Chlorination of Alkanes (AQA A Level Chemistry)
Revision Note
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 Tips and Tricks
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.
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