Catalysis of Organic..

168Nitrobenzene Hydrogenationfurther supporting evidence for Haber’s reaction scheme, the mechanism is stillnot well understood and has never been fully delineated.PhNO 2 PhNO PhNHOH PhNH 2PhNHOHPhNH 2Ph-NO=N-PhazoxybenzenePh-N=N-PhazobenzenePh-NH-NH-PhhydrazobenzeneScheme 1. Haber mechanism.Although nitrobenzene, nitrosobenzene and azobenzene are often observedin nitrobenzene hydrogenation, we are aware of no studies of competitivereactions. In this paper we report on the competitive hydrogenations and theirmechanistic implications.Results and DiscussionThe hydrogenation of nitrobenzene progressed to aniline without any significantby-product formation, only trace amounts of azobenzene were formed (< 1 %)as the reaction went to completion. NMR analysis showed no detectable phenylhydroxylamine in solution. The hydrogen uptake displayed a smooth curve andthe rate of hydrogen consumption coincided with the rate of aniline production.The rate of hydrogenation of nitrobenzene to aniline was 15.5 mmol.min -1 .g -1 .In the hydrogenation of azobenzene, the rate of hydrogen consumptionfollowed a smooth curve. The reaction profile showed a direct transformation toaniline with no by-product formation or intermediates detected. The rate ofaniline production (8.3 mmol.min -1 .g -1 ) was half the rate of nitrobenzene toaniline.Unlike the hydrogenation of nitrobenzene, the hydrogen uptake curve fornitrosobenzene displayed two distinct stages. Both these stages proceeded at asignificantly slower rate than the rate of hydrogen up-take during hydrogenationof nitrobenzene. This two-stage hydrogen up-take curve has also been reportedby Smith et al. (10) over palladium/silica catalysts. Also, the rate of anilineproduction from nitrosobenzene was 0.35 mmol.min -1 .g -1 , which was much lessthan the rate of hydrogen consumption. Therefore aniline was produced 50times slower from nitrosobenzene than nitrobenzene. The nitrosobenzene