pp. 7-8

In a hydrogenation reaction, the hydrogen is added to a double bond, but it will not add to a single bond.  Consequently, hydrogenation will remove unsaturation due to a pi-bond, but will leave a ring structure untouched.  In this manner it is possible to differentiate between unsaturation caused by a pi-bond and unsaturation caused by a ring system.

In organic chemistry, it is always very important to understand not only that reactions occur, but why reactions occur.  Knowing orbital overlap and electron distribution give us good insight into where the electron density on any molecule is located.  Knowing the mechanism of reactions (how reactants are transformed into products) is the next step in adding to your knowledge base.  We have already studied one reaction mechanism (Metal Insertion) and now we will study our second mechanism (Hydrogenation).  By studying mechanisms in-depth, we will learn how electrons flow from area to area, and we will become adept at determining which reactions will take place and where they will take place.  We will also learn that all mechanisms, which help us follow the flow of electrons from reactants to products, are all related in that the show movement of electrons from high density to low density electron areas,  Learning how one mechanism works will give insight as to how ALL mechanisms work.

(For a larger version of this movie, please click here.)

The mechanism for hydrogenation is also shown below:

Alkene Stability:

       Measuring Relative Energies in Alkene Isomers:

Chemists are able to abstract more from the Hydrogenation Reaction than just the number of pi-bonds in a molecule.  Hydrogenation can also give the relative energy of alkene isomers.  In chapter one, we looked at Constitutional Isomers (click here for a review) (Molecules that have the same molecular formula, but differ in atom connectivity.)  With the introduction of a multiple bond, another type of isomer may also be formed, a Geometric Isomer.  A Geometric Isomer is one in which the molecular formula of two molecules is the same, the skeletal connectivity is the same, but the molecules differ by rotation about a double bond.  Examples are shown below: