Alkanes are the simplest hydrocarbons and are formed only of Carbon and Hydrogen. They are only made up of single bonds and are often known as saturated molecules (Like a sponge is saturated if it’s full of water alkanes are saturated because they are full of hydrogen)


Alkanes only undergo three types of reactions, combustion, cracking and radical substitution.


Combustion of alkanes is one of the most important reactions in human history. We burn alkanes to release heat and energy to release power for many applications.

Complete combustion of an alkane to Carbon dioxide and water

Radical Substitution

Radical substitution of an alkane is a three stage reaction. Initiation, Propagation and Termination.


UV light causes homolytic fission (breaking the bond) causing the formation of a radical


A Chlorine radical removes a hydrogen from an alkane to form an alkyl radical in the first stage of propagation

The alkyl radical removes a chlorine to form a halogenoalkane and leaves a chlorine radical


Two alkyl radicals combine. This could happen again and again

A chlorine radical and an alkyl radical combine to form a halogenoalkane

Two chlorine radicals combine to form another Chlorine molecule

Exam tip: Remember 123 – initiation 1 step, propagation 2 steps, termination 3 steps.