Phenylhydroxylamine, from nitrobenzene

In neutral or alkaline solution the conditions are altered so as to favour the immediate precursor of the final product of hydrogenation, namely, phenylhydroxylamine. This compound is obtained from nitrobenzene, suspended in ammonium chloride solution, by reduction with zinc dust. Zinc dust can decompose water with the formation of Zn(0H)2 if a substance is present which takes up the liberated hydrogen. Molecular, i e. ordinary, oxygen is capable of doing this and is thereby converted into hydrogen peroxide (M. Traube) ... 

When reduced by electrolysis, nitrobenzene and its homologues yield the same products as may be obtained by the various chemical methods of reduction. Aniline, azobenzene, azoxybenzene, hydrazobenzene, and -amino-phenol, as well as phenylhydroxylamine, can thus be obtained from nitrobenzene, and most, if not all, of these products could be prepared satisfactorily on an industrial scale by electrolysis, by adjusting the manner of working so that economy of energy is combined with maximum yields. Many of these products demand a comparatively high price, so that low power cost is not so important in this class of manufacture as high percentage yields. 

Condensation Products with Aldehydes.—Gattermann1 has obtained direct proof of the intermediate formation of phenylhydroxylamine in the preparation of amidophenol by adding bcnzaldehyde to the solution at the beginning of the electrolysis. He was thus able to isolate a condensation product of phenylhydroxylamine with benzaldehyde. In this way he obtained from nitrobenzene benzylidene-phenylhydroxylamine,... 

Electrolytic reduction, as in the preparation of hydrazobenzene from nitrobenzene, or of p-aminophenol from nitrobenzene. The latter reduction involves a simultaneous rearrangement of the intermediate phenylhydroxylamine. 

Nitrosobenzene (3) can be prepared by oxidation of aniline by H2SO5 (see Section 7.2.2) and by oxidation of phenylhydroxylamine (4) with potassium dichromate. Phenylhydroxylamine is available from nitrobenzene by reduction with zinc dust and aqueous ammonium chloride (Scheme 7.10). 

The preparation of N-phenylhydroxylamine in high yields from nitrobenzene under catalytic transfer hydrogenation conditions is also possible utilizing wet 5% rhodium on carbon and hydrazine hydrate. Unfortunately, the transition metal catalysts tend to be expensive and the high temperatures required can be detrimental, particularly when the resulting hydroxylamines are explosive in nature. ... 

Zinc and Weak Alkali The mechanism of reduction with zinc and strongly alkaline solutions leads to the formation of azoxybenzene as the first stable product in the reduction of nitrobenzene. Bamberger has shown that, when faintly alkaline systems are employed, the principal initial stable product of reduction is N-phenylhydroxylamine. The systems —zinc + calcium chloride and zinc + ammonium chloride—are not neutral but quite alkaline to phenolphthalein. In the preparation of N-phenylhy-droxylamine from nitrobenzene by means of zinc dust and aqueous calcium chloride, the reduction liquid has a pH Between 10.5 and 11.7, which corresponds to the alkalinity of 0.01 N alkali. When NH4CI is substituted for CaCb in the reducing system, the pH is between 8 and 9. 

By reduction, it is possible to pass from nitrobenzene to aniline through the intermediate compounds nitrosobenzene and phenyl-hydroxylamine, and by oxidation in the reverse direction. It has been shown that nitrosobenzene and phenylhydroxylamine are formed in the electro-reduction of nitrobenzene, but it is impossible, at present, to effect some of the transformations indicated above in such a way that the reactions can be used as preparative methods. Some, however, can be used for this purpose. Phenylhydroxylamine, for example, is oxidized in the cold by potassium bichromate and sulphuric acid to nitrosobenzene, and is converted by reducng agents into aniline. In certain cases it is possible, by selecting the proper reagent, to effect the change indicated by two of the steps noted above. Phenylhydroxylamine is best prepared by treating with zinc dust a solution of nitrobenzene in dilute alcohol which contains a small amount of calcium chloride. 

The electrochemical reduction of nitrobenzene is believed to be operating commercially in India where aniline is the required product, and in Japan where 4-aminophenol is the desired product.The latter process relies on the in situ rearrangement of phenylhydroxylamine, the conditions of electrolysis being selected to maximize the production of this intermediate in the process. Also at BASF it is believed that a pilot plant synthesis of 4-anisidine from nitrobenzene is being operated. This process relies not only on in situ rearrangement of phenylhydroxylamine but also on in situ methylation of... 

The chemical production of aminophenols via the reduction of nitrobenzene occurs in two stages. Nitrobenzene [98-95-3] is first selectively reduced with hydrogen in the presence of Raney copper to phenylhydroxylamine in an organic solvent such as 2-propanol (37). With the addition of dilute sulfuric acid, nucleophilic attack by water on the aromatic ring of /V-phenylhydroxylamine [100-65-2] takes place to form 2- and 4-aminophenol. The by-product, 4,4 -diaminodiphenyl ether [13174-32-8] presumably arises in a similar manner from attack on the ring by a molecule of 4-aminophenol (38,39). Aniline [62-53-3] is produced via further reduction (40,41). 

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. 

In aliphatic amines (diethylamine or triethylamine) the intramolecular hydrogen abstraction is quenched almost completely. Instead, smooth photoreduction of the nitro group without participation of the side chain is observed with 1,3,5-tri-fezf-butyl-2-nitrobenzene (5) and 14, R = C(CH3)3 ). Products derived from the respective phenylhydroxylamines were isolated in both cases. Again, an electron transfer, which does not seem to suffer from steric restrictions, is operative (see also Section A. 1.3). 

Pertechnetate in neutral and alkaline media can be extracted into solutions of tetra-alkylammonium iodides in benzene or chloroform. With tetra-n-heptylammo-nium iodide (7.5 x 10 M) in benzene distribution coefficients up to 18 can be obtained . A solution of fV-benzoyl-iV-phenylhydroxylamine (10 M) in chloroform can be used to extract pertechnetate from perchloric acid solution with a distribution coefficient of more than 200, if the concentration of HCIO is higher than 6 M The distribution of TcO between solutions of trilauryl-ammonium nitrate in o-xylene and aqueous solutions of nitrate has been measured. In 1 M (H, Li) NOj and 0.015 M trilaurylammonium nitrate the overall equilibrium constant has been found to be log K = 2.20 at 25 °C. The experiments support an ion exchange reaction . Pertechnetate can also be extracted with rhodamine-B hydrochloride into organic solvents. The extraction coefficient of Tc (VII) between nitrobenzene containing 0.005 %of rhodamine-B hydrochloride and aqueous alcoholic " Tc solution containing 0.0025 % of the hydrochloride, amounts to more than 5x10 at pH 4.7 . 

Ffgure 11.2. Flow celt for conversion of nilrobenzencs to the nitrosobenzene. Both of the porous electrodes are constructed from carbon fibre. They arc fed with constant current as indicated with ib, = 2 i,.. The feedstock containing nitrobenzene is introduced at a rate coiresponding to the current ib for reduction to phenylhydroxylamine. The outflow contains nitrosobenzene, sec ref (62j. 

The well-known photorearrangement of urtAo-nitrobenzaldehyde to orfAo-nitrosobenzoic acid458 may well involve hydrogen abstraction as the initial photoprocess. Nitrobenzene is photoreduced in reactive solvents to iV-phenylhydroxylamine,459 and the intermolecular hydrogen abstraction apparently proceeds from the triplet state, since, when perfluoronaphthalene is present in the sample, the ESR spectrum of the PhN02H radical is replaced by the typical triplet ESR spectrum of the naphthalene.460... 

The moist phenylhydroxylamine obtained from 1000 g. of nitrobenzene, by the method described on page 57, is weighed and dissolved in 4.5 1. of ordinary ether (Note 1). The ether-insoluble material (sodium chloride and water) is also weighed, the difference between the two weighings being a fairly accurate measure of the amount of phenylhydroxylamine in solution. 

Cupferron has usually been prepared from a mixture of alkyl nitrite and /3-phenylhydroxylamine in the presence of ammonia in ether or benzene solution,6 but it has also been made by the zinc dust reduction of nitrobenzene in the presence of amyl nitrite and ammonium hydroxide solution.7... 

Zinc residues from reduction of nitrobenzene to /V-phenylhydroxylamine are often pyrophoric and must be kept wet during disposal. 

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