Nitrobenzene, cathodic reduction

Titanium ions can also he used as redox catalysts for the indirect cathodic reduction of nitro compounds (417). The electroreduction is carried out in an H20-H2S04/Ti(S04)2-(Pb/Cu) system at 45 80°C under 5 20Am . Nitrobenzene, dinitrobenzene, nitrotoluene, 2,4-dinitrotoluene, 2-nitro-m-xylene, nitro-phenol, 2,4-dinitrophenol, nitrophenetole, o-nitroanisole, 4-nitrochlorotoluene, ni-trobenzenesulfonic acid, and 4,4 -dinitro-stilbene-2,2 -disulfonic acid can all be reduced by this procedure to the corresponding amino compounds (418) in good yields (Scheme 146) [513-516]. Tin... 

The counterpart of anodically electrocatalyzed oxidation by redox oxides, namely the cathodic reduction of organic substrates by surface-coup led redox system with sufficiently negative redox potential, is almost unknown. Beck reports that specially prepared TiO coating on Ti-electrodes can be reduced cathodically and that the electrogenerated Ti(III) and Ti(II) species do in fact reduce nitrobenzene to aniline (207). 

Non-Reversible Processes. —Reactions of the non-reversible type, i.e., with systems which do not give reversible equilibrium potentials, occur most frequently with un-ionized organic compounds the cathodic reduction of nitrobenzene to aniline and the anodic oxidation of alcohol to acetic acid are instances of this type of process. A number of inorganic reactions, such as the electrolytic reduction of nitric acid and nitrates to hydroxylamine and ammonia, and the anodic oxidation of chromic ions to chromate, are also probably irreversible in character. Although the problems of electrolytic oxidation and reduction have been the subject of much experimental investigation, the exact mechanisms of the reactions involved are still in dispute. For example, the electrolytic reduction of the compound RO to R may be represented by... 

In 1898, in a study of the cathodic reduction of nitrobenzene, Haber discovered that by varying the electrode potential he could obtain either azoxybenzene or hydrazobenzene. This led him to make the following significant conclusions The electric current up to this time has been regarded in organic electrochemistry as a... 

BeckF, Gabriel W (1985) Heterogeneous redox catalysis at titanium/titanium dioxide cathodes. Reduction of nitrobenzene. Angew Chem 97 765-767... 

A process for the direct reduction of nitrobenzene to -p-ammophenol, an important intermediate for the production of dyes, depends on the above interesting transformation. Nitrobenzene in alcoholic solution is mixed with concentrated sulphuric acid and electrolysed with a lead cathode. This process proves that phenylhydroxylamine is also an intermediate in the reduction of nitrobenzene in acid solution, as was mentioned above. Here, as a result of the rapidity of the rearrangement which takes place, it is not converted into aniline. 

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... 

Conversion of substituted nitrobenzenes to the arylhydroxylamine is easily achieved by reduction in neutral or slightly acid solution. In the first classical experiments, Haber [35] used a platinum cathode and ammonia ammonium chloride buffer and die process was improved by Brand [57] using either a nickel or silvered copper cathode in an acetate buffer. The hydroxylamine can also be obtained from reduction in dilute sulphuric acid provided tire temperature is kept below 15° C to suppress furtlier reduction [58]. This electrochemical route to arylhydroxylamines due to Brand is superior to the chemical reduction using zinc dust and ammonium chloride solution. The latter process is known to give variable yields depending on... 

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. 

The use of a fixed-bed cell with Cu particles as the cathode 576 has been suggested for the synthesis. p-Aminophenol, produced by electrochemical reduction of nitrobenzene, was used for the synthesis of hydroquinone 577). According to recent work, the addition of emulsifiers, for example, trialkylamine oxides 578), is supposed to suppress the formation of aniline as a byproduct. The electrosynthesis of p-amino-phenol from nitrobenzene is carried out industrially in India 276). 

The catalytic effect of copper is shown in the reduction of nitrobenzene, which at a copper cathode is reduced to aniline, but while copper sponge under ordinary chemical conditions will reduce phenyl-hydroxylamine to aniline it has no effect upon nitrobenzene, and the inference is that in electrolytic reduction phenylhydroxylamine may be first formed by electrolysis, and this substance is then converted to aniline largely by the catalytic effect of the copper cathode. 

This work was extended by Elbs and his pupils,1 and the processes were protected by patents.2 Subsequently alcohol was dispensed with and aqueous caustic soda employed. For example, an emulsion of nitrobenzene in 10 per cent, aqueous sodium hydroxide may be reduced with a cathode of lead or nickel in a porous earthenware cell, with a current density of 10-12 amps, per dm.2 An anode of graphite or lead may be employed in an outer containing vessel filled with sodium hydroxide solution or sodium sulphate. Azo- or hydrazobenzene is obtained according to the quantity of electricity passed through, and the azobenzene emulsion can be transformed into benzidine by acidifying the cathode liquor and completing the reduction.4... 

Elbs, on the contrary, obtained entirely different results when he electrolytically reduced p-nitrotoluene and nitrobenzene in acid and in alkaline solution with other cathode metals. There were formed in the reduction of nitrobenzene in alkaline solution at a lead or mercury cathode varying quantities of azoxy- and azobenzene, the former mostly preponderating. p-Nitrotoluene behaves similarly if reduced in the same manner, —p-azoxy- and p-azololuem being produced. The reduction takes place much more slowly and less completely in this case than when nitrobenzene is used. Haussermann observed the same with o-nitrotoluene, o-Nitrophenol behaves quite differently the chief product is o-amidophenol, besides red and brown substances which could not be obtained pure. In the reduction of nitrobenzene in sulphuric-acid solution Elbs employed a zinc cathode and obtained chiefly aniline. 

It is very evident, from all these observations, what influence the cathode material exercises on the obtainable reduction phase of nitrobenzene and its derivatives. There is no lack of attempts to explain this influence. The expressed opinions can be grouped under three points of view ... 

In the reduction of nitrobenzene in a 2% aqueous sodium-hydroxide solution, according to previous publications, azoxy-benzene is formed at platinum and nickel electrodes, azobenzeno at lead, tin, and zinc cathodes, and aniline at copper cathodes especially in the presence of copper powder. It was found that, in an unchangeable experimental arrangement, a cathodo potential of 1.8 volts, as measured in connection with the deci-normal electrode, could be carried out with all the chosen cathodes and additions. At this constant potential, by using different metals and adding various metallic hydroxides, the whole reduction was carried out and the nature and quantity of the reduction products determined in each case. It turned out that the emphasized differences in the results disappeared and that, with an equal potential of all cathodes, similar yields of azoxybenzene and aniline and traces of azobenzene resulted. The cathodes were of platinum, copper, copper and copper powder, tin, platinum with addition of stannous hydroxide zinc, platinum with addition of zinc hydroxide, lead, platinum with addition of lead hydroxide, and nickel. The yields of azoxybenzene varied from 41-65% of aniline 23-53%. 

Considering the trifling quantity of the product started with which had to be chosen in order to at all carry out the experiments, and considering the difficulty with which air accurate quantitative separation and determination of the reduction products could be carried out, the proposition that, can be laid down as a sure result of the above is that the cathode potential ns the measure for the reduction energy for nitrobenzene when a 2% sodium-hydroxide solution is employed as electrolyte. 

Lob,3 convinced of the futility of thus being able to obtain a good yield of benzidine by a direct reduction of nitrobenzene in acid solution, sought to carry out the benzidine process by a careful realization of the conditions theoretically required— primary preparation of azoxy- or azobenzene in the best quantitative yields, i.e. in electrolytes, containing alkali or alkali-salt, then reducing these products in acid solution. Two processes thus resulted. In the first one the electrolytic reduction was carried out to azobenzene in alcoholic-alkaline solution, then the cathode solution was acidified with sulphuric acid, and the further reduction and molecular rearrangement combined in one operation. The second process, which was... 

Sometimes a cathodic substitution reaction takes place during the reduction of ortho-substituted nitrobenzenes thus the reduction of 2-nitrophenoxyacetic acid (21) in 2 N hydrochloric acid (50% ethanol) yielded 5-chloro-2H-l,4-benzoxazin-3(4H)-one (22), Y = C1) rather than the expected cyclic hydroxamic acids02 in the presence of another nucleophile Y (e.g., thiocyanate) ring substitution with this reagent occurs. 

In addition to the method described, which is the one commonly used, the reduction can also be effected with iron shavings. This method, however, is difficult to carry out. There is also the electrolytic method in which nitrobenzene, in suspension in sodium hydroxide solution, is reduced to l drazobenzene at the cathode. So far as is known, this method is used only by the GeseUschaft fur Chemische Industrie in Basel. It does not require zinc dust and gives good yields. The best results are obtained when the specific gravity of the hydroxide solution equals that of m trobenzene. 

Interesting results were obtained by Lob and Moore (1904) in connection with the electrolytic reduction of nitrobenzene in an alkaline medium. They employed different cathode materials, but adjusted the current density so that the potential was the same, viz., — 1.5 volts, in each case. The main products were azoxybenzene and aniline, and the current efficiencies for these two substances are given in Table LXXXIV... 

Nitrobenzene is reduced in aqueous medium at mercury and gold at low pH in two waves, and at high pH in one wave [94]. In CV in neutral buffer is found a four-electron reduction peak to phenylhydroxylamine, which on the anodic sweep is oxidized to nitrosobenzene on the second cathodic sweep the reduction of nitrosobenzene is observed. The response at... 

In 1898 Haber published a textbook on electrochemistry and promoted his research to relate chemical research to industrial processes, showing his results on electrolytic oxidation and reduction. That same year he explained the reduction of nitrobenzene in stages at the cathode, which became the model for other similar reduction processes. He continued electrochemical... 

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