Hydrogenation of nitrosobenzene

A similar nitrene intermediate can also be postulated in the mechanism of nitrosobenzene hydrogenation. Indeed a standard way of producing Ph-N is from the reaction of Ph-NO with PPhs (17). In the hydrogenation of nitrosobenzene the principal product in the early stages is azoxybenzene [7, 18]. It was suggested that azoxybenzene was formed by the following sequence ... 

FIGURE 2.39 Product distribution during hydrogenation of nitrosobenzene over Pd black at ambient temperatures in methanol. 

A mild structure sensitivity accompanies hydrogenation of nitrosobenzene over a series of dispersed 1% Pd/Si02.294 The lower dispersed catalysts (larger particle sizes) catalyze faster rates, suggesting that planes are more active than either edges or corners for catalyzing the hydrogenation of nitrosobenzene. 

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. 

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. 

In the solid state almost all nitroso-compounds are colourless,1 but when fused or in solution they are blue or green. Determinations of the molecular weight of nitrosobenzene in liquid hydrogen cyanide have shown (Piloty) that the colourless form is bimolecular. The NO-groups of two molecules are loosely united in one of the ways indicated by the following formulae ... 

Atomic charges, effective charges at reacting centres, and HOMO and LUMO energies have been calculated for nitrobenzene, nitrosobenzene, A-phenylhydrazine, diphenyldiazine, A,A -diphenyldiazine-A-oxide, and A,A -diphenyUiydrazine, compared with kinetic data for the hydrogenation of these compounds, and used to propose a mechanism for the hydrogenation of nitrobenzene. 

The enthalpy of reaction 38 is ca 13 kJ mol" for the case of cyclohexanone wherein R R = —(CH2)5—. This difference is compatible with intermolecular hydrogen bonding as would be found for the benzoquinone oxime tautomer. In order to understand the energetics of reaction 39, we derived the solid phase enthalpy of formation of nitrosobenzene... 

The hydrogenation of nitrobenzene, nitrosobenzene and azobenzene has been studied singly and competitively. A kinetic isotope effect was observed with nitrobenzene but not with nitrosobenzene. Nitrosobenzene inhibits nitrobenzene hydrogenation in a competitive reaction, whereas azobenzene and nitrobenzene co-react but at lower rates. Taken together a more detailed mechanistic understanding has been obtained. 

The hydrogenation of nitrobenzene and nitrosobenzene were also studied using deuterium. The hydrogen/deuterium up-take graphs are shown in Figures 2 and 3. 

The hydrogenation of nitrobenzene and nitrosobenzene are complex and a range of factors can influence by-product reactions, e.g. hydrogen availability, support acid/base properties (13,14). In this study we have examine competitive hydrogenation between nitrobenzene, nitrosobenzene and azobenzene. This methodology coupled with the use of deuterium has further elucidated the mechanism of these reactions. 

Surprisingly, V-containing molecular sieves were not active in the oxidation of aniline with hydrogen peroxide, even when the reaction was performed in various solvents. This could be easily understood over VAPO-5 because of the hydrophobic character of the framework. Nevertheless, the lack of activity over VS-1 was more unexpected. In contrast the use of TBHP converted aniline into nitrobenzene (NB) over VAPO-5 and V-HMS. Traces of nitrosobenzene (NSB) were detected at the beginning of the reaction but AZY was never observed. 

The preparation of nitrosobenzene by oxidation of phenylhydroxylamine with dichromate-sulfuric acid has been described by Gattermann and Wieland.la 2,6-Dihaloanilines can be oxidized to nitroso compounds in good yield by a mixture of 30% hydrogen peroxide and glacial acetic acid.205 Nitroso compounds are also obtained in good yield by oxidation of aryl-hydroxylamines with diethyl azodicarboxylate.206... 

The same group was able to perform a similar cycloaddition reaction but with complete reversal of the regioselectivity (Scheme 11.45) [126]. They revealed that the O vs. N selectivity is dependent on the presence or absence of a weak hydrogen-bond donor in the catalyst (OH group), which apparentiy coordinates the oxygen atom of nitrosobenzene to facilitate the formation of iV-nitroso aldol products. Nucleophilic attack of the preformed enamine is postulated to occur from the Re face as described in Scheme 11.45. For steric reasons the subsequent intramolecular 0-Michael addition just occurs when R are CH3 groups. 

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 ... 

The arylhydroxylamines are weak bases which dissolve in dilute acids to form salts. The instability of phenylhydroxylamine has three causes the effect of atmospheric oxygen, of alkalis, and of acids. On exposure to air the compound, especially if impure, is oxidised to nitrosobenzene, which can be detected in decomposing phenylhydroxylamine by its pungent odour. As is often the case, the velocity of this process, which is known as autoxidation, is increased by alkalis it is accompanied by the production of hydrogen peroxide according to the equation ... 

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