Electrochemical reduction nitro compounds

Electrolytic Reductions. Both nitro compounds and nitriles can be reduced electrochemically. One advantage of electrochemical reduction is the cleanness of the operation. Since there are a minimum of by-products, both waste disposal and purification of the product are greatiy simplified. However, unless very cheap electricity is available, these processes are generally too expensive to compete with the traditional chemical methods. 

Compounds. When nitro compounds are reduced by electrochemical methods a number of products are possible depending on such factors as the nature of the electrode, the electrode potential, and the reaction media. For the reduction of nitroben2ene these products include aniline, /)-aminopheno1,j -ch1oroani1ine, phenyUiydroxylamine, a2oxyben2ene, a2oben2ene, and hydra2oben2ene (60). 

Neta.1 Ama.lga.ms. Alkali metal amalgams function in a manner similar to a mercury cathode in an electrochemical reaction (63). However, it is more difficult to control the reducing power of an amalgam. In the reduction of nitro compounds with an NH4(Hg) amalgam, a variety of products are possible. Aliphatic nitro compounds are reduced to the hydroxylamines, whereas aromatic nitro compounds can give amino, hydra2o, a2o, or a2oxy compounds. 

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... 

Nitro compounds can be further reduced to hydrazo compounds with zinc and sodium hydroxide, with hydrazine hydrate and Raney nickel,or with LiAlH4 mixed with a metal chloride such as TiCU or VCl3. The reduction has also been accomplished electrochemically. 

Electrochemically generated nickel is very selective for the reduction of aromatic nitro compounds into anilines, in which alkenyl, alkynyl, halo, cyano, formyl, and benzyloxy groups are not affected.84 Sodium sulfide has been used for the selective reduction of aromatic nitro group in the presence of aliphatic nitro groups (Eq. 6.44).85... 

Electrochemical reduction of benzylic nitro compounds (27) in an ethanolic aqueous acetic acid buffer (35 65) affords a mixture of the corresponding oxime and hydroxylamine (equation 6)48. The hydroxylamine can subsequently be oxidized back to the oxime (28) (via the intermediate nitroso compound) conversions as high as 90% can be obtained. 

Electrochemical reduction of the a, -unsaturated nitro compounds 32 in acetonitrile containing tetraethylammonium tosylate affords the corresponding hydrodimer in 37%... 

As can be seen from the preceding discussion, the existence of nitroso compounds as intermediates in the electrochemical reduction of nitro compounds is mostly inferential nitroso compounds are easier to reduce than nitro compounds. Hence, they should be reduced as quickly as they are formed and would not be expected to be isolable. However, nitroso compounds have occasionally been isolated in unusual structural cases54 and the nitrosobenzene radical anion has been identified by ESR spectroscopy in at least one instance64. It is possible to prepare nitroso compounds by a two-step sequence one reduces the nitro compound electrochemically to the hydroxylamine, then electrochemi-cally oxidizes the hydroxylamine to the nitroso compound65. 

The mechanism of the global 4-electron electrochemical reduction of aromatic nitro compounds to hydroxylamines in aqueous medium shown in reaction 37 was investigated by polarography and cyclic voltametry. The nitro group is converted first to a dihy-droxylamine, that on dehydration yields a nitroso group the latter is further reduced to a... 

Electrochemical reductions of several ortho-substituted nitro compounds of the general formula 0-O2NC6H4CH = CR R (Scheme 73) [47,106, 107] or o-O2NC6H4COCHRIR2 (Scheme 74) [108], in which and are electron-... 

Barek et al. have reported on the determination of AT-nitroso compounds, azo compounds, heterocychcs, aromatic nitro compounds, heterocychc amines and even benzyl chloride using electrochemical methods such as voltammetry and polarog-raphy. The nitro and AT-nitroso compounds work particularly well in reductive mode [47, 48]. For appropriate analytes, adsorptive stripping voltammetry and anodic stripping voltammetry can give orders of magnitude lower detection hmits than are available from HPLC with electrochemical detection [48]. 

Furthermore, aromatic nitro compounds may be reduced to hydroxylamines by tita-nium(in) chloride or ammonium sulfide ", as well as by electrochemical reduction . 

In this paper, the selectivity of the ECH method for the reduction of nitro compounds to the corresponding amines on RCu electrodes will be compared with that of reduction by RCu alloy powder in alkaline aqueous ethanol. In the latter method (termed chemical catalytic hydrogenation (CCH)), chemisorbed hydrogen is generated in situ but by reduction of water by aluminium (by leaching of the alloy) (equation [12]). The reductions by in situ leaching must be carried out in a basic medium in order to ensure the conversion of insoluble Al(OH)3 into soluble aluminate (equation [12]). The selectivity and efficiency of the electrochemical reduction of 5-nitro-indoles, -benzofurane, and -benzothiophene at RCu electrodes in neutral and alkaline aqueous ethanol will also be compared with that of the classical reduction with zinc in acidic medium. 

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