Monomers nitroso compounds

Dimeric nitroso compounds with tertiary alkyl groups show more tendency toward dissociation into the monomers. For example, 2-methyl-2-nitrosopropane is so volatile in the form of the monomer that it can hardly be isolated from organic solvents. For the prepara-... 

Nitro- and nitroso-compounds,170171 amines, and thiols induce the decomposition of diacyl peroxides in what may be written as an overall redox reaction. Certain monomers have been reported to cause induced decomposition of BPO. These include AN,172 A -vinylcarbazole,17,177 Ar-vinylimidazole178 and NVP.177... 

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants (kjkp) are provided in Table 5.6. Absolute rate constants (kj) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

PE spectroscopic studies of C-nitroso compounds have sometimes been hampered by these properties, but also the dimer-monomer transformation has been studied by this technique125,126 (vide infra). 

A study of a series of C-nitroso compounds, including monomers as well as dimers, by field desorption has demonstrated the superiority of this technique to this class of compounds118. All the compounds display intense molecular ions118. The method has a significant potential for studies of the equilibrium between mixed and pure C-nitroso compounds, since the amount of pure and mixed dimers present in a solution apparently can be visualized by the relative abundances of the respective molecular ions see Scheme 43. Determination of the concentrations versus time may resolve the kinetics of the dimer formation118. 

Nitroso derivatives (with the nitroso group bound to a carbon atom) can exist in three molecular forms176177 the monomer 63 and the dimers 64 and 65, Z and E, respectively. Aliphatic C-nitroso compounds are mainly dimers178. Aromatic nitroso derivatives, in solution, may be monomers or dimers, depending on the concentration and temperature, and on the substituent on the aromatic ring. Nitrosobenzene itself is in the E dimer form in the solid state179. 

There is a tendency toward alternation in the copolymerization of ethylene with carbon monoxide. Copolymerizations of carbon monoxide with tetrafluoroethylene, vinyl acetate, vinyl chloride, and acrylonitrile have been reported but with few details [Starkweather, 1987]. The reactions of alkenes with oxygen and quinones are not well defined in terms of the stoichiometry of the products. These reactions are better classified as retardation or inhibition reactions because of the very slow copolymerization rates (Sec. 3-7a). Other copolymerizations include the reaction of alkene monomers with sulfur and nitroso compounds [Green et al., 1967 Miyata and Sawada, 1988]. 

In contrast to a straightforward and predictable decomposition pattern of photolysis with >400 nm light, irradiation of nitrosamides under nitrogen or helium with a Pyrex filter (>280 nm) is complicated by the formation of oxidized products derived from substrate and solvent, as shown in Table I, such as nitrates XXXIII-XXXV and nitro compound XXXVI, at the expense of the yields of C-nitroso compounds (19,20). Subsequently, it is established that secondary photoreactions occur in which the C-nitroso dimer XIX ( max 280-300 nm) is photolysed to give nitrate XXXIII and N-hexylacetamide in a 1 3 ratio (21). The stoichiometry indicates the disproportionation of C-nitroso monomer XVIII to the redox products. The reaction is believed to occur by a primary photodissociation of XVIII to the C-radical and nitric oxide followed by addition of two nitric oxides on XVIII and rearrangement-decomposition as shown below in analogy... 

Nitroso Compounds Primary and secondary aliphatic C-nitroso compounds are usually unstable and rearrange to oximes or dimerize. Tertiary and aromatic nitroso compounds are reasonably stable, existing as monomers in the gaseous phase or in dilute solution and as dimers in neat samples. Monomeric, tertiary, aliphatic nitroso compounds show N=0 absorption in the 1585-1539 cm1 region aromatic monomers absorb between 1511 and 1495 cm-1. 

The formation of the oxime (2) probably results from the presence of adventitious traces of hydroxylic material which catalyze the isomerization of the nitroso monomer, the primary product of the Barton reaction, to an oxime. More direct evidence for the formation of nitroso compounds in the photochemical rearrangement of nitrites is provided by the isolation of nitroso dimers. ... 

Trifluoronitrosomethane is unusual among nitroso compounds in that it exists as a monomer that is deep blue and is therefore one of the few highly coloured simple polyfluoro compounds. On photolysis, a species derived from radical coupling is formed [276] (Figure 8.109). 

Aliphatic primary and secondary nitroso compounds are not stable but tautomerize to oximes. In nitrosocyclohexane the special steric conditions favor a dimerization rather than a tautomerization. The dimer is reduced polarographically in a six-electron reaction to A, A -dicyclohexylhydrazine [71]. Aliphatic Miitro compounds are dimers in the solid state, but dissociates in solution to a degree depending on the solvent the monomer is easier reducible than the dimer [72]. Only the monomer reacts as a spin trap the rate constant for the dissociation in MeCN is 1.5 x 10" s" [72a]. 

The o- and p-nitrosophenols enjoy the possibility of resonance stabilization by jt-electron donation from the phenolic hydroxyl group to the nitroso group, and the o-isomer could also be stabilized by an intramolecular hydrogen bond. These species are also tautomeric with benzoquinone oximes. All of this could confound interpretation of enthalpy of formation values if only they were available—there are seemingly no measured enthalpy of formation values for o-nitrosophenol. The value for p-nitrosophenol will be discussed later in Section VI because of tautomeric ambiguity. The m-species lacks the stabilizing conjugate NO/OH interaction, and so the monomer-dimer equilibrium as found in other nitroso compounds becomes problematic—should the measurement of enthalpy of combustion be available. 

However, the potential range within which the spin trap is electro-inactive is considerably reduced. This, the commonly used trap nitroso-t.-butane is reduced at a potential -0.98 V (vs. Ag/AgI) at mercury in dimethyl-formamide. The range may be extended by substitution of appropriate aryl groups for butane [114]. Nitroso compounds also present a problem in that some compounds have been shown to undergo a monomer/dimer equilibrium in solution in which only the monomeric form acts as a radical trap. Hence, for nitroso compounds, it is necessary to understand this equilibrium, as well as the electrochemical properties of the trap, before it may be used in the investigation of electrode reactions. 

A large number of other substances are also active inhibitors. These include oxygen, NO (one of the most effective inhibitors, so much so that some highly reactive monomers can be distilled only under an atmosphere of NO), aromatic nitro compounds, numerous nitroso compounds, sulfur compounds, amines, phenols, aldehydes, and carbamates. An interesting inhibitor is molecular oxygen. Being a diradical, oxygen reacts with chain radicals to form the relatively unreac-tive peroxy radical ... 

Solution Aliphatic nitroso compounds exist in the solid phase as dimers which is their most stable form. Resonance stabilization allows N,N-dimethylaniline to remain as a monomer. 

Trapping can involve either nitroxides followed by. separation and characterization or tlie use of nitroso compounds and subsequent structural analysis by ESR. As an example of the former, the trapping of the radicals from the reaction of t-butoxy radicals and methyl methacrylate (MMA) by l,l,3,3-tetramethylisoindolinyl-2-oxy (1) is shown (Scheme 1). Alkyoxyamines were isolated by conventional techniques and their pathways deduced. The methyl radical, formed by P-scission of the t-butoxy radical, is trapped as the methoxyamine, which in turn can add a further monomer unit in a thermally activated step growth addition to form (2). As an example of the latter, the radicals from the same reaction are now trapped by 2-methyl-2-nitrosopropane as tlie corresponding nitroxyl radicals. 

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