Mechanism of the carbon catalyzed reduction of nitrobenzene by hydrazine

The reduction of nitroaromatics to anilines by hydrazine is catalyzed by carbons. The carbon serves as an adsorbent and electrical conductor to enable the reaction to occur. Hydrazine is a two-electron reducing agent as shown by trapping the diimide intermediate with norbornene. The nitrobenzene reduction is a four-electron process proceeding first to phenylhydroxylamine which was observed in low concentrations in the reacting system by NMR. Phenylhydroxylamine is reduced to aniline in a second step. The two-electron intermediate, nitrosobenzene, gave different products than nitrobenzene under the reaction conditions and could not be trapped, supporting the expected four-electron pathway. By serving as an adsorbent and collecting hydrazine on their surfaces, carbons make it possible to execute a four-electron reduction using a two-electron reducing agent. This role of carbon was confirmed by successfully operating an emf cell using graphite or carbon paste electrodes immersed in solutions of hydrazine and nitrobenzene. The initial rates of reduction of substituted nitrobenzenes showed only a tiny substituent effect and proceed at nearly the same rate as the carbon-catalyzed decomposition of hydrazine leading to the conclusion that hydrazine reaction at the carbon surface is the rate determining step. (C) 2000 Elsevier Science Ltd. All rights reserved.