How do alkenes react with hydrogen in hydrogenation reactions?

In hydrogenation reactions, alkenes react with hydrogen to form alkanes.

In a bit more detail, hydrogenation is a type of chemical reaction where hydrogen is added across the double bond of an alkene to form an alkane. This reaction is typically catalysed by a metal catalyst such as nickel, platinum or palladium. The catalyst is necessary to lower the activation energy of the reaction, making it easier for the hydrogen atoms to break the double bond of the alkene and attach themselves to the carbon atoms.

The general equation for this reaction is: alkene + hydrogen → alkane. For example, if ethene (C2H4) is reacted with hydrogen (H2) in the presence of a nickel catalyst, the product is ethane (C2H6). The double bond in the ethene molecule is broken, and the hydrogen atoms attach themselves to the carbon atoms, forming a single bond.

This reaction is exothermic, meaning it releases energy in the form of heat. The breaking of the double bond in the alkene and the formation of the single bonds in the alkane releases more energy than is required to break the initial hydrogen-hydrogen bond, resulting in a net release of energy.

Hydrogenation reactions are important in the industrial production of many substances. For example, they are used in the food industry to convert unsaturated fats (which have double bonds) into saturated fats (which have single bonds), a process that results in the formation of margarine from vegetable oils.

In summary, hydrogenation is a reaction where hydrogen is added to an alkene to form an alkane, typically in the presence of a metal catalyst. This reaction is exothermic and has many practical applications in industry.