Final answer:
When an alkene reacts with hydrogen in the presence of a Pd catalyst, a hydrogenation reaction occurs, converting the alkene into an alkane. The reaction typically requires high pressures of hydrogen gas and involves the syn addition of hydrogen across the double bond with both hydrogen atoms adding from the same face.
Step-by-step explanation:
When an alkene is treated with hydrogen (H₂) in the presence of a Pd catalyst, a hydrogenation reaction occurs. This process involves the addition of hydrogen across the alkene's double bond, converting the unsaturated hydrocarbon into a saturated alkane. The catalyst, typically palladium on carbon (Pd/C), generates active sites that allow the hydrogen molecules to dissociate and the individual hydrogen atoms to add to the carbon atoms of the double bond.
The fact that this reaction often requires high pressures of H₂ gas highlights the need for appropriate reaction conditions to ensure the reaction's efficiency. Alkenes are planar around the C=C bond, and during the catalytic hydrogenation, both hydrogen atoms add to the same face of the alkene, facing the catalyst surface, leading to the syn addition of hydrogen.
The final product formed after the hydrogenation of an alkene using H₂ and a Pd catalyst is an alkane. An example is the hydrogenation of ethene (C2H4), which yields ethane (C2H6), a fully saturated hydrocarbon.