Nitrosomethane dimer

Nitrosomethane (1) is known to be less stable than its isomer formaldoxime 2 and original attempts to isolate this species failed owing to its facile isomerization to the oxime 2. Already Bamberger and Seligman considered in 1903 that it would be difficult to isolate nitrosomethane after oxidation of methylamine due to its rapid isomerization to 2. Hence, 2 is always present in the synthesis of the nitrosomethane. Nitrosomethane is produced in the pyrolysis or photolysis of tcrf-butyl nitrite and by the reaction of methyl radicals with nitric oxide. Early results were confusing since the final product obtained is dimeric nitrosomethane. It was first isolated in 1948 by Coe and Doumani from the photolysis of gaseous ferf-butyl nitrite according to the overall reaction shown in equation 2. 

Batt, Gowenlock and Trotman carried out a detailed study of the pyrolysis and photolysis of terf-butyl nitrite and established that dimeric nitrosomethane exists in two isomeric forms, cis and trans (Scheme 5). Monomeric nitrosomethane could be generated by heating the dimer in the gas phase (the activation energy for dissociation was found to be ca 90 kJmoH )". Also ultraviolet irradiation dissociates the dimer, leaving monomeric 1. Vibrational analysis of monomeric 1 is summarized in Table 4. 

Many nitrosoalkanes and nitrosoarenes exist as dimers at room temperature. In 1976, Gowenlock and Trotman carried out a unique investigation on the isolation of E- and Z-isomers of the nitrosoalkane dimer." They prepared two isomers, A and B, of dimeric nitrosomethane generated by the photolysis and pyrol) is of t-butyl nitrite vapor by trapping them on the surface of the apparatus. The less-stable isomer A was transformed into isomer B upon heating or by dissolving it in a solvent. They formulated the A and B forms as Z- and E-isomers of diazine dioxides 3. The diazine dioxide structure for the nitroso dimers was later confirmed by NMR spectroscopy by Freeman." ... 

Gowenlock, B.G. and Trotman, J., Geometrical isomerism of dimeric nitrosomethane, /. Chem. 

The Hel PE spectrum of nitrosomethane (Figure 13) was first studied by Bergmann and Bock125,127. This compound as well as several other aliphatic and aromatic C-nitroso compounds were investigated by Pfab and coworkers126,128, however several of them were dimers. 

FIGURE 13. PE spectra of nitrosomethane (b) and its cfr-dimer (a) displaying the temperature-dependent decomposition of cw-(MeNO)2 into MeNO. Reproduced (modified) with permission from Reference 125... 

While highly reactive formaldonitrone (3) was detected only recently, both the nitrosomethane (1) and the formaldehyde oxime (2) are also reactive compounds in the condensed phase 2 undergoes facile oligomerization , whereas 1 forms a dimer and isomerizes to 2 . Deprotonation of any of these isomers (1, 2, 3) would result in the corresponding simplest nitrosomethanide anion [H2C=N—O]. ... 

The dimer of nitrosomethane (azo-dioxide), (CHsNO) , exists at ambient temperatures in two isomeric forms, cis and trans (Figure 8), which undergo isomerization relatively easily. rrawi-(CH3—NO)2, having a planar azo-dioxide moiety, crystallizes in the orthorhombic space group Cmcm with Z = 4. The experimentally determined bond lengths are rf(N—O) = 1.25 and rf(N—C) = 1.57 A, respectively (calculated data, see Figure 8). ... 

FIGURE 9. Resonance scheme for dimeric trans nitrosomethane... 

As in other areas of the chemistry of nitroso compounds, the tacit assumption has been that the rearrangement of aliphatic nitroso compounds bearing hydrogens on the carbon atom which also carries the nitroso group rearranges to the oxime virtually instantaneously. Yet, recently, the existence of all types of aliphatic nitroso compounds has been well established. Without going into details here, nitrosomethane, nitrosocyclohexane, and nitrosodibenzoyl-methane have been prepared and appear to exist as reasonably stable dimers. 

The first preparation of nitrosomethane dimer (m.p. 122°-122.2°C corr.) was carried out by the photolytic decomposition of /-butyl nitrite at 25°C and at a pressure of 50 mm using a 300 watt mercury vapor lamp (General Electric Uviarc ) as a source of radiation [36],... 

A related procedure, which may be of value from the preparative standpoint, involves the preparation of /rans-nitrosomethane dimer by adding a solution of diacetyl peroxide in sec-butyl nitrite to warm sec-butyl nitrite [50]. From the product of the reaction it has been assumed that this preparation involves the generation of free methyl radicals which react with the nitrite to give nitrosomethane and alkoxy radicals. The latter disproportionate to ketones and alcohols, while the nitroso compound dimerizes. 

Nitro-compounds fRNOj) are isomeric with nitrites, but their electronic structure, excited states and photochemistry are very different. There is no very low-lying (n.jt ) state, and nitroalkanes show n — 3i absorption with a maximum around 275 nm ( —201 mol - cm In cyclohexane solution, nitromethane (CH1NOi) is photoreduced to nitrosomethane(CH,NO, but nitroethane under the same conditions gives rise to a nitroso-dimer derived from the solvent CS.47). The latter process is probably initiated by cleavage of the carbon-nitrogen bond in the nitroalkane. In basic solution (when the nitroalkane is converted to a nitronate anion) irradiation can lead to efficient formation of a hydroxamic acid (S.48), and this reaction most likely proceeds through formation of an intermediate three-mem bered cyclic species. 

Isomerization of nitroso compounds to the corresponding oxime is rapid at temperatures above the melting point of the dimer or in the presence of metallic or other reactive surfaces or in certain non-aqueous solvents . For the nitroso-methane-formaldoxime isomerization the activation energy was estimated as 30-40 kcal.mole . The thermal gas-phase isomerization of nitrosomethane, reaction (1), has been studied by a flow technique at low pressures, and found to be highly sensitive to reaction conditions, especially to the nature and extent of the surfaces. Under conditions favourable to the homogeneous gas phase process, 27.1 kcal.mole and log /l(sec ) = 8.9. The low pre-exponential factor was interpreted in terms of a cyclic transition state, viz. 

The rate of decay of nitrosomethane (formed in the photolysis of azomethane-nitric oxide mixtures at 25° C) to form the stable dimer, reaction (2), was measured by infrared techniques and the homogeneous second order rate coefficient = 87 l.mole. sec was derived. The cis and trans iomers of nitrosomethane have been characterized and the experimental conditions which favour the formation of each of the dimers and the rearrangement to formaldoxime have been described ... 

Acetone and the dimer of nitrosomethane (CH3NO) were identifled as major products in the early studies of Thompson and Dainton (1937) and Coe and Doumani (1948), and this led to their conclusion that the primary dissociation occurred in process (HI). Later experiments proved definitively that these products were derived from secondary... 

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