Question

Olefins are also hydrogenated by
A. Zinc & $HCl$
B. Nascent hydrogen
C. Raney $Ni$ & ${{H}_{2}}$
D. Ether solution containing Lithium Hydride

Answer 1

Hint: When hydrogen is added to the carbon-carbon double bond is called hydrogenation. The overall result of such addition is the reductive removal of the carbon-carbon double bond. This process is done by adding a metal catalyst with molecular hydrogen (${{H}_{2}}$). For example Sabatier and Senderens reaction by which olefins can be hydrogenated.

Complete Step by Step Answer:
Olefines are generally alkenes containing carbon-carbon double bonds. In olefins, alkene consists of a sigma ($\sigma $) and a pi bond ($\Pi $) bond. As we know sigma bond is stronger than a pi bond, hence pi bond is more reactive than a sigma bond. Any reagents can be added to the carbon by breaking the double bond.

Such an addition reaction occurs through the hydrogenation process, in which two hydrogens are added across the double bond and resulting in a product alkane that is thermodynamically stable. The overall reaction is exothermic I.e a high amount of heat is released.

But reaction only with hydrogen molecules is a very slow process. Hence a small quantity of metal catalysts is added to increase the rate of reaction. Catalysts that are insoluble in reacting mixture and also inactive are used for example Raney nickel, Palladium with charcoal, and $Pt{{O}_{2}}$.
The hydrogenation process when Raney nickel with ${{H}_{2}}$ at temperature range ${{250}^{{\mathrm O}}}-{{300}^{{\mathrm O}}}C$are added to the olefins is called Sabatier and Senderens reaction.

But zinc $HCl$ is used for the reduction of aldehydes and ketones to a hydrocarbon. Lithium hydride is used to synthesise $LiAl{{H}_{4}}$which is a stronger reducing agent and thus reduces aldehydes, ketones, acids, esters, and amides but cannot reduce olefins. Nascent hydrogen reduces alkyl halides to alkane. Therefore olefins can be hydrogenated by Raney nickel and hydrogen.
Thus, option (C) is correct.

Note: Raney nickel is an alloy of nickel and aluminium. This is prepared by heating aluminium to a molten stage and then adding nickel into this mixture. Raney nickel is used widely for hydrogenation for its high range of oxygen absorbing capacity and good reactivity.