Addition of hydrogen to alkenes is an exothermic (releasing heat energy) reaction requiring the use of a transition metal catalyst (usually Pd, Pt, Ni, or Rh) due to the high energy barriers to direct reaction between alkenes and hydrogen gas.
Catalytic hydrogenation af alkynes with transition metal catalysts results in alkanes (just like the catalytic hydrogenation of alkenes) unless specially deactivated catalysts are used.
Catalytic hydrogenation occurs on the surface of the metal catalyst. Transition metals adsorb hydrogen molecules and pi systems. The pi system (alkene or alkyne) adds hydrogen atoms in a stepwise manner as shown in the still image below.
The mechanism of hydrogenation provides a spatial bias for adding both hydrogen atoms to the same face of the pi system. This bias results in the formation of certain stereoisomers in greater amounts than others. This stereoselectivity is demonstrated below for the catalytic hydrogenation of both geometric isomers of 3,4-dimethyl-3-hexene.
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