Reactions of Alkanes

Reactions of alkanes.

Alkanes by their very nature tend to be unreactive molecules. They only undergo two reactions

1. Combustion

2. Free radical substitution.


Any alkane will undergo combustion in excess oxygen to form carbon dioxide and water.

CH4(g) + 2O2(g)    CO2(g)  + 2H2O(l)

Free radical Substitution.

Overall a halogen atom replaces a hydrogen atom on an alkane.

Here are a couple of key terms that you will need:

Free radical: Species with an unpaired electron

Homolytic fission: The bond splits equally so that each atom receives one electron. In order for homolytic fission to occur UV light is needed.

The you tube video explains the process of free radical substitution using chlorine and methane. Other halogens can be used like bromine and they could also use other alkanes like ethane.

The problem is that further substitution can occur. Once the Chloromethane is formed it is possible for it to undergo further reactions with chlorine free radicals. Each time this happens one of the hydrogen atoms of methane will be removed and it will be replaced with a chlorine atom. The video below shows how further substitution can occur:

AQA tend to take a slightly different approach. The tend to start by using a halogenated alkane rather than a standard alkane. I have set up a you tube video to show how these types of questions can be dealt with. You will need to be able to write initiation, propagation and termination reactions for halogenated alkanes rather than just standard alkanes.





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