4.6.6 Addition of Hydrogen

Metal catalyst such as platinum, palladium, or nickel, can be used to add hydrogen to the double bond of an alkene to form an alkane. The H-H bond is so strong so that the energy barrier to the reaction would be enormous that can be decreased by the catalyst and breaks H-H bond. Platinum and palladium are used in a finely divided state adsorbed on charcoal (Pt/C, Pd/C).

Addition of hydrogen occurs to the double bond of 2-butene in the presence of platinum charcoal to give butane (Scheme 19).

Scheme 19

Since the metal catalysts are insoluble in the reaction mixture, they are classified as heterogeneous catalysts , which can easily be separated from the reaction mixture by filtration. One face of the alkene π-bond binds to the catalyst, which has hydrogen adsorbed on its surface. Hydrogen inserts into the π-bond, and the product is released from the catalyst. The two hydrogen atoms add with syn stereochemistry (Scheme 20).

Scheme 20

Homogeneous catalysts, such as Wilkinson's catalyst, also catalyze the hydrogenation of carbon-carbon double bonds (Scheme 21).

Scheme 21

Wilkinson's catalyst adds a hydrogen molecule across the double bond of an alkene with syn -stereochemistry where hydrogenation of deutriated cyclohexene leads to syn -additon to give syn -deutriated cyclohexane. Wilkinson's catalyst is not chiral, but can be converted into a chiral catalyst by replacing its triphenylphosphine (PPh₃) groups with chiral phosphines. This chiral Wilkinson's catalyst is capable of converting optically inactive starting materials to optically active products. Such a process is called asymmetric induction or enantioselective synthesis. For example, hydrogenation of optically inactive 3,7-dimethylocta-2,6-dien-1-ol with chiral Wilkinson's catalyst gives, 3,7-dimethyloct-6-en-1-ol, an optically active compound (Scheme 22).

Scheme 22