Here’s the mechanism of free radical halogenation explained entirely in words:
1. Initiation
The reaction begins when a halogen molecule (like chlorine or bromine) is exposed to heat or light. This energy causes the halogen molecule to split into two highly reactive halogen atoms, each with an unpaired electron. These are called halogen radicals and are what drive the reaction forward.
2. Propagation
This is the main part of the reaction, where the actual substitution happens, and it occurs in two steps that keep repeating:
- A halogen radical attacks a hydrogen atom on a hydrocarbon, removing it and leaving behind a carbon radical (a carbon atom with an unpaired electron). At the same time, a hydrogen halide molecule is formed.
- The newly formed carbon radical reacts with another halogen molecule. This produces a halogenated hydrocarbon and regenerates a halogen radical, which can then continue the process with another hydrocarbon molecule.
Because the halogen radical is regenerated, the reaction can continue as a chain reaction, producing many molecules of the halogenated product.
3. Termination
Eventually, radicals collide with each other and combine, which stops the chain reaction. For example, two halogen radicals can join to reform a halogen molecule, two carbon radicals can form a carbon-carbon bond, or a carbon radical can combine with a halogen radical to form a halogenated hydrocarbon.
Key Concepts
- The reaction relies on radicals, which are atoms or molecules with unpaired electrons.
- The stability of radicals determines which hydrogen atoms are most likely to be replaced (tertiary radicals are most stable, then secondary, then primary).
- Bromine radicals are more selective than chlorine radicals, meaning they prefer attacking the more stable carbon radical.