Electroreduction of nitrobenzene

The preparative electroreduction of nitrobenzene (443) is performed in an aqueous HCl/ZnCl2-(C/Al) system with... 

Ion-pair formation in nonaqueous media was frequently observed in electrochemical studies (see [98]). There is no space here to discuss these problems. For illustration we mention only the study carried out in the present author s laboratory, on ion-pairing in the electroreduction of nitrobenzene and related compounds [99,100]. Ion-pairing of intermediates may in some cases change the mechanism of electrode reactions. 

In many descriptions of electrochemical preparations of organic substances, only the overall current and voltage applied across the cell have been specified. It must be emphasized that this information is generally inadequate for a proper electrochemical specification of the experimental conditions and a characterization of the reaction mechanism. Under constant current conditions, as consumption of the reactant occurs, the potential normally becomes increased (greater polarization) until eventually some new electrode process becomes predominant (see Section 5.1). This may either be decomposition of the solvent or supporting electrolyte or, in some cases, a further reaction with the substrate involved in the electroorganic preparation. In the latter case, it is clear that the preparation will yield more than one principal product. A classical case, first investigated by Haber, is the electroreduction of nitrobenzene referred to above and also the Kolbe reaction. ... 

The electroreduction of nitrobenzene received early attention, probably because of the need for dyestuffs precursors. A patent for making aniline by this procedure was granted over a century ago. Depending upon the conditions, nitrobenzene can yield a variety of reduction products. By his introduction of controlled-potential electrolysis near the end of last century, Fritz Haber (1868-193 ) was able to elucidate the electrochemical and chemical steps that give rise to these products. The Luggin capillary, named for Haber s friend and colleague, dates from this period (22). 

Limitations to the acceptance of organic electrochemistry, particularly as a synthetic technique, may have been connected with the fact that electrode reactions are normally two-dimensional, i.e., they are restricted to a surface and therefore require mass transport (see elsewhere in this chapter) and also because many reactions yield a complex mixture of products when the electrolyses are carried out using a constant current. However, as early as 1898, Haber had pointed out the importance of control of the electrode potential for the overall process, in his work where nitrosobenzene, phenylhy-droxylamine and aniline were isolated selectively from the reduction of nitrobenzene. However, design of suitable controlled-potential equipment proved to be a practical barrier, even in laboratory studies, until 1942, when the potentiostat—an instrument capable of automatically controlling the electrode potential—was introduced.Without question, this instrument has facilitated electro-organic syntheses, mechanistic studies, and specific electrooxidation and electroreduction processes. More modern and electronically... 

The electroreduction of an approximately 2mM solution of nitrobenzene in acetonitrile/0.1 M tetrabutylammonium perchlorate was studied using a platinum microdisc electrode of radius, r, 24 fim. A steady-state limiting current of —4.05 X 10 A was observed at long time, while at short times the following current (f)-time (t) data were collected. 

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... 

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