO formed on temperature, to determine the difference in activation energies \((\Delta E)\) and the ratio of the steric factors \((f_1\) and \(f_2)\) of these paths:

\[ \frac{\Sigma \mathrm{RNO}_2}{\Sigma \mathrm{RONO}} = \frac{f_1}{f_2} e^{\Delta E/RT}. \]

Alkyl radicals were obtained by the reaction of H atoms with \(\mathrm{C_2H_4}\). A stream of molecular hydrogen containing H atoms was drawn from the discharge tube into the reactor, where \(\mathrm{C_2H_4}\) and \(\mathrm{NO_2}\) were admixed to it through two inlets. The distance from the point of introduction of \(\mathrm{C_2H_4}\) to the point of introduction of \(\mathrm{NO_2}\) could be varied, and thus the point at which the radical \(\dot{\mathrm{R}}\) met \(\mathrm{NO_2}\) along the length of the reactor was changed.

In the products of different series of experiments the following were determined:

\[ \mathrm{CH_4,\quad C_2H_6,\quad C_2H_4,\quad CO,\quad CO_2,\quad NO,\quad RONO,\quad RNO_2,\quad RCHO.} \]

The hydrocarbon gases were analyzed chromatographically on charcoal and silica-gel columns; CO, \(\mathrm{CO_2}\), and NO were analyzed by absorption in solutions of cuprous chloride, alkali, and hydrogen peroxide, respectively. Alkylnitrites were determined by the polarographic method developed by us \((^3)\). We also developed a method for the polarographic determination of nitroparaffins, alkylnitrites, and aldehydes in their simultaneous presence.

At first, preliminary study was made of processes which, evidently, may proceed in the reactor along with the main process under investigation, \(\dot{\mathrm{R}}+\mathrm{NO_2}\). In the interaction of H atoms with \(\mathrm{C_2H_4}\), only \(\mathrm{CH_4}\) and \(\mathrm{C_2H_6}\) were found. The heavier hydrocarbons found in this case in a number of works \((^{4-6})\) in various amounts depending on the experimental conditions were not detected under our conditions \((P_{\mathrm{H_2}}=2.3\ \mathrm{mm},\ P_{\mathrm{C_2H_4}}=0.05\ \mathrm{mm},\ I=0.07\text{–}0.75\ \mathrm{A},\ t=20\text{–}90^\circ)\). This discrepancy should apparently be attributed to the high concentration of H atoms in our experiments.

It was further shown that, when H atoms interact with \(\mathrm{NO_2}\) \((P_{\mathrm{H_2}}=2.3\ \mathrm{mm},\ P_{\mathrm{NO_2}}=0.08\ \mathrm{mm},\ I=0.75\ \mathrm{A},\ t=20^\circ)\), NO is formed. Finally, it was shown that under our conditions of low pressures and low temperatures there is no interaction of \(\mathrm{C_2H_4}\) with \(\mathrm{NO_2}\).

The first series of experiments on the study of the interaction of \(\dot{\mathrm{R}}\) with \(\mathrm{NO_2}\) was carried out with \(\mathrm{C_2H_4}\) and \(\mathrm{NO_2}\) introduced at one and the same place in the reactor \((P_{\mathrm{H_2}}=2.3\ \mathrm{mm},\ P_{\mathrm{C_2H_4}}=0.05\ \mathrm{mm},\ P_{\mathrm{NO_2}}=0.08\ \mathrm{mm},\ I=0.75\ \mathrm{A},\ v_{\text{stream}}=1\ \mathrm{m/sec})\). In this case the reaction \(\dot{\mathrm{R}}+\mathrm{NO_2}\) proceeded in the presence of an excess of H atoms. The results of two typical experiments of this series are given in Table 1.

Table 1

Composition of the products of the interaction of \(\dot{\mathrm{R}}\) with \(\mathrm{NO_2}\) in the presence of H atoms, \(t=20^\circ\) (mole percent)

As is seen from Table 1, both \(\mathrm{RNO_2}\) and \(\mathrm{RONO}\) are detected in the products of the experiments, i.e., two pathways of interaction of \(\dot{\mathrm{R}}\) with \(\mathrm{NO_2}\) are indeed realized. Since thermal decomposition of \(\mathrm{RNO_2}\) and \(\mathrm{RONO}\) at room temperature is excluded, the formation of oxidation products (\(\mathrm{CO}\), \(\mathrm{CO_2}\), and \(\mathrm{HCHO}\)) could be regarded as evidence for the correctness of Gray’s ideas on the decomposition of \(\mathrm{RONO}\) at the moment of formation. In this case it may be assumed that, as a result of the interaction of the radical \(\dot{\mathrm{R}}\mathrm{O}\) formed with H atoms and \(\mathrm{NO_2}\), aldehyde, CO, and \(\mathrm{CO_2}\) are obtained, while the presence of \(\mathrm{RONO}\) in the products would be explained by its secondary formation from the alkoxy radical and NO, produced by the reaction of \(\mathrm{NO_2}\) with H atoms. However, special experiments carried out to study the interaction of knowingly introduced \(\mathrm{C_2H_5ONO}\) with H atoms (but without \(\mathrm{NO_2}\)) showed that in this case aldehyde and CO are also obtained. In addition, it was found that \(\mathrm{CH_3NO_2}\) also interacts with H atoms, the degree of conversion under our conditions reaching 60%.

Thus, both for testing Gray’s hypothesis and for finding the true ratio between the \(RNO_2\) and \(RONO\) formed, it was necessary to carry out the reaction under study,

\[ \dot R + NO_2 \]

in the absence of an excess of H atoms. This was achieved by moving the point of introduction of nitrogen dioxide 90 mm away from the point of meeting of ethylene and atomic hydrogen, increasing the amount of ethylene fed to \(P_{C_2H_4}=0.16\) mm, and decreasing the current in the discharge tube to 0.07 A. All this led to a decrease in the concentration of atomic hydrogen at the point where \(\dot R\) met \(NO_2\). As it turned out, in carrying out such an experiment, CO, \(CO_2\), and aldehydes are absent from the products of the experiment; this means that interaction of \(RONO\) with atomic hydrogen does not occur.

Table 2

Behavior of \(CH_3NO_2\) under experimental conditions without an excess of H atoms

To prove that under these conditions there is also no interaction of atomic hydrogen with \(RNO_2\), we introduced a known amount of \(CH_3NO_2\) into the reaction, feeding it together with \(NO_2\). As is seen from Table 2, the amount of \(CH_3NO_2\) found in such experiments is equal to the sum of the amount formed in the ordinary experiment under the same conditions (i.e., without addition of \(CH_3NO_2\)) and the amount artificially added.

Table 3

Composition of the products of interaction of \(\dot R\) with \(NO_2\) without an excess of H atoms

Thus, under the conditions found, there is no interaction of atomic hydrogen either with nitroparaffin or with alkyl nitrite.

The results of two series of experiments, carried out at four different temperatures, are presented in Table 3.

CO, \(CO_2\), and HCHO were absent from the products of the experiments at all temperatures (at 96° traces of aldehyde were found by the qualitative reaction with dimedone, which apparently indicates the beginning of thermal decomposition of \(RONO\) at this temperature). Thus, in this case the products of the interaction

\[ \dot R + NO_2 \]

are only \(RNO_2\) and \(RONO\).

Fig. 1. Determination of the difference in activation energy of the two paths of interaction of \(R\) with \(NO_2\).
\(a\)—first series of experiments; \(b\)—second series of experiments.

Following Gray’s ideas about the decomposition of \(RONO\) at the moment of its formation by the reaction \(\dot R + NO_2\), it is impossible to explain its presence in the products of the experiments described above. The “secondary” nitrite, according to Gray, is formed by equation (3). However, under the conditions of our experiments in the reaction zone there may be present only

only insignificant amounts of NO could exist in a large excess of NO₂. Consequently, as a result of the secondary reaction of RO with NO₂, only alkyl nitrate RONO₂ could be formed. Polarographic analysis does not make it possible to distinguish RONO from RONO₂, since both are reduced at the same half-wave potential. Therefore we carried out a spectrophotometric analysis of the products of the experiments, which confirmed the presence of alkyl nitrite in them. In this case the amount of alkyl nitrite determined spectrophotometrically is equal to the total amount of nitrite and nitrate determined polarographically, i.e., in fact only alkyl nitrite is found in the products. This result makes it possible to speak of the incorrectness of Gray’s ideas about the decomposition of nitrite at the moment of its formation from $\dot{R}$ and NO₂. Apparently, distribution of the excess energy among bonds (without their rupture) and its dissipation in subsequent deactivating collisions are possible.

Table 4

Difference in activation energies and ratio of steric factors for two pathways of the interaction of $\dot{R}$ with NO₂

Using the temperature dependence of the ratio $\dfrac{\mathrm{RNO_2}}{\mathrm{RONO}}$, the difference in activation energies of the two pathways of the interaction of $\dot{R}$ with NO₂ can be determined; it is approximately equal to 1 kcal/mol (Fig. 1 and Table 4). From the same dependence we find the ratio of the steric factors $f_1/f_2 \simeq 0.6$.

Table 5

Effect of the surface on the interaction of $\dot{R}$ with NO₂

Thus, the formation of alkyl nitrite requires a somewhat greater activation energy and has a larger steric factor than the formation of nitroparaffin.

To check the role of the surface in the reaction $\dot{R} + \mathrm{NO_2}$, experiments were carried out in a reactor whose surface was coated with KCl and K₂B₄O₇. No change in the composition or amounts of the products was found, which indicates the homogeneous character of the reaction of alkyl radicals with nitrogen dioxide (Table 5).

Received
18 VII 1958

CITED LITERATURE

1. G. B. Bachman, L. M. Addison et al., J. Org. Chem., 17, 906 (1952).
2. P. Gray, Trans. Farad. Soc., 51, 1367 (1955).
3. I. V. Patsevich, A. V. Topchiev, V. Ya. Shtern, Zhurn. anal. khim., 13, 5 (1958).
4. G. K. Lavrovskaya, R. E. Mardaleishvili, V. V. Voevodskii, Problems of Chemical Kinetics and Reactivity, Publishing House of the Academy of Sciences of the USSR, 1955.
5. W. J. Moore, H. S. Taylor, J. Chem. Phys., 8, 466 (1940).
6. J. C. Jungers, H. S. Taylor, J. Chem. Phys., 6, 325 (1938).