8.14 : Reduction of Alkenes: Catalytic Hydrogenation

Hydrogenation of alkenes is a reduction process wherein the addition of molecular hydrogen breaks the weak π bond of an alkene to form two C–H σ bonds of an alkane.

Intrinsically, hydrogenation of alkenes has a large energy barrier, making the reaction unfavorable at room temperature.

The reaction can be altered to give a low-energy pathway using a transition-metal catalyst — usually containing palladium, platinum, and nickel.

The catalyst is heterogeneous; that is, finely divided solid dispersed on the surface of inert support, such as charcoal.

Hydrogenation begins with the adsorption of molecular hydrogen to the surface of the metal catalyst.

Interaction of molecular hydrogen with the metal cleaves the H–H bond to give individually adsorbed hydrogen atoms.

Next, an alkene adsorbs by coordinating one of the faces of its π bond to the metal surface.

This is followed by the sequential insertion of two hydrogen atoms into the π bond, giving the reduced product that simultaneously releases from the catalyst surface.

As the hydrogens transfer to the same face of the π bond, hydrogenation has syn stereochemistry.

Consider the hydrogenation of an alkene generating two new chiral centers. 

Though the chirality in the molecule reveals the possibility of four stereoisomers, due to syn addition, only one pair of enantiomers is predominantly formed, making the reaction stereospecific.

Additionally, the steric environment around the double bond governs its approach towards the catalyst.

For instance, the double bond in α-pinene is under the steric influence of the methyl group attached to the four-membered ring, inhibiting hydrogen insertion from the sterically hindered side.

Hence, the hydrogen insertion into the π bond of α-pinene takes place exclusively from its bottom face, forming a single product.