Addition of hydrogen to alkenes and alkynes catalytic hydrogenation

3 Addition of hydrogen to alkenes and alkynes catalytic hydrogenation 

The addition of hydrogen to carbon-carbon multiple bonds (reduction) may be achieved using gaseous hydrogen in the presence of a finely divided noble metal catalyst. This is termed catalytic hydrogenation. It is not a radical reaction as we have seen 

The catalyst used is typically platinum, palladium, rhodium, or ruthenium, or sometimes an appropriate derivative. Precise details of the reaction remain vague, but we believe the catalyst surface binds to both the substrate, e.g. an alkene, and hydrogen, weakening or breaking the jr bond of the alkene and the a bond of hydrogen. Sequential addition of hydrogen atoms to the alkene carbons then occurs and generates the alkane, which is then released from the surface. 

Catalytic hydrogenation delivers hydrogen to one face of the alkene the consequence is syn addition of hydrogen. This is a departure from our usual observations with ionic mechanisms, where the groups typically add to a double bond with anti stereochemistry (see Section 8.1.2). 

The stereochemical consequences of this are illustrated in the following examples. 

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