Overview

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.

Procedure

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.