Mechanism: Free Radical Substitution
Chain reactions
- A free radical is a species with one (or more than one) unpaired electrons
The diagram shows a free radical which has one unpaired electron
- A free radical reaction is a reaction involving free radicals and is a three-step reaction:
- Initiation is the first step and involves breaking a covalent bond using energy from ultraviolet (UV) light from the sun to form two free radicals
- The propagation step is the second step in which the free radical can attack reactant molecules to form even more radicals
- These in turn can again attack other molecules to form more free radicals and so on
- In the termination step, two free radicals react together to form a product molecule
The diagram shows the different stages of a general radical chain reaction
- In an exam, the type of reaction will be a specific alkane and halogen
- For example, ethane and chlorine
The Free Radical Substitution Reaction of Ethane and Chlorine
Examiner Tip
Remember:Initiation = a molecule forming two radicalsPropagation = always two steps, a molecule and a radical reacting to form a different molecule and radical combinationTermination = two radicals reacting to form a moleculeRead the question carefully - sometimes they specify which termination step you should include.
