What conditions are necessary for alkenes to react with hydrogen?

Alkenes react with hydrogen in a process known as hydrogenation, which typically requires the presence of a catalyst to facilitate the reaction. The catalyst, often a metal such as palladium, platinum, or nickel, lowers the activation energy needed for the reaction to occur, allowing the hydrogen to add across the double bond of the alkene more efficiently at lower temperatures and pressures.

While high temperatures can be beneficial in some reactions, they are not strictly necessary for the hydrogenation of alkenes, especially when a catalyst is employed. A large excess of hydrogen is not required either; the reaction can proceed effectively with a controlled amount of hydrogen in the presence of the catalyst. Thus, the requirement for a catalyst is essential for the hydrogenation of alkenes to take place effectively.