Free Radical Substitution
There are THREE main stages!
This is where a molecule, such as a halogen in Group 7 is exposed to UV light and its bonds break. This then forms TWO radicals (a species with unpaired electrons).
Since as both atoms gained one electron each, this is known as 'Homolytic fission'.
There are always TWO steps in the propagation stage. The chlorine radicals will react with an organic compound, to form more and more radicals...
Let's use methane as our organic compound.
- The chlorine radical reacts with the methane to produce a methyl radical and hydrochloric acid.
- The chlorine radical breaks one of the hydrogen bonds with the carbon atom, and forms a new bond with the hydrogen to form HCl.
- The hydrogen donates one of its electrons to the chlorine radical, and the carbon atom takes its electron back, hence, it forms a CH₃ ・radical!
- The CH₃ ・ radical reacts with a chlorine molecule (Cl₂).
- Why not another Cl ・radical?
- Because not every chlorine molecule will turn into a radical; and there is a greater chance that the CH₃ ・radical will react with a Cl₂ molecule. The CH₃ ・radical is VERY reactive, so quickly reacts with the chlorine to form CH₃Cl.
- This means that the chlorine molecule donated one of its electrons to the carbon, and the carbon donated its unpaired electron (radical) to form the bond.
- Therefore, through homolytic fission, another Cl ・radical is formed.
- This reaction could keep repeating until every hydrogen was replaced by a chlorine.
At the end of the reaction, there cannot be any radicals left over. Therefore, they must react with something to form a product. In free radical substitution, a variety of products can be formed. Let's look at our two equations from the propagation stage.
We can see from our equations that we have two Cl ・radicals, and two CH₃ ・ radicals. They can react together to form a variety of products!
This is the end of topic!
Drafted by Cherry (Chemistry)