Advices For Studying Organic Chemistry

7.14 HYDROGENATION: THE FUNCTION OF THE CATALYST

1. Hydrogenation of an alkene is an exothermic reaction (∆Η° ≈ –120 kJ mol–1).

R–CH=CH–R + H 2

hydrogenation

R–CH 2 –CH 2 –R + heat

1) Hydrogenation reactions usually have high free energies of activation.

2) The reaction of an alkene with molecular hydrogen does not take place at room

temperature in the absence of a catalyst, but it often does take place at room

temperature when a metal catalyst is added.

Figure 7.8 Free-energy diagram for the hydrogenation of an alkene in the
presenceof a catalyst and the hypothetical reaction in the absence of a
catalyst. The free energy of activation [∆G‡(1)] is very much larger
than the largest free energy of activation for the catalyzed reaction
[∆G‡(2)].

2. The most commonly used catalysts for hydrogenation (finely divided platinum,
   nickel, palladium, rhodium, and ruthenium) apparently serve to adsorb hydrogen
   molecules on their surface.

1. Unpaired electrons on the surface of the metal pair with the electrons of