Free-radical substitution does not stop at one substitution. Once some chloromethane has formed, chlorine radicals can also remove a hydrogen atom from it, in more propagation steps:
Cl• + CH3Cl → CH2Cl• + HCl
CH2Cl• + Cl2 → CH2Cl2 + Cl•
This can continue until every hydrogen atom has been replaced:
CH4 → CH3Cl → CH2Cl2 → CHCl3 → CCl4
chloromethane, dichloromethane, trichloromethane, tetrachloromethane.Worked example: write the overall equation for making trichloromethane from methane. Three H atoms are replaced, so three Cl2 are used and three HCl are formed:
CH4 + 3Cl2 → CHCl3 + 3HCl
Controlling the products
An excess of methane makes it more likely that a Cl• collides with CH4 rather than CH3Cl, so chloromethane is the main product.
An excess of chlorine favours further substitution, giving more CCl4.
The products still have to be separated, by fractional distillation, because they have different boiling points.Longer alkanes
With larger alkanes, substitution can happen at any carbon atom. Propane gives both 1-chloropropane and 2-chloropropane, as well as many products with more than one chlorine.Termination steps also add unwanted products, such as ethane and chloroethane.Because it gives a mixture of products and a low yield of any single one, free-radical substitution is a poor way to make one particular halogenoalkane.
This can continue until every hydrogen atom has been replaced:
chloromethane, dichloromethane, trichloromethane, tetrachloromethane.Worked example: write the overall equation for making trichloromethane from methane. Three H atoms are replaced, so three Cl2 are used and three HCl are formed:
Controlling the products
An excess of methane makes it more likely that a Cl• collides with CH4 rather than CH3Cl, so chloromethane is the main product.
An excess of chlorine favours further substitution, giving more CCl4.
The products still have to be separated, by fractional distillation, because they have different boiling points.Longer alkanes
With larger alkanes, substitution can happen at any carbon atom. Propane gives both 1-chloropropane and 2-chloropropane, as well as many products with more than one chlorine.Termination steps also add unwanted products, such as ethane and chloroethane.Because it gives a mixture of products and a low yield of any single one, free-radical substitution is a poor way to make one particular halogenoalkane.