Lecture Notes on the Hydrogenation of Alkenes
 
Hydrogenation of an alkene yields an alkane.

 
 

THE MECHANISM OF HYDROGENATION



This reaction is an example of a heterogeneous catalysis process.

The reaction takes place on the surface of the metal.
STEP 1:

Hydrogen molecules react with the metal atoms at the catalyst surface.

The relatively strong H-H sigma bond is broken and replaced with two weak metal-H bonds.
STEP 2:

The pi bond of the alkene interacts with the metal catalyst weakening the bond.

A hydrogen atom is transferred from the catalyst surface to one of the carbons of the double bond.
STEP 3:

The pi bond of the alkene interacts with the metal catalyst weakening the bond.

A second hydrogen atom is transferred from the catalyst surface forming the alkane.
The alkane is released from the catalyst's surface allowing the catalyst to accedpt additional hydrogen and alkene molecules.

HEATS OF HYDROGENATION

Heats of hydrogenation can be used to measure the relative stability of isomeric alkenes.

Heats of hydrogenation correlate with structure just like heats of combustion do.

The greater the heat of hydrogenation, the less stable the alkene.
INCREASING ALKENE STABILITY
 
In the addition both groups can add to the same face of the alkene (syn addition) or they can add to opposite faces (anti addition).

Hydrogenation of alkenes is a syn addition.