Alkanes react with chlorine or bromine in ultraviolet (UV) light. A hydrogen atom is replaced by a halogen atom, so the reaction is a substitution. It happens by a free-radical mechanism with three stages.
CH4 + Cl2 → CH3Cl + HCl
1. Initiation
UV light provides the energy to break the Cl-Cl bond by homolytic fission, forming chlorine radicals:
Cl2 → 2Cl•
2. Propagation
A radical reacts with a molecule to form a product and a new radical:
Cl• + CH4 → CH3• + HCl
CH3• + Cl2 → CH3Cl + Cl•
The Cl• made in the second step starts the first step again, so this is a chain reaction. One chlorine radical can convert thousands of methane molecules.3. Termination
Two radicals join, removing radicals and stopping the chain:
Cl• + Cl• → Cl2
CH3• + Cl• → CH3Cl
CH3• + CH3• → C2H6
Small amounts of ethane in the product are evidence for this mechanism.
Worked example: give the initiation step and overall equation for methane with bromine. Initiation: Br2 → 2Br•. Overall: CH4 + Br2 → CH3Br + HBr. The propagation steps follow the same pattern as for chlorine.
1. Initiation
UV light provides the energy to break the Cl-Cl bond by homolytic fission, forming chlorine radicals:
2. Propagation
A radical reacts with a molecule to form a product and a new radical:
The Cl• made in the second step starts the first step again, so this is a chain reaction. One chlorine radical can convert thousands of methane molecules.3. Termination
Two radicals join, removing radicals and stopping the chain:
Small amounts of ethane in the product are evidence for this mechanism.
Worked example: give the initiation step and overall equation for methane with bromine. Initiation: Br2 → 2Br•. Overall: CH4 + Br2 → CH3Br + HBr. The propagation steps follow the same pattern as for chlorine.