Catalysis of Organic..

Gelder, Jackson and Lok 173and that in this sequence it is probable that the final coupling reaction is the ratedeterminingstep and such would not show a kinetic isotope effect whendeuterium was used instead of hydrogen.This behavior also suggests that during nitrosobenzene hydrogenation thereis always a surface concentration of strongly adsorbed material that inhibitsnitrobenzene adsorption on the palladium. There is rapid up-take ofnitrosobenzene on the catalyst but most is adsorbed on the carbon support. Onlywhen this strongly bound surface species is removed from the palladium doesnitrobenzene hydrogenation initiate. This removes the possibility that the massmissing in nitrosobenzene hydrogenation is phenyl hydroxylamine in solution.The catalyst retains this mass during nitrosobenzene hydrogenation. Wepropose that this surface species is Ph-NOH(a), the reaction of ntrosobenzenewith a surface saturated with adsorbed hydrogen, which can be considered anadsorbed form of phenyl hydroxylamine.Nitrobenzene hydrogenation in deuterium revealed that there was a kineticisotope effect indicating that a hydrogen bond making step was rate determining.The difference in the nitrobenzene and nitrosobenzene hydrogenation suggeststhat the rate-determining step in nitrobenzene hydrogenation must lie before theformation of Ph-NOH(a). In this way the surface concentration of Ph-NOH(a) iskept low. At low surface concentrations, this intermediate reacts with adsorbedhydrogen to continue to aniline. At high concentrations, i.e. duringnitrosobenzene hydrogenation, Ph-NOH(a) dimerizes to produce azoxybenzeneand water. This is shown in Scheme 2.Ph-NO 2Ph-N(OH)H+H+HPh-NO+HPh-NOH(a)+Ph-NOH-H 2 OPh-NO=N-PhPh-NH 2 Ph-N=N-PhPh-NH(a)Ph-NH-NH-PhScheme 2. Overview of NB and NSB hydrogenation (hashed lines denotemultiple surface steps).