Alkanes are saturated hydrocarbons containing only $C−C$ and $C-H$ bonds. For alkanes each carbon atom is bonded to four other atoms.

Halogenation of alkanes

Alkanes react with halogens in the presence of ultraviolet radiation or at a temperature of around 300°. This reaction is called a radical substitution reaction.

$CH_{4} + Cl_{2} \rightarrow CH_{3}Cl + HCl$


A radical is a species with an unpaired electron.

Mechanism of chlorination

A mechanism is a sequence of steps, showing the path taken by electrons in a reaction. the mechanism for radical substitution occurs in three stages:

  1. Initiation

    In this stage, the $Cl-Cl$ bond in a chlorine molecule is broken by homolytic fission, forming two chlorine radicals. UV radiation provides the energy for the bond fission.
    $$Cl_{2} \rightarrow Cl\cdot + Cl\cdot$$

    Homolytic Fission

    This is the splitting of a bond to produce two radicals each with a unpaired electron.

  2. Propagation

    • In the first stage, the alkane reacts with a chlorine radical. A single $C-H$ bond is broken forming a $ \cdot CH_{3}$ radical. Hydrogen chloride is also formed:
      $$CH_{4} + Cl\cdot \rightarrow CH_3{\cdot} + HCl $$
    • In the second stage, the methyl radical reacts with a chlorine molecule forming chloroethane and a chlorine radical which can then be used again in the first propagation stage.
      $$CH_3{\cdot} + Cl_2 \rightarrow CH_3Cl + Cl{\cdot} $$
  3. Termination

    In the termination stage, two radicals combine to form a molecule. In this example, the following molecules can be formed:

$$Cl{\cdot} + Cl{\cdot} \rightarrow Cl_{2} \\
CH_{3}{\cdot} + Cl{\cdot} \rightarrow CH_{3}Cl \\
CH_{3}{\cdot} + CH_{3}{\cdot} \rightarrow C_{2}H_{6}$$

The termination stage removes the radicals, stopping the reaction.

© Andrew Deniszczyc, 2023