Alkenes and alkynes are both unsaturated hydrocarbons, meaning they contain carbon-carbon double or triple bonds, respectively. The difference in their properties and reactivity towards addition reactions can be attributed to the number of pi bonds and their bond strength.1. Number of pi bonds: Alkenes have one pi bond C=C while alkynes have two pi bonds CC . This difference in the number of pi bonds affects their reactivity towards addition reactions. Alkenes generally undergo one addition reaction, whereas alkynes can undergo two addition reactions due to the presence of two pi bonds.2. Bond strength: The carbon-carbon triple bond in alkynes is stronger than the carbon-carbon double bond in alkenes. This makes alkynes less reactive towards addition reactions compared to alkenes. However, once the first addition reaction occurs in alkynes, the resulting product has a double bond, making it more reactive towards a second addition reaction.Example of an addition reaction for alkenes:Reaction: HydrogenationAlkene: Ethene C2H4 Reagent: H2 in the presence of a catalyst e.g., Pt, Pd, or Ni Product: Ethane C2H6 Mechanism: In the hydrogenation reaction, the pi bond of the alkene is broken, and two hydrogen atoms are added to the carbon atoms, forming a single bond between the carbons. The catalyst helps in the adsorption of hydrogen and the alkene on its surface, facilitating the reaction.Example of an addition reaction for alkynes:Reaction: HydrohalogenationAlkyne: Ethyne C2H2 Reagent: HX X = Cl, Br, or I Product: Vinyl halide C2H3X after the first addition; geminal dihalide C2H2X2 after the second additionMechanism: In the hydrohalogenation reaction, the first addition involves the electrophilic attack of the hydrogen atom H+ from the hydrohalogen HX on one of the carbon atoms in the triple bond, forming a carbocation intermediate. The halogen atom X- then attacks the carbocation, forming a vinyl halide with a double bond. The second addition follows a similar mechanism, with the H+ attacking the double bond, forming another carbocation intermediate, which is then attacked by the X- to form the geminal dihalide product.In summary, the difference in the properties of alkenes and alkynes, such as the number of pi bonds and bond strength, affects their reactivity towards addition reactions. Alkenes generally undergo one addition reaction, while alkynes can undergo two addition reactions due to the presence of two pi bonds.