Alkenes undergo a chemical reaction known as hydrogenation, which transforms their double bonds into single bonds.

Hydrogenation is a reaction involving molecular hydrogen ($H_2$) and another compound or element, typically facilitated by a catalyst such as nickel, palladium, or platinum. This process is commonly utilized to reduce or saturate organic compounds, with alkenes serving as prime examples of such compounds that can be hydrogenated.

In the case of alkenes, the reaction with hydrogen is classified as an addition reaction. During this process, hydrogen atoms are ‘added’ to the carbon atoms involved in the alkene’s double bond. This addition effectively breaks the double bond, resulting in the formation of a saturated hydrocarbon, known as an alkane. The general equation representing this reaction can be expressed as:

For instance, when ethene ($C_2H_4$) reacts with hydrogen in the presence of a nickel catalyst, the resulting product is ethane ($C_2H_6$). The reaction is represented by the following equation:

The role of the catalyst is essential in this reaction. It provides a surface for the reactants to interact, thereby lowering the energy barrier required for the reaction to proceed. On the catalyst’s surface, hydrogen molecules are adsorbed, which facilitates the breaking of the H-H bonds and enables the hydrogen atoms to react with the alkene.

This hydrogenation reaction is exothermic, meaning it releases heat. This phenomenon occurs because the energy necessary to break the bonds in the reactants is less than the energy released when new bonds form in the products.

Hydrogenation has substantial industrial applications, particularly in the production of margarine from vegetable oils. In this process, unsaturated fats from oils are reacted with hydrogen to yield saturated fats, which are solid at room temperature. Additionally, this reaction is utilized within the petrochemical industry to convert alkenes into alkanes, which serve as fuels and are also important in the production of plastics.