Substitution reaction is whereby one atom or organic molecule of a group of atoms is directly replaced
with another.
In the presence of sunlight, chlorine breaks the Cl-Cl bonds forming Cl- free radicals that form
chloromethane.
CH4 + Cl2
CH3Cl + HCl
The hydrogens are replaced by chlronie atoms in the methane. Resulting to;
CH3Cl + Cl2
CH2Cl2 + HCl
dichloromethane
CH2Cl2 + Cl2
CHCl3 + HCl
trichloromethane
CHCl3 + Cl2
CCl4 + HCl
Tetrachloromethane (if enough chlorine is used).