The Disproportionation of Hydroxylamine

Disproportionation reactions of free NH2OH have earlier been reported to produce N2, N20, and NH3 (104). Given that these processes are very slow, it has been proposed that they should have originated from metal ion impurities, suggesting that previous coordination of NH2OH is a prerequisite to disproportionation (105). Unfortunately, the redox properties of coordinated 

The NH2OH disproportionation reaction catalyzed by NP (75) was considered in a detailed kinetic study, performed spectrophotometri-cally and gas volumetrically (108). The found products were N2, N20, and NH3, and a mechanistic interpretation identified a fast route, probably effected by radicals derived from a bridged hydroxylamine dinuclear complex. The slow route was associated with the intermediacy of a nitroside complex. 

The electrophilic reaction of NP with sulfite (the Boedeker reaction) has been studied, and follows a similar reaction pattern as with other reactants [Eqs. (5) and (6)]. The red color shows up at 475 nm, and this decays in an unknown way with formation of [Fe(CN)5S03]5 as the 

The [Fe(CN)5S03]5 ion has been prepared and characterized as a sodium salt (112). By reacting with one equivalent of the oxidizing 

The electrophilic reactions of NP with SH- and several SR- have been studied and reviewed (28). The nature of the reversible addition reactions [Eq. (5)] are reasonably well understood for the thiolates. A kinetic study including some bioinorganically relevant nucleophiles (cysteine, glutathione) was performed by using stopped-flow and T-jump techniques (77). The rate constants for the forward and reverse processes in Eq. (5) were in the range 103-104 M-1 s-1 and 101 K)3 s-1 at 25 °C, respectively. 

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The Oxidation of Amines and Alcohols The Disproportionation of Hydroxylamine Miscellaneous Reactions... 

Another mechanism of nitroxyl radical regeneration was proposed and discussed in the literature [67-71]. The alkoxyamine AmOR is thermally unstable. At elevated temperatures it dissociates with cleavage of the R—O bond, which leads to the appearance of an [AmO + R ] radical pair in the cage of polymer. The disproportionation of this radical pair gives hydroxylamine and alkene. The peroxyl radical reacts rapidly with hydroxylamine thus... 

Depending on the type of iron catalyst, the reaction seems to take different mechanistic pathways. According to Johannsen and Jorgensen s results, the catalytic cycle starts with the formation of nitrosobenzene 32 either by disproportionation of hydroxylamine 29a to 32 and aniline in the presence of oxo iron(IV) phthalocyanine (PcFe4+=0) or by oxidation of 29a [131]. The second step, a hetero-ene reaction between the alkene 1 and nitrosobenzene 32, yields the allylic hydroxylamine 33, which is subsequently reduced by iron(II) phthalocyanine to afford the desired allylic amine 30 with regeneration of oxo iron(IV) phthalocyanine (Scheme 3.36). That means the nitrogen transfer proceeds as an off-metal reaction. The other byproduct, azoxybenzene, is probably formed by reaction of 29a with nitrosobenzene 32. 

A less restrictive implementation of the procedure, find-all-pathways, on the initial chemical species set, would employ the. operators K or K and would generate several additional reactions of interest. Among these additional reactions of particular interest are, the decomposition reaction of nitrobenzene and the disproportionation of nitrobenzene with N-phenyl hydroxylamine forming aniline and nitrobenzene ... 

The products of the superoxidized state may be explained if one considers the possibility that occasionally during turnover an active site at Mn is reduced by only one electron to Mn Mn . The following two-electron oxidation step would then yield the Mn Mn form of the enzyme, which is only slowly reduced back to an active Mn" form. Such chemistry was shown to be viable by the introduction of the one-electron reductant hydroxylamine to catalase followed by the addition of hydrogen peroxide (23). These catalases exhibit saturation kinetics and complete the disproportionation of hydrogen peroxide at a rate of 5.75 x 10 s for L. plantarum (23), 1.7 X 10 ... 

Such a metal-assisted disproportionation of hydroxylamine may be initiated by a bimolecular reaction with uncoordinated hydroxylamine or by intramolecular electron transfer. The presence of a reducible metal allows a concurrent three electron oxidation of hydroxylamine to nitric oxide with the two electron reduction of hydroxylamine to ammonia, Eqs. (4.45) and (4.46). Ultimately, the reaction proved to be an efficient synthetic route to the ferrous nitrosyl adduct, which rapidly precipitates from solution in high yield. 

The solution of hydroxylamine is a very weak base, it disproportionates slowly in addic solutions and rapidly in alkalis. 

The substituted hydroxylamine C NOPP from reaction 7) can take part in various dark reactions, even at ambient temperature. From a study of the low molecular weight model I in the liquid phase, two decomposition pathways are possible (reaction 8) (12). The products from the disproportionation reaction 8a were only observed in the absence of a radical trap such as O2. In a given solvent ks kaa-Uo (solvent air saturated and degassed respectively). Both k8a and ke were found to increase by an order of magnitude on going from a non-polar solvent (iso-octane) to a polar solvent (methanol or tert.-butyl hydro peroxide, BuOOH). 

Hydrogenation catalyst, Acid, Fuel Riesthuis, P. et al., J. Loss Prev. Process Ind., 1997, 10(10), 67 In the presence of precious metal hydrogenation catalyst, hydroxylamine salts may disproportionate and form dinitrogen monoxide. Such a mixture is present in a process whereby the hydroxyamine is formed by hydrogenation of nitrate. An explosion in the degassing line, after a period of abnormal operation, was attributed to nitrous oxide build-up. Fuel, in the form of hydrogen and methane diluent, was already present. 

In the reaction profile shown in Figure 1 (similar to that shown by Smith et al. (10)) the initial product was azoxybenzene. However this figure is deceptive firstly azoxybenzene may be produced by a non-catalyzed reaction between nitrosobenzene and phenyl hydroxylamine (10), secondly the figure does not show the mass balance. Indeed at 10 min when all nitrosobenzene has been removed from the solution the amount of azoxybenzene formed was 18.6 mmol, equivalent to 37.2 mmol of reacted nitrosobenzene. Therefore, 42.8 mmol of the original 80 mmol of nitrosobenzene (53.5 %) were unaccounted for. It is possible that the missing mass is in the form of phenyl hydroxylamine in solution, which continues to disproportionate to produce aniline and nitrosobenzene and subsequently azoxybenzene and azobenzene. However as we shall subsequently discover this interpretation is unsustainable. 

Fe(CN)4NO]3- ion upon one-electron reduction, followed by a proton-induced disproportionation reaction to regenerate [Fe(CN)4 NO]2-, along with the formation of [Fe(CN)4NH2OH]2-. The overall reaction involves a four-electron reduction of NP to the hydroxylamine complex. 

Figure 2. The four-step mechanism for hydrogen peroxide disproportionation by the Mn catalse proposed by Penner-Hahn (22) is shown above the solid line. Our proposed mechanism for the inactivation of the Mn catalase by hydroxylamine in the presence of hydrogen peroxide is shown below the solid line.

Attention was paid to the reactivity of PP with aliphatic nitroso compounds too [235]. It was postulated that the stabilizing effect was based on formation of a PP bound nitroxide and its disproportionation into the respective derivatives of hydroxylamine and nitrone. 

Aromatic hydroxylamines are frequently produced during the hydrogenation of aryl nitro groups. Generally, these are undesired intermediates because when they are present in excessive amounts, a potentially explosive situation caused by the exothermic disproportionation of the hydroxylamine can result. 2 This is usually not a problem, but care should be exercised to prevent the accumulation of large amounts of the hydroxylamine, particularly when the... 

In contrast to [CrO(OH2)5]2+ (Section 4.6.4.3.1), the reactions of (72) (including the formation of deprotonated or partially aquated species), as well as those of the related CrIV oxo-carboxylato complexes, can be followed easily by UV-visible spectroscopy.208,242 Detailed mechanistic studies of the reduction or disproportionation reactions of (72) in weakly acidic aqueous media have been performed (reviewed in 1994).240 More recent studies include those of the reactions of (72) with Sn11, In1, Ge11,244 hydrazine, 5 and hydroxylamine.246... 

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