Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of the metal catalyst. It begins with the adsorption of the hydrogen onto the metal surface, followed by the cleavage of the H–H bonds to give individual metal–hydrogen bonds. The alkene then complexes with the catalyst surface by using its p orbitals to overlap with the empty metal orbitals of the catalyst. The two hydrogen atoms then insert into the π bond sequentially through syn addition (addition to the same face of the π bond) to give the reduced product — the alkane. The alkane formed is no longer bound to the metal and diffuses away from the catalyst's surface.
The process of hydrogenation is exothermic. The heat released is called the heat of hydrogenation (ΔH°), and it helps predict the relative stabilities of alkenes. For example, although the hydrogenation of both cis-2-butene and trans-2-butene gives the same product — butane, trans-2-butene is more stable than cis-2-butene. This can be explained based on the heat of hydrogenation of the two isomers. The cis isomer (ΔH° = −28.6 kcal/mol) has a slightly higher heat of hydrogenation compared to the trans isomer (ΔH° = −27.6 kcal/mol). In cis-2-butene, the steric repulsion between the two methyl groups lying on the same side of the double bond makes it less stable, which is reflected in its larger heat of hydrogenation.
