How do alkenes react with hydrogen?

Alkenes react with hydrogen in a process called hydrogenation, which converts the double bonds into single bonds.

Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Alkenes are a perfect example of such organic compounds that can be reduced.

In the case of alkenes, the reaction with hydrogen is an addition reaction, as the hydrogen atoms 'add' to the carbon atoms in the double bond of the alkene. This breaks the double bond and forms a saturated hydrocarbon, an alkane. The general equation for this reaction is: Alkene + Hydrogen → Alkane.

For example, if ethene (C2H4) is reacted with hydrogen in the presence of a nickel catalyst, the product is ethane (C2H6). The equation for this reaction is: C2H4 + H2 → C2H6.

The catalyst is crucial for this reaction. It provides a surface for the reactants to come together, reducing the energy needed for the reaction to occur. The hydrogen molecules are adsorbed onto the surface of the catalyst, breaking the H-H bonds and allowing the hydrogen atoms to react with the alkene.

This reaction is exothermic, meaning it releases heat. This is because the energy required to break the bonds in the reactants is less than the energy released when new bonds are formed in the products.

Hydrogenation has significant industrial applications, particularly in the production of margarine from vegetable oils. The oils, which are unsaturated fats, are reacted with hydrogen to produce saturated fats, which are solid at room temperature. This process is also used in the petrochemical industry to convert alkenes to alkanes, which are used as fuels and in the production of plastics.