Further Substitution

Free-radical substitution does not stop after one hydrogen has been replaced. A chlorine radical can remove a hydrogen from chloromethane, leading to further substitution.For example: CH₃Cl + Cl• → •CH₂Cl + HCl, then •CH₂Cl + Cl₂ → CH₂Cl₂ + Cl•Repeating this gives trichloromethane, CHCl₃, and tetrachloromethane, CCl₄. The overall equation for complete substitution is CH₄ + 4Cl₂ → CCl₄ + 4HCl.The reaction therefore gives a mixture of products, which is a disadvantage. Termination steps also make other products, such as ethane or chloroethane.Using excess methane makes it more likely that a chlorine radical hits a methane molecule, so it favours CH₃Cl. The products can be separated by fractional distillation.