Addition Reactions of Alkenes

All members of the alkene homologous series share the same functional group (at least one C=C bond). This means that they react in similar ways, opening up the C=C bond to allow the two carbon atoms to bond to another atom or molecule. As this involves adding another atom or molecule to the alkene, it is called an addition reaction.

  • No other products are formed in the process.

Let’s look at how alkenes react with hydrogen, water and halogens.


Alkenes can react with hydrogen to form an alkane, in a process called hydrogenation:

Alkene + Hydrogen Alkane

These reactions require a catalyst. An example of a hydrogenation reaction is:

Ethene + Hydrogen → Ethane


In the presence of heat, alkenes can react with steam to form an alcohol in a process called hydration:

Alkene + Water Alcohol

These reactions require a temperature of around 300°C, a pressure of around 60 – 70 atm and a catalyst. An example of a hydration reaction is:

Ethene + Water → Ethanol

Afterwards, the water and ethanol are separated by fractional distillation.


Alkenes can also react with halogens, such as chlorine, in a process called halogenation:

Alkene + Halogen Dihaloalkane

An example of a halogenation reaction is:

Ethene + Chlorine → Dichloroethane