Substitution Reaction

  1. Substitution reaction is a reaction where one atom (or a group of atoms) in a molecule is replaced by another atom (or a group of atoms).
  2. Substitution reaction of alkanes occurs when an alkane is mixed with a halogen in the presence of sunlight (ultraviolet light).
  3. Since the substitution reaction of alkanes is the reaction between alkanes and halogen, hence it is also called the halogenation reaction.
  4. Substitution reaction is a slow reaction.
MUST Know!
Substitution reaction of alkanes take place only in the presence of ultraviolet light (such as sunlight).

Reaction between Methane and Chlorine


  1. We can see that, in the reaction, each hydrogen atom in the alkane molecule is substituted one by one by a chlorine atom. 
  2. The sunlight or UV light is needed to break covalent bond in halogen and alkane molecules to produce hydrogen and chlorine atom.
  3. Alkanes also react with bromine vapour in the presence of UV light, but with a much lower rate.


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