In spite of its favorable deltaHº ethylene and hydrogen do not react even heated at 200ºC under high pressure. A metal catalyst is necessary in order to homolitically break the H-H bond.
Hydrogen splits at the metal suface leading to two electrophilic, very reactive hydrogen atoms.
The "pi" cloud of the double C=C bond is thus easily attacked yielding the corresponding alkane.
Hydrogenation with heterogenous catalysts is stereospecific, in that the two hydrogens come from the same side of the double C=C bond.
If the two faces of the doble C=C bond are not equivalent, the least sterically hindered will be preferred.
