The addition of hydrogen to an alkene or alkyne is called hydrogenation. The reaction involves the breaking of a carbon-carbon double or triple bond and the formation of new carbon-hydrogen bonds. The difference in the reaction mechanism between the addition of hydrogen to an alkene and an alkyne lies in the number of bonds present and the number of hydrogenation steps required to reach the final product.1. Hydrogenation of Alkenes:Alkenes contain a carbon-carbon double bond C=C , which consists of one bond and one bond. In the hydrogenation of alkenes, hydrogen H2 is added across the double bond in the presence of a catalyst, usually a transition metal like palladium Pd , platinum Pt , or nickel Ni . The catalyst helps to lower the activation energy of the reaction.The reaction mechanism for the hydrogenation of alkenes involves the following steps:a. Adsorption: Hydrogen molecules H2 adsorb onto the surface of the catalyst and dissociate into individual hydrogen atoms.b. Formation of a complex: The alkene molecule adsorbs onto the catalyst surface, and the bond of the alkene interacts with the hydrogen atoms on the catalyst surface.c. Formation of the product: The hydrogen atoms on the catalyst surface form new bonds with the carbon atoms of the alkene, breaking the bond in the process. The resulting product is an alkane, with all carbon-carbon single bonds.2. Hydrogenation of Alkynes:Alkynes contain a carbon-carbon triple bond CC , which consists of one bond and two bonds. The hydrogenation of alkynes can proceed in two steps, depending on the reaction conditions and the catalyst used.a. Partial hydrogenation: In the presence of a poisoned catalyst e.g., Lindlar's catalyst, which is palladium on calcium carbonate with lead or quinoline as a poison , alkynes undergo partial hydrogenation to form alkenes with a cis configuration. The poisoned catalyst prevents the reaction from proceeding further to form alkanes.b. Complete hydrogenation: In the presence of an unpoisoned catalyst e.g., platinum or nickel , alkynes undergo complete hydrogenation to form alkanes. This process involves two sequential hydrogenation steps, similar to the mechanism described for alkenes. First, one bond of the alkyne is hydrogenated to form an alkene, and then the remaining bond of the alkene is hydrogenated to form an alkane.In summary, the difference in the reaction mechanism between the addition of hydrogen to an alkene and an alkyne lies in the number of bonds present in the reactants and the number of hydrogenation steps required to reach the final product. Alkenes undergo a single hydrogenation step to form alkanes, while alkynes can undergo either partial hydrogenation to form cis-alkenes or complete hydrogenation to form alkanes, depending on the reaction conditions and the catalyst used.