Alkanes

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.

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

Radical

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.