Azoxybenzene, reduction

Further nitration gives w-dinilrobenzene sulphonation gives w-nitrobenzene sulphonic acid. Reduction gives first azoxybenzene, then azobenzene and aniline depending upon the conditions. Used in the dyestufTs industry as such or as aniline. 

N-phenylhydroxylamine, PhNHOH and further reduction can give azoxybenzene, azobenzene, hydrazobenzene and aniline. The most important outlet commercially for the nitro-compounds is the complete reduction to the amines for conversion to dyestufTs. This is usually done in one stage with iron and a small amount of hydrochloric acid. 

The azoxybenzene in turn, when heated with iron filings, readily undergoes C4HjNO NC,Hs + Fe = C.HjNtNC.Hs + FeO further reduction to azobenzene. 

Azoxybenzene is readily prepared by reduction of nitrobenzene in an alkaline medium with dextrose or sodium arsenite ... 

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. 

Fig. 1. Reduction products of nitrobenzene (1) nitrosobenzene [98-95-3] (2) /V-pbenylbydroxyl amine [100-65-2] (3) aniline [62-53-3] (4) azoxybenzene...

In the reduction of nitro compounds to amines, several of the iatermediate species are stable and under the right conditions, it is possible to stop the reduction at these iatermediate stages and isolate the products (see Figure 1, where R = CgH ). Nitrosoben2ene [586-96-9] C H NO, can be obtained by electrochemical reduction of nitrobenzene [98-95-3]. Phenylhydroxylamine, C H NHOH, is obtained when nitrobenzene reacts with ziac dust and calcium chloride ia an alcohoHc solution. When a similar reaction is carried out with iron or ziac ia an acidic solution, aniline is the reduction product. Hydrazobenzene [122-66-7] formed when nitrobenzene reacts with ziac dust ia an alkaline solution. Azoxybenzene [495-48-7], C22H2QN2O, is... 

Azoxybenzene from Nitrobenzene by Electrolysis.— Nitrobenzene can be conveniently converted into azoxybenzene by electrolytic reduction. The apparatus required is shown in Fig. 77. 

Because aromatic nitro compounds such as nitrobenzene had been reduced by hexamethyldisilane 857 at 240 °C to give azobenzene and aniline [84], we slowly added hexamethyldisilane 857 in THF to a solution of nitrobenzene and 0.05 equivalents of Bu4NF-2-3H20 and obtained, via the probable intermediates 1000-1002, azobenzene in 84% yield [85]. Because azoxybenzene 961 affords azobenzene in 95% yield, azoxybenzene 961 is a probable intermediate in the reduction of nitrobenzene [85] (Scheme 7.26). 

A similar reduction of nitrobenzene with (Me3Si)2Hg to give azobenzene and azoxybenzene has been described [86]. The dehydration of tetrabutylammonium fluoride di- or trihydrate by hexamethyldisilane 857 is discussed in Chapter 13. 

The highly oxophilic nature of the cobalt powder was readily demonstrated by its reaction with nitrobenzene at room temperature. Reductive coupling was quickly effected by 2 to give azo- and azoxy derivatives. Nitrobenzene reacted with 2 to give azobenzene in yields up to 37%. In some cases small amounts of azoxybenzene were also formed. With 1,4-diiodonitrobenzene, 2 reacted to give low yields of 4,4-diiodoazoxy-benzene and 4,4-diiodoazobenzene. 

During the reductive carbonylation of azoxybenzene to N-Ph urethane a possible key intermediate was isolated, i.e., (151).577... 

The least powerful method of reduction, boiling nitrobenzene with sodium methoxide in solution in methyl alcohol, provides azoxybenzene in excellent yield (Zinin) the methoxide is converted into formate. (Write the equation.)... 

Since azoxybenzene is attacked by more powerful reducing agents, e.g. zinc dust and sodium hydroxide solution or ammonia, the use of such agents converts nitrobenzene to azobenzene and hydrazobenzene, by passing at once beyond the azoxybenzene stage. The three reduction products with paired " nitrogen atoms, therefore, stand in very close genetic relation to each other. 

Azoxybenzene (1 g.) is dissolved in 5 c.c. of alcohol, the solution is heated to boiling, and 3 c.c. of 50 per cent sodium hydroxide solution and 2-3 g. of zinc dust are added with shaking. At first the mixture becomes red, because of the formation of azobenzene, but on more prolonged boiling a colourless solution is obtained just as in the reduction of nitrobenzene. When this stage has been reached, the mixture is filtered with suction through a small Buchner funnel and the hydrazobenzene is finally isolated in the manner described on p. 183 et seq. 

The addition of hydroxyde ion to nitrosobenzene produces azoxybenzene186. Three techniques (electronic absorption spectroscopy, linear sweep voltammetry and d.c. polarography) have been used to study the equilibrium between nitrosobenzene and hydroxyde ions. The probable reaction pathway to obtain azoxybenzene is indicated by Scheme 4. The importance of the nitroso group in the reduction of nitro derivatives by alkoxide ions, when the electron-transfer mechanism is operating, has been explained187. 

The catalytic effect of tetra-n-butylammonium fluoride in the homogeneous reduction of heterocyclic A-oxides and nitroarenes by hexamethyldisilane in tetra-hydrofuran can occur with EXPLOSIVE violence, but can be controlled by the slow addition of the disilane to the A-oxide (or nitroarene) and tetra-n-butylammonium fluoride to yield the parent heterocycle (>70%) (or azobenzene 84%). In a similar manner, azoxybenzene is converted into azobenzene (95%), and 4-nitropyridine-l-oxide, is reduced to azoxypyridine-l,l -dioxide (78%), with minor amounts of azopyridine-1, l -dioxide and azopyridine-1-oxide [5,6]. 

Photolytic. Irradiation of trifluralin in hexane by laboratory light produced a,a,a-trifluoro-2,6-dinitro-A-propyl-jo-toluidine and a,a,a-trifluoro-2,6-dinitro-p-toluidine. The sunlight irradiation of trifluralin in water yielded a,a,a-trifluoro-A, 7 -dipropyl-5-nitrotoluene-3,4-diamine, a,a,a-trifluoro-A/ ,A/ -dipropyltoluene-3,4,5-triamine, 2-ethyl-7-nitro-5-(trifluoromethyl)benzimidazole, 2,3-dihydroxy-2-ethyl-7-nitro-l-propyl-5-(trifluoromethyl)benzimidazoline, and 2-ethyl-7-nitro-5-(trifluoromethyl)benzimidazole. 2-Amino-6-nitro-a,a,a-trifluoro-p-toluidine and 2-ethyl-5-nitro-7-(trifluoromethyl)benzimidazole also were reported as major products under acidic and basic conditions, respectively (Crosby and Leitis, 1973). In a later study, Leitis and Crosby (1974) reported that trifluralin in aqueous solutions was very unstable to sunlight, especially in the presence of methanol. The photodecomposition of trifluralin involved oxidative TV-dealkylation, nitro reduction, and reductive cyclization. The principal photodecomposition products of trifluralin were 2-amino-6-nitro-a,a,a-trifluoro-jo-toluidine, 2-ethyl-7-nitro-5-(trifluoromethyl)benzimida-zole 3-oxide, 2,3-dihydroxy-2-ethyl-7-nitro-l-propyl-5-(trifluoromethyl)benzimidazole, and two azoxybenzenes. Under alkaline conditions, the principal photodecomposition product was 2-ethyl-7-nitro-5-(trifluoromethyl)-benzimidazole (Leitis and Crosby, 1974). 

The behavior of the same azoxybenzene is studied in homogeneous conditions— when the dipotassium salt of cyclooctatetraene dianion (CgHgKj) acts as a dissolved electrode. In this case, the reduction of azoxybenzene stops at the very first stage, that is, after the transfer of one electron only (Todres et al. 1975). The initial one-electron reduction produces the azoxybenzene anion-radicals, which are not reduced further despite the presence of residual electron donor in the solution. The ESR method does not reveal these anion-radicals although one-electron oxidation by phenoxyl radicals quantitatively regenerates azoxybenzene and produces the corresponding potassium phenolate molecules in a quantitative yield. Treatment with water leads to a 100% yield of azobenzene (Scheme 2.14). 

The diamagnetic complex is not reduced further by the cyclooctatetraene dianion. This prevents the conversion of the azoxybenzene anion-radicals into azodianions. Potassium cation plays an important role in this limitation of the reduction process, which, generally, proceeds readily (the... 

The removal of potassium cations makes the results of the liquid-phase and electrode reactions similar. In the presence of crown ether, the eight-membered complex depicted in Scheme 2.16 is destroyed. The unprotected anion-radicals of azoxybenzene are further reduced by cyclooctatet-raene dianion, losing oxygen and transforming into azodianion. The same particle is formed in the electrode reaction shown in Scheme 2.13. In the chemical reduction, stabilization of azodianion is reached by protonation. Namely, addition of sulfuric acid to the reaction results in the formation of hydrazobenzene, which instantly rearranges into benzidine (4,4 -diamino-l,l"-diphenyl). The latter was isolated from the reaction, which proceeded in the presence of crown ether. 

Azoxybenzene was synthesized in 85% yield by reduction of nitrobenzene with sodium arsenite [221]. Nitrotoluenes and 2,5-dichloronitrobenzene were converted to the corresponding azoxy compounds by heating to 60-90° with hexoses (yields up to 74%) [316. Some ring-substituted nitrobenzenes were converted to azoxy compounds, some other to azo compounds by sodium bis 2-methoxyethoxy)aluminum hydride [575]. 

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... 

Both the copper and nickel surfaces are efficient for the electrochemical reduction of nitrobenzene to aniline. With time however, the properties of this surface are transformed to those of the polycrystalline metal and azoxybenzene becomes as major reduction product [49]. 

Nitrosobenzene and phenylhydroxylamine condense rapidly in alkaline solution to give azoxybenzene. During the reduction of nitrobenzenes at high pH, the phenylhydroxylamine scavenges nitroso compound so that the azoxybenzene becomes... 

They can be isolated in good yields by reduction of the nitrobenzene in aqueous ethanolic sodium acetate under reflux, passing around 10% excess electric charge [103]. Any hydrazobenzene formed is rapidly oxidised back to the azobenzene by air during work-up. Azoxybenzene is formed first and then reduced to azobenzene and finally hydrazobenzene at the cathode. A solution electron transfer reaction between azoxybenzene and the hydrazobenzene reforms azobenzene. 

Fermenting yeast is able to reduce added nitrobenzene to the corresponding amine, aniline, to quite a considerable extent. Part of the added nitrobenzene remains unattacked, but 70% of it could be converted to aniline. Since a direct reduction of the nitro group to the amino group is improbable, Neuberg and Welde tried phytochemical treatment of the possible intermediaries, namely, nitrosobenzene and phenylhydroxylamine on the one hand and azoxybenzene and azobenzene on the other hand. 

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