Catalysis of Organic..

170Nitrobenzene Hydrogenationhydrogen uptake (mol)0.50.450.40.350.30.250.20.150.10.0500 50 100 150 200 250Time (min)deuteriumhydrogenFigure 2. Hydrogen/deuterium up-take during nitrobenzene hydrogenation0.140.12hydrogen uptake (mol)0.10.080.060.040.0200 20 40 60 80 100 120 140 160Time (minutes)DeuteriumhydrogenFigure 3. Hydrogen/deuterium uptake during nitrosobenzene hydrogenationFrom figure 2 it can clearly be seen that there was a kinetic isotope effectwith the reaction in H 2 occurring much more rapidly than the reaction in D 2 .The value of the kinetic isotope coefficient (η) was calculated as 1.9. Therefore,it follows that the rate-determining step in the hydrogenation of nitrobenzenemust involve a bond-breaking or bond-forming reaction with hydrogen.However when nitrosobenzene was reacted with deuterium (Figure 3) the uptakeis considerably different. During the first 15 minutes of reaction, the rates ofhydrogenation and deuteration were almost identical. After 15 minutes reactiontime, an inverse kinetic isotope effect was evident as the reaction proceeded