Electrolytic reduction of Nitrobenzene

It is prepared by reduction of nitrobenzene with iron and NaOH. It is also prepared by an electrolytic reduction of nitrobenzene. It is widely used for the preparation of benzidine. 

About 250 ml of a reaction mixture obtained by the electrolytic reduction of nitrobenzene in sulfuric acid solution and containing about 23 grams of p-aminophenol by assay is neutralized while at a temperature of 60° to 65°C, to a pH of 4.5 with calcium carbonate. The calcium sulfate precipitate which forms is filtered off, the precipitate washed with hot water at about 65°C and the filtrate and wash water then combined. The solution is then extracted twice with 25 ml portions of benzene and the aqueous phase is treated with 0.5 part by weight, for each part of p-aminophenol present, of activated carbon and the latter filtered off. The activated carbon is regenerated by treatment with hot dilute caustic followed by a hot dilute acid wash, and reused a minimum of three times. 

The electrolytic reduction of nitrobenzene can be carried out conveniently according to the methods described by K. Elbs in tJbungs-beispidefur die dektrolytische Darstdlung chemischer Praparate (Halle a. S 1911). 

Although phenylhydroxylamine may be prepared by catalytic reduction,1 by the oxidation of aniline,2 and by electrolytic reduction of nitrobenzene,3 the most feasible method is still based upon the original zinc reduction method of Bamberger 4 and of Wohl.5 Various solvents and catalysts have been used in this reduction, and copper-coated and amalgamated zinc, as well as aluminium amalgam,6 have been substituted for zinc dust. The method herein recommended is essentially one previously described 7 but it has been found 8 that cooling is not an essential, as claimed in the patents. The preparation of the oxalate is also a more recent contribution.9... 

A short time after the publication of the interesting experiments of Gattermann, A. A. Noyes and A. A. Clement3 made known their studies on the electrolytic reduction of nitrobenzene in sulphuric-acid solution. 

Chloraniline.—Lob 4 has found that p- arid o-chloraniline are obtained by the electrolytic reduction of nitrobenzene suspended in fuming hydrochloric acid, nitrobenzene dissolved in alcoholic hydrochloric acid, and nitrobenzene dissolved in mixtures of hydrochloric and acetic acids. With hydrobromic acid the corresponding bromanilines are formed. 

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... 

TABLE LXXXIV. ELECTROLYTIC REDUCTION OF NITROBENZENE AT CONSTANT POTENTIAL... 

Directions have been given for the electrolytic reduction of nitrobenzene to azobenzene. Azoxy compounds are also formed by this technique. ... 

Derivation (1) By reduction of p-nitrophenol with iron filings and hydrochloric acid (2) by electrolytic reduction of nitrobenzene in concentrated sulfuric acid and treatment with an alkali to free the base. Also available as the hydrochloride. 

Aminophenols, NH2.C6H4.OH.—The three aminophenols may be prepared by reducing the three nitrophenols. p-Amino-phenol (m.p. 170°) is manufactured by the electrolytic reduction of nitrobenzene dissolved in sulphuric acid. Phenylhydroxyla-... 

The electrolytic reduction of nitrobenzene is to be carried out very carefully. 

Electrolytic reduction of nitrobenzene to />-anisidine in a methanol medium... 

Fritz Haber (Breslau, 9 December 1868-Basel, 29 January 1934) studied in Berlin, Heidelberg and Charlottenberg, and worked at first on organic chemistry. In 1894 he became assistant to Bunte at the Technical High School at Karlsruhe, where he became associate professor (1898) and (1906) professor of technical chemistry. Whilst at Karlsruhe he investigated the synthesis of ammonia from its elements (1905, 1915) which afterwards (with the collaboration of Carl Bosch) led to the development of the manufacture of synthetic ammonia by the Badische Co. at Ludwigshafen, although the reaction under pressure (the technical process) was first carried out by Nernst (see above). In 1911 Haber became director of the Kaiser Wilhelm Institute of Physical Chemistry and Electrochemistry at Berlin-Dahlem. He received the Nobel Prize in 1919. He worked on chemical equilibria in flames (1895 f.), the electrolytic reduction of nitrobenzene (1898 f.), autoxidation (1900 f.), the synthesis of nitric oxide in the electric arc (1908 f.), and on many branches of electrochemistry. His books contain useful material, the one on thermodynamics an unsuccessful approach to the Nernst heat theorem. 

Although formally similar to the previous processes, this reaction (Bamberger, 1894) which is often carried through in one step by electrolytic reduction of nitrobenzenes in acid conditions , involves nucleophilic attack at the para (or less favourably the ortho) positions by the solvent (reaction 94). Redox reactions catalysed by oxygen... 

Dimethylaminobenzaldehyde has been made by the condensation of chloral with dimethylaniline, and subsequent hydrolysis 1 by the hydrolysis of tetramethyldiaminobenzhydrol with acetic acid 2 by the condensation of dimethylaniline, formaldehyde and m-sulfo-/>-tolyI hydroxylamine followed by hydrolysis 3 by the electrolytic reduction of a mixture of sodium nitrobenzene sulfonate, dimethylaniline and formaldehyde, and subsequent hydrolysis 4 by the reduction of a mixture of dimethylaniline, formaldehyde and sodium nitrobenzene sulfonate with iron and hydrochloric acid, followed by hydrolysis 5 by the condensation of alloxan with dimethylaniline followed by hydrolysis 6 by the condensation of dimethylaniline, formaldehyde and sodium -toluidine sulfonate in the presence of hydrochloric acid and potassium dichromate followed by hydrolysis.7 The most satisfactory method, however, is the condensation of dimethylaniline, formaldehyde and nitroso dimethylaniline, followed by hydrolysis,8 a method which was first described by E. Noelting and later perfected in detail by L. Baumann. 

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... 

Azoxybenzene has been prepared by reduction of nitrobenzene with alcoholic potassium hydroxide,1 with sodium amalgam,2 with hydrogen in the presence of lead oxide,3 with methyl alcohol and sodium hydroxide,4 with sodium methylate and methyl alcohol,5 and by electrolytic reduction 6 by oxidation of azobenzene with chromic anhydride 7 by treatment of /9-phenylhydroxylamine with alkaline potassium permanganate,8 with nitrobenzene,9 with mineral adds,10 and with mercury acetamide,11 and by oxidation of aniline with hydrogen peroxide,12 and with acid permanganate solution in the presence of formaldehyde.13 The procedure described above is a slight modification of one described in the literature.14... 

Thus far the solvent systems we have discussed are typical protic organic media, such as, for example, water-ethanol mixtures containing an added supporting electrolyte. These solvents are presumably quite homogeneous on a microscopic level. However, a number of solvents have been developed in recent years which are heterogeneous on a microscopic scale. Micellar media are one example of such solvents. The electrochemical reduction of nitrobenzene in aqueous solutions containing polyoxyethylene lauryl ether, a substance known to produce neutral micelles, produces azobenzene (4) even at pH somewhat less than 723. This is apparently the first case of formation of a dimeric product from electrolysis of nitrobenzene (1) in acidic media. Another striking example of this phenomenon... 

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. 

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. 

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... 

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... 

The nature of the electrolyte sometimes has an important influence on the products of electrolytic reduction. The alkalinity or acidity, for example, plays an essential part in determining the nature of the substance obtained in the reduction of nitrobenzene in this case the effect is mainly due to the influence of the hydrogen ion concentration on various possible side reactions. The formation of azoxybenzene, for example, in an alkaline electrolyte is due to the reaction between phenyl-hydroxylamine and nitrosobenzene, viz.,... 

The electrolytic reduction of nitro compounds has played an important role in the development of organic electrochemistry in the classic period about 1900 [1]. It was the investigation of the stepwise reduction of nitrobenzene that led Haber [2] to the idea that the deciding factor in determining the product was the potential of the working electrode, and he was also the first to apply the principle to organic electrolysis. 

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... 

Considerable study has been directed toward the reaction mechanism of catalytic reduction under different conditions. Nord has shown that the reduction of nitrobenzene by hydrogen gas in the presence of colloidal platinum gives rise to the same intermediate compounds that are obtained by metal and acid or electrolytic reductions. The following shows the course of the reaction ... 

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