CHEMISTRY

Corresponding Member of the Academy of Sciences of the USSR G. A. RAZUVAEV, S. F. ZHIL’TSOV,
O. N. DRUZHKOV, G. G. PETUKHOV

OXIDATION OF ALKYL ORGANOMERCURY COMPOUNDS

Earlier we showed that secondary alkyl organomercury compounds—dicyclohexyl-(¹) and diisopropylmercury (²) (DIPM)—are oxidized at ordinary temperature by oxygen both without a solvent and in solutions, with liberation of mercury and formation of the corresponding alcohols, ketones, and hydrocarbons. In that work the main attention was directed to the oxidation and detachment from mercury of radicals. In the present communication we have dealt with a more detailed investigation of the process, in which special attention was given to elucidating the formation of any oxidized organomercury compounds. The oxidation of symmetrical mercury compounds with primary (benzyl-), secondary (isopropyl-), and tertiary (tert-amyl-) alkyl radicals was studied. The reaction products were analyzed by chemical and physicochemical methods, with the use of mass spectrometry, gas chromatography, and infrared spectroscopy.

The oxidation of DIPM in benzene, cyclohexane, cyclohexene, CHCl₃, and CCl₄ was studied in detail. The principal reaction products in benzene are isopropylmercury isopropylate, isopropylmercury hydroxide, acetone, isopropyl alcohol, and mercury. In the gaseous space above the reaction mixture, propylene and traces of propane were detected. Benzene took almost no part in the reaction. The presence of its derivatives (phenylisopropylmercury, diphenylmercury, diphenyl, phenol, and cumene) was detected in small quantity by mass spectrometry. In a specially conducted experiment with C¹⁴-benzene it was established that their total yield was not more than 5 mol.% relative to the initial DIPM. Isopropylmercury isopropylate and isopropylmercury hydroxide were identified by low-voltage mass spectrometry, and their amount was determined in total by titration of hydroxides with 0.1 N HCl with methyl orange after preliminary hydrolysis of the reaction product with water. After the hydrolysis carried out, isopropyl alcohol was detected in the extract, which confirms the formation of iso-

Table 1

Oxidation of 0.007, 0.0031, and 0.0031 moles of DIPM, respectively, in 0.11, 0.046, and 0.046 moles of C₆H₆, C₆H₁₂, and C₆H₁₀ at 50° C

* Relative to reacted DIPM.
** Calculated as the C₆H₉ group.

propylmercury isopropoxide during oxidation of DIPR (Table 1). It was established that their content at first increased and then remained constant during prolonged oxidation at 50°.

A similar picture in the ratio of the same reaction products (Table 1) is observed when this compound is oxidized in cyclohexane.

In both the first and the second solvent, approximately twice as much ketone as alcohol is formed. This can be explained by the comparatively easy oxidation of the alcohol formed to acetone. Formation of the latter was in fact observed at the expense of the solvent during oxidation of dicyclohexylmercury (¹) and DIPR (²) in isopropanol. Oxidation of DIPR in cyclohexene proceeds differently: the yield of isopropyl alcohol is higher than that of acetone; propane is formed instead of propylene; the balance for isopropyl radicals indicates the absence of mercury alkoxy compounds and the presence of hydroxide; in the course of the reaction this solvent undergoes considerable oxidation (Table 1). Mass-spectrometric analysis showed the absence of isopropylmercury isopropoxide.

We believe that the formation of hydroxide in all solvents occurs as a result of the easy hydrolysis of isopropylmercury isopropoxide by the moisture present or by water formed in the reaction. The latter is, in all probability, one of the products of cyclohexene oxidation. Upon hydrolysis of the alkoxy compound, isopropyl alcohol should be obtained along with mercury hydroxide. Its considerable yield in comparison with acetone is indeed observed in the oxidation of DIPR in cyclohexene.

The participation of CHCl₃ in the reaction was indicated earlier (²). We repeated the experiment, carrying it out at 50° (Table 2). It was noted that the mercury liberated at the beginning of the oxidation disappeared, evidently being converted into calomel, whose presence was established. Among products not indicated earlier, propane, propylene, CO₂, and CO should be noted, the last two being formed at the expense of the solvent.

Table 2

Oxidation of 0.0032 and 0.005 moles of DIPR, respectively, in 0.062 and 0.083 moles of CHCl₃ and CCl₄

Oxidation of DIPR in CCl₄ proceeds somewhat differently. The yield of alcohol and acetone is lower than in CHCl₃, while the greater part of the isopropyl radicals reacts with the solvent to form 2-chloropropane. In addition to CO and CO₂, formation of CHCl₃ and hexachloroethane at the expense of CCl₄ is observed (Table 2).

Tertiary-alkyl mercury compounds are also readily oxidized. Ditert-amylmercury behaves analogously to DIPR in the corresponding solvents. Table 3 gives the results of its oxidation in CHCl₃, CCl₄, benzene, and cyclohexane. In C₆H₆ and ц-C₆H₁₂ we were unable to titrate hydroxide. The product decomposed with water with liberation of mercury and did not have an alkaline reaction. Evidently, the alkoxy compound or hydroxide of tert-amylmercury is unstable in water and decomposes with liberation of mercury.

Consequently, not only secondary- but also tertiary-alkyl mercury compounds are affected by oxygen. Using dibenzylmercury as an example, the possibility of oxidation of primary mercurialkyls was shown qualitatively. In the oxidation of this compound in CHCl₃, CCl₄, and C₆H₆, one of the reaction products is benzaldehyde. Thus, in the reaction of 0.0064 moles

Table 3

Oxidation of 0.0055, 0.0055, 0.0019, and 0.0029 moles of ditertiaryamylmercury, respectively, in 0.062, 0.075, 0.056, and 0.074 moles of CCl₄, CHCl₃, C₆H₆, and C₆H₁₂ at 40° C

On oxidation of dibenzylmercury in 0.23 mole of C₆H₆ at 50° for 120 h, 0.0016 mole (25%) of mercury was isolated and 0.002 mole (31%) of benzaldehyde was formed.

Apparently, susceptibility to oxidation is a general property of alkyl organomercury compounds, which, to one degree or another, under definite conditions are affected by oxygen.

The mechanism of oxidation of organomercury compounds is rather complex. For a complete understanding of it, kinetic studies are required; however, on the basis of the results obtained the following conclusions may be drawn. The oxidation process proceeds with formation of an intermediate organomercury peroxide compound:

\[ \mathrm{R_2Hg + O_2 \to [R_2Hg \cdot O_2] \to [ROO{-}HgR] \begin{cases} \xrightarrow[\mathrm{R_2Hg}]{} 2\mathrm{ROHgR} \quad (a)\\ \longrightarrow \mathrm{RO_2^{\cdot} + RHg^{\cdot}} \quad (b) \end{cases}} \tag{1} \]

The attack of oxygen directed at the metal is accompanied by subsequent nucleophilic migration of the alkyl group from the metal to oxygen with formation of the indicated peroxide. The latter reacts with the unoxidized organomercury compound (a), giving an alkoxy compound, or partially decomposes (b), owing to its instability, into peroxyalkyl and alkylmercury radicals. A similar scheme is given by many authors to explain the oxidation of organometallic compounds of various metals (³).

The peroxyalkyl radical apparently reacts as an oxidizing agent with the starting compound, with formation of an alcohol, ketone, and alkylmercury radical.

\[ \mathrm{RO_2^{\cdot} + R_2Hg \to ROH + R_{-H}{=}O + RHg^{\cdot}} \tag{2} \]

For a tert-alkyl peroxide radical, its decomposition with destruction of the carbon skeleton at the tertiary carbon atom into a ketone (in our case, the formation of acetone is observed) and an alkoxy radical is characteristic:

\[ \mathrm{C_2H_5(CH_3)_2CO_2^{\cdot} \to (CH_3)_2CO + C_2H_5O^{\cdot}} \tag{3} \]

It is known from the literature (⁴) that such a process takes place and proceeds readily on a surface; in the present case it is promoted by the mercury liberated in the course of the reaction. From the data of Table 3 it is seen that acetone is formed in considerable amounts. The fate of the ethoxyl radical was not established by us.

Evidence for the participation of the alkylmercury radical in the reaction is the significant yield of alkylmercury chloride in CHCl₃ and CCl₄, whose formation—

can be represented by the equation:

\[ \begin{gathered} \mathrm{RHg}^{\bullet} \ \xrightarrow[\ ]{\mathrm{CCl}_{4}(\mathrm{CHCl}_{3})}\ \mathrm{RHgCl}. \\ \downarrow \\ \mathrm{Hg}+\mathrm{R}^{\bullet} \end{gathered} \tag{4} \]

This radical also partially decomposes into mercury and an alkyl radical. The participation of the latter is manifested in its interaction with \(\mathrm{CHCl}_{3}\), with formation of a hydrocarbon, or with \(\mathrm{CCl}_{4}\), with formation of an alkyl chloride:

\[ \mathrm{R}^{\bullet} \begin{cases} \xrightarrow{\mathrm{CHCl}_{3}} \mathrm{RH}+\mathrm{Cl}_{3}\mathrm{C}^{\bullet} \\ \xrightarrow{\mathrm{CCl}_{4}} \mathrm{RCl}+\mathrm{Cl}_{3}\mathrm{C}^{\bullet} \end{cases} \tag{5} \]

As already indicated, in \(\mathrm{CCl}_{4}\) the yield of oxidation products of diisopropylmercury is lower than in \(\mathrm{CHCl}_{3}\), while isopropyl chloride is formed in a fairly significant amount. In the present case, in addition to reactions (4) and (5), one should assume an interaction, initiated by an intermediate peroxide compound, of dialkylmercury with the solvent, with formation of alkylmercury chloride and alkyl chloride. It is known \((^{5})\) that acyl peroxides initiate the interaction of organomercury compounds with \(\mathrm{CCl}_{4}\).

In \(\mathrm{CHCl}_{3}\) and \(\mathrm{CCl}_{4}\), secondary dichloro- and trichloromethyl radicals, respectively, should be formed. In the case of the former, we did not detect the product of its dimerization. It may be assumed that it is oxidized to \(\mathrm{CO}_{2}\) and \(\mathrm{CO}\), which are present in the reaction products. In \(\mathrm{CCl}_{4}\), formation of \(\mathrm{CO}_{2}\) and \(\mathrm{CO}\) is likewise observed, as well as, in the case of diisopropylmercury, chloroform and hexachloroethane. \(\mathrm{CHCl}_{3}\), evidently, is formed by interaction of the trichloromethyl radical chiefly with the isopropyl alcohol formed \((^{6})\), and to a lesser extent with diisopropylmercury, as indicated by the insignificant yield of propylene.

Thus, it has been established experimentally that alkyl organomercury compounds are capable of being oxidized, the ease of oxidation increasing in the order of increasing nucleophilicity in the following sequence:

\[ \mathrm{CH}_{3}\mathrm{CH}_{2}-\mathrm{CH}_{2}-,\quad \mathrm{C}_{6}\mathrm{H}_{5}-\mathrm{CH}_{2}-,\quad (\mathrm{CH}_{3})_{2}-\mathrm{CH}-,\quad \mathrm{C}_{2}\mathrm{H}_{5}(\mathrm{CH}_{3})_{2}-\mathrm{C}-, \]

i.e., it is evidently due to the different ability of alkyl radicals to undergo nucleophilic rearrangement in the intermediate complex \([\mathrm{R}_{2}\mathrm{Hg}\cdot \mathrm{O}_{2}]\).

Scientific Research Institute of Chemistry
at Gorky State University
named after N. I. Lobachevsky

Received
5 VII 1963

CITED LITERATURE

1. G. A. Razuvaev, G. G. Petukhov et al., DAN, 135, 87 (1960); 144, 810 (1962).
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