In the presence of UV light, free radical substitution occurs to halogenate alkanes.

Mechanism of Free Radical Substitution:

1. Initiation:

UV light provides energy to break the bond in a halogen molecule (e.g., Cl₂ or Br₂) into two halogen atoms (radicals) through homolytic cleavage.

Example:

\text{Cl}_2 \xrightarrow{\text{UV light}} 2\text{Cl}^\cdot

2. Propagation:

A halogen radical reacts with an alkane, abstracting a hydrogen atom to form a hydrogen halide and an alkyl radical.

\text{CH}_4 + \text{Cl}^\cdot \rightarrow \text{CH}_3^\cdot + \text{HCl}

\text{CH}_3^\cdot + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}^\cdot

3. Termination:

Two radicals combine to form a stable molecule, stopping the chain reaction.

\text{Cl}^\cdot + \text{Cl}^\cdot \rightarrow \text{Cl}_2

\text{CH}_3^\cdot + \text{Cl}^\cdot \rightarrow \text{CH}_3\text{Cl}

]

This reaction continues until the supply of reactants or UV light is exhausted.