Question

How can I explain the mechanism for catalytic hydrogenation?

Answer 1

Hint: The process which involves gain of electrons (or hydrogen atom) by an atom or molecule is called reduction. Catalytic hydrogenation of alkene is an electrophilic addition reaction.

Complete step by step solution:
The substance which donates electrons in chemical reactions is called reducing agent. There are many reducing organic compounds like $(H_2/Pd)LiAl{H}_4,NaB{H}_4$ which causes reduction of organic compounds like alkene, and other carbonyl compounds.
In catalytic hydrogenation alkene gets reduced to alkenes with the help of in the presence of metal like platinum, palladium, nickel or rhodium. In this reaction new sigma bonds are formed. In this reaction a hydrogen atom takes place in alkene. This reaction also forms an experimental basis of heat of hydrogenation which is used to explain the stability of isomeric alkenes.
Alkynes are less reactive to catalytic hydrogenation in respect to alkene, because of the presence of stronger pi bond. Catalytic hydrogenation of alkenes takes place in three steps:
a hydrogen atom forms attachment with the metal ion.
alkene shows an approach toward the hydrogen atom on the metal surface. Metal catalysts attract the alkene and transfer hydrogen atoms to the alkene.
Alkene reacts with the H-atom and gets reduced by forming two new bonds. Two alkene molecules and gives after catalytic hydrogenation.
The surface of the catalyst adsorbs both the alkene and the hydrogen. This weakens both the $$C=C\pi$$ bond and the $H-H$ bond and provides a lower-energy pathway for the reaction.

Note: It reduces alkene, aldehyde and ketone but cannot reduce the carboxylic acid, ester and amides. It reduces aldehyde and ketones into corresponding alcohols. $(H_2/Pd)$ acts as a weak reducing agent. It reduces alkene, aldehyde and ketone but cannot reduce the carboxylic acid, ester and amides. It reduces aldehyde and ketones into corresponding alcohols.