Nitrosobenzene to Nitrobenzene

The reaction of nitrosobenzene with r-butyl hydroperoxide in benzene or cyclohexane is initiated by hydrocarbon-soluble ]3-diketonates of a number of transition metals [190]. In all cases the reaction product is nitrobenzene which is formed rapidly and quantitatively, equation (138). 

The induction periods observed, the inhibition by traces of 2,6-di-r-butyl-4-methyl-phenol, and the fractional reaction orders appearing in the rate laws suggested a series of radical chain processes. 

Catalytic action by the pivaloylmethane chelate of Co(II), [Co(dpm)2] appears to occur via a two step initiation sequence in which the 2+ state of the metal reacts with hydroperoxide yielding r-BuO , equation (139), after which the 3+ state reacts with a second hydroperoxide, yielding r-BuOO , equation (140). 

The oxidation of Co(II) in the absence of nitrosobenzene has been found to be first order in each reaction component, equation (141). 

The oxidation of nitrosobenzene catalyzed by [Co(dpm)2] closely follows the rate expression (142), where [CoJ = total concentration of added cobalt. 

---

Iron(III) and manganese(III) porphyrins have been found to be effective catalysts for the oxidation of nitroso to nitro compounds, as in, for example, the conversion of nitrosobenzene to nitrobenzene with iodosylbenzene as the oxygen source. 5)... 

Add 4 4 g. of recrystaUised -phenylhydroxylamine to a mixture of 20 ml. of concentrated sulphuric acid and 60 g. of ice contained in a 1 litre beaker cooled in a freezing mixture. Dilute the solution with 400 ml. of water, and boil until a sample, tested with dichromate solution, gives the smell of quinone and not of nitrosobenzene or nitrobenzene (ca. 10-15 minutes). Neutralise the cold reaction mixture with sodium bicarbonate, saturate with salt, extract twice with ether, and dry the ethereal extract with anhydrous magnesium or sodium sulphate. Distil off the ether p-aminophenol, m.p. 186°, remains. The yield is 4-3 g. 

The mechanism of electrochemical reduction of nitrosobenzene to phenylhydroxylamine in aqueous medium has been examined in the pH range from 0.4 to 13, by polaro-graphic and cyclic voltametry. The two-electron process has been explained in terms of a nine-membered square scheme involving protonations and electron transfer steps565. This process is part of the overall reduction of nitrobenzene to phenylhydroxylamine, shown in reaction 37 (Section VI.B.2). Nitrosobenzene undergoes spontaneous reaction at pH > 13, yielding azoxybenzene471. 

Reduction of substituted nitrobenzenes under alkaline conditions, usually with aqueous sodium acetate as electrolyte and a nickel cathode, is the classical method due to Elbs [45] for the formation of azo- and azoxy-compounds. Protons are used in the electrochemical reaction so that the catholyte becomes alkaline and under these conditions, phenylhydroxylamine reacts rapidly with nitrosobenzene to form azoxybenzene. Finely divided copper has long been known to catalyse the reduction of nitrobenzene to aniline in alkaline solution at the expense of azoxybenzene production [46]. Modem work confirms that whereas reduction of nitrobenzene at polycrystalline copper in alkaline solution gives mainly azoxybenzene, if the electrode is pre-oxidised in alkaline solution and then reduced just prior to the addition of nitrobenzene, high yields of aniline are obtained with good current efficiency... 

Azobenzene was also present during the hydrogenation of nitrosobenzene and a stepwise hydrogenation process of nitrosobenzene to azoxybenzene to azobenzene to aniline is a credible route. No azoxybenzene was observed during the hydrogenation of nitrobenzene. 

In this way nitrosobenzene may be obtained from aniline, as has been shown by Bamberger and Tschimer [220], Nitrosobenzene may then be oxidized to nitrobenzene. 

Oxidation of manganous salts in acid solution to red manganic salts is characteristic of true perphosphates. They should not give the characteristic tests of hydrogen peroxide with chromic acid or titanic acid. They oxidise acid aniline solutions to nitrosobenzene and gradually to nitrobenzene. 

Acid-catalyzed electron transfer plays an important role in reduction of not only carbonyl compounds but also other substrates such as O2 [90, 91], N02 [92], nitrobenzene derivatives [93, 94], nitrosobenzene derivatives [93, 94] and sulfoxides [95, 96], The ket value for the photoinduced electron transfer from [Ru(bpy)3] + to nitrobenzene increases parabolically with increase in [HCIO4] [94]. This indicates that PhN02 is doubly protonated in the photoinduced electron transfer reaction to give PhN02H2 + (Eq. 9) ... 

Another interesting example of formation of aminyloxides is the thermal decomposition of benzoylperoxide in benzene in presence of nitrobenzene. Reduction of nitrobenzene to nitrosobenzene to some extent and subsequent spin-trapping of phenyl radical gives diphenylaminyloxide83. ... 

The presence of p-nitrophenol in the urine can be used to indicate exposure to nitrobenzene (Ikeda and Kita 1964). Measurement of p-nitrophenol, however, cannot be used to determine the level of nitrobenzene exposure or if harmful effects can be expected to occur. The nitrobenzene metabolites, nitrosobenzene and phenylhydroxylamine, have been found to bind with hemoglobin in the blood of... 

Nitrosobenzene can be catalytically oxidized to nitrobenzene at 1 bar 0 in acetone, in the presence of cobaloximedi) derivatives as... 

Nitrosobenzene can be converted to nitrobenzene with cobaloxime(II) as catayst. 

Formation of nitrosobenzene may be explained by disproportionation of the radical anion of nitrobenzene to nitrobenzene and nitrosobenzene, reaction (R13). 

Trapping of nitrenes by oxygen has been observed in the deoxygenation of nitrosobenzene, thus leading to nitrobenzene. Photolysis of ferrocenyl azide in benzene or cyclohexane under oxygen provides on of the best methods of preparing nitroferrocene ... 

The radical anion [Fc3(CO)n] reacts with nitro- or nitrosobenzene to yield, after workup, a mixture of azo- and azoxybenzene [250], In order to obtain more stable derivatives, pentachloronitrobenzene was used in place of unsubstituted nitrobenzene. The reaction was found to be very sensitive to the experimental conditions (solvent, identity of the countercation, rate of the addition of the reagents and their concentrations). Apart from Fe(CO)5, a product which was almost always formed was a cluster identified as fFe3(CO)9(p3-NC6Cl5)] . Also formed in several reactions was [Fe3(CO)n] (eq.31) ... 

The reduction of the nitro group to yield aniline is the most commercially important reaction of nitrobenzene. Usually the reaction is carried out by the catalytic hydrogenation of nitrobenzene, either in the gas phase or in solution, or by using iron borings and dilute hydrochloric acid (the Bechamp process). Depending on the conditions, the reduction of nitrobenzene can lead to a variety of products. The series of reduction products is shown in Figure 1 (see Amines byreduction). Nitrosobenzene, /V-pbenylbydroxylamine, and aniline are primary reduction products. Azoxybenzene is formed by the condensation of nitrosobenzene and /V-pbenylbydroxylamine in alkaline solutions, and azoxybenzene can be reduced to form azobenzene and hydrazobenzene. The reduction products of nitrobenzene under various conditions ate given in Table 2. 

Oxidation. Aromatic amines can undergo a variety of oxidation reactions, depending on the oxidizing agent and the reaction conditions. For example, oxidation of aniline can lead to formation of phenyUiydroxylamine, nitrosobenzene, nitrobenzene, azobenzene, azoxybenzene or -benzoquinone. Oxidation was of great importance in the early stages of the development of aniline and the manufacture of synthetic dyes, such as aniline black and Perkin s mauve. 

Phenazine mono-N-oxides have also been prepared from nitrobenzene derivatives. Condensation of nitrobenzene with aniline using dry NaOH at 120-130 °C results in modest yields of phenazine 5-oxide, although the precise mechanism of this reaction is not well understood (57HC(ll)l) with unsymmetrical substrates it is not possible to predict which of the isomeric fV-oxides will be produced. Nitrosobenzene derivatives also function as a source of phenazine mono-fV-oxides thus, if 4-chloronitrosobenzene is treated with sulfuric acid in acetic acid at 20 °C the fV-oxide is formed (Scheme 21). 

Note that the nitrene now allows a common intermediate that can be hydrogenated to anihne from both nitrobenzene and nitrosobenzene. It is also interesting to speculate on another route to form a nitrene on the surface. Phenyl hydroxylamine is a known intermediate whose concentration in solution is highly dependent upon reaction condihons. On adsorption of phenyl hydroxylamine one can write the following ... 

---