Catalysis of Organic..

Gelder, Jackson and Lok 16720. Competitive Hydrogenation ofNitrobenzene, Nitrosobenzene and AzobenzeneAbstractElaine A. Gelder 1 , S. David Jackson 1 , C. Martin Lok 21 WestCHEM, Department of Chemistry, The University,Glasgow, G12 8QQ Scotland2 Johnson Matthey Catalysts, Belasis Avenue,Billingham, Cleveland, TS23 1LB, U.K.sdj@chem.gla.ac.ukThe hydrogenation of nitrobenzene, nitrosobenzene and azobenzene has beenstudied singly and competitively. A kinetic isotope effect was observed withnitrobenzene but not with nitrosobenzene. Nitrosobenzene inhibits nitrobenzenehydrogenation in a competitive reaction, whereas azobenzene and nitrobenzeneco-react but at lower rates. Taken together a more detailed mechanisticunderstanding has been obtained.IntroductionThe catalytic hydrogenation of nitrobenzene is an industrially importantreaction, utilized in the production of aniline for the plastics industry.Commercially, the reaction is carried out in the gas phase over a nickel orcopper based catalyst (1). However, the transformation is extremely facile andis carried out under relatively mild conditions. For this reason, hydrogenationoccurs rapidly over most metals and is often employed as a standard referencereaction for comparing the activities of other hydrogenation catalysts (2 – 5).Despite the large volume of literature available citing the use of this reaction,very little has been published regarding actual mechanistic detail. Haber’sinitial scheme was published in 1898 (6) and proposed that nitrobenzene (NB)was transformed to aniline (A) in a three-step process involving nitrosobenzene(NSB) and phenylhydroxylamine intermediates (Scheme 1). In addition, it wasalso proposed that azobenzene (AZO) and azoxybenzene (AZOXY) by-productscould be formed via reaction of the two intermediate species. This mechanismhas been widely accepted since and a number of studies have reported theidentification of the suggested reaction intermediates during hydrogenation (7,8). In addition, Figueras and Coq (9) have also described the hydrogenationbehavior of these intermediates and by products and found azobenzene tohydrogenate through to aniline. While these studies have appeared to provide