Chapter 11.3, Problem 11.30P

Interpretation Introduction

Interpretation:

The product has to be predicted for the given reaction.

Concept Introduction:

Addition reactions are the one in which the groups are added to an unsaturated double bond.  The double bond is destroyed because the groups are added across the double bond.  If the considered alkene is a symmetrical one, then there is no difference in the product obtained but if the alkene is an unsymmetrical one, then regiochemistry decides the products that are obtained.

When two similar groups are added across a double bond, regiochemistry is irrelevant.  When adding two different groups across the double bond of an unsymmetrical alkene, the regiochemistry becomes relevant.  In simple words, it can be said that, when two different groups are added across a double bond, then regiochemistry becomes relevant.

For Hydrogenation reaction, the regiochemistry will be irrelevant always.  This is because the same group is added to the double bond.  The stereochemistry of hydrogenation reaction becomes relevant.  Two pairs of enantiomers can be obtained maximum.  Hydrogenation reaction is accomplished by using hydrogen and a metal catalyst namely, Platinum, Palladium or Nickel.  Hydrogen interacts with the metal catalyst surface which effectively breaks the $\text{H}-\text{H}$ bonds.

The individual hydrogen atoms are absorbed to the surface of metal.  The addition of the hydrogen atoms takes place across the alkene.  The alkene first coordinates with the metal surface.  Then the hydrogen atoms are added to the double bond.  If the hydrogen atoms are added on same side of alkene, it is known as syn addition and if it is on opposite side of alkene means it is known as anti-addition.  In syn addition only pair of enantiomers are obtained.

The formed compounds might be a meso compound also.  In this case, two enantiomers are not possible.  Instead only one structure is drawn.  This is because they have plane of symmetry.

The complete hydrogenation reaction profile can be summarized as shown below,