Radical substitution of alkanes
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Question
What is a radical?
Answer
A species with an **unpaired electron**, written with a dot (e.g. Cl•, •CH_{3}); very reactive.
Question
What is homolytic fission?
Answer
A bond breaks **evenly** — **one electron goes to each** atom, forming two **radicals**.
Question
What is heterolytic fission?
Answer
A bond breaks **unevenly** — **both electrons go to one** atom, forming **ions** (a cation and an anion).
Question
Homolytic vs heterolytic — which makes radicals?
Answer
**Homolytic** fission makes radicals; **heterolytic** fission makes ions.
Question
What is radical substitution?
Answer
An alkane reacts with a halogen in **UV light**, replacing an H atom with a halogen atom, via a radical chain.
Question
What happens in the initiation step?
Answer
**UV light** breaks the halogen molecule by **homolytic** fission, e.g. Cl_{2} → 2 Cl•.
Question
What happens in propagation?
Answer
A radical reacts to give a product **and a new radical**, so the chain continues (radical count unchanged).
Question
Write the two propagation steps for CH_{4} + Cl_{2}.
Answer
Cl• + CH_{4} → •CH_{3} + HCl, then •CH_{3} + Cl_{2} → CH_{3}Cl + Cl•.
Question
What happens in termination?
Answer
**Two radicals combine** into one molecule, removing radicals and **stopping** the chain (e.g. •CH_{3} + Cl• → CH_{3}Cl).
Question
Why is UV light needed?
Answer
It supplies the energy to break the halogen bond **homolytically** and create the first radicals.
Question
Why is it called a chain reaction?
Answer
Each propagation step **regenerates** a radical, so one initiation triggers many cycles (and a mixture of products).
Question
How is a radical drawn?
Answer
With a **dot (•)** next to it, showing the single unpaired electron (e.g. Cl•).
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Topic 6.3 hub
Electron sharing reactions
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