Organic Chemistry: Catalytic Hydrogenation (Reduction) of Alkenes. The Horiuti-Polanyi Mechanism

Hinzugefügt: 2026-05-24

[00:00:00]The mechanism for this next reaction may look a little different at first. We just came from a learning about various oxidative cleavage reactions of the double bond. Yet, throughout this reaction, we are going to reduce the double bond, fully saturating the alkene to an alkane using both hydrogen gas and a metal catalyst such as nickel, platinum, or palladium. The summary of this reaction is that the hydrogen gas and the alkene will react on the metal catalyst surface to give us the saturated alkane product. But, this reaction does not have a typical arrow pushing mechanism. It does have three main steps. The first step involves things adhering onto the metallic catalyst. In this case, we'll focus on palladium. In the first step, hydrogen gas dissociates onto the surface into individual hydrogen atoms. Then, the alkene adheres onto the surface through pi interactions between the double bond and palladium. This is the first step.

[00:01:01]The second step involves a hydrogen attaching itself to one of the double bond carbons. The intermediate is held onto the metallic surface up until another hydrogen gets added to the adjacent carbon, freeing the saturated molecule from the surface. Specifically, both hydrogens are added syn addition since they interact with the same face interacting with the metallic surface.

[00:01:24]Hopefully, that helped as an introduction to catalytic reduction of alkenes.