The big idea: Alkanes are saturated and fairly unreactive, but in ultraviolet (UV) light they react with halogens (Cl2, Br2) by radical substitution — a hydrogen atom is swapped for a halogen atom.
The whole reaction is driven by radicals. A radical is a species with an unpaired electron, written with a dot, for example Cl• or •CH3. Radicals are very reactive because that lone electron 'wants' to pair up.
Radicals are made by homolytic fission — breaking a covalent bond so that one of the two shared electrons goes to each atom. This is the opposite of heterolytic fission, where both shared electrons go to one atom (making ions, not radicals).
Spot the difference fast: - Homolytic → same to each → two radicals (Cl• + Cl•). - Heterolytic → different (one keeps both) → ions (H⁺ + Br⁻).
A radical always carries a dot (•) for its unpaired electron.
Exam questions often ask you to contrast the two ways a bond can break, or to identify a diagram of each. The key is where the two bonding electrons go.
Homolytic fission
- The bond breaks evenly — one electron goes to each atom.
- Forms two radicals (each has an unpaired electron, shown •).
- Cl–Cl → Cl• + Cl•
- Shown by single-barbed (fish-hook) curly arrows — each moves one electron.
Heterolytic fission
- The bond breaks unevenly — both electrons go to one atom.
- Forms ions (a cation and an anion), not radicals.
- H–Br → H⁺ + Br⁻
- Shown by a full (double-barbed) curly arrow — it moves the whole pair.
Why UV starts the reaction: The Cl–Cl bond is relatively weak, and a UV photon carries just enough energy to break it homolytically into two chlorine radicals. Without UV (in the dark) no radicals form, so the reaction does not start — this is why the reaction is described as photochemical.
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Radical substitution always proceeds in three named stages: initiation, propagation and termination. The example below is the chlorination of methane, CH4 + Cl2 → CH3Cl + HCl.
Chlorination of methane: UV breaks Cl2 homolytically (initiation), then a chain of propagation steps regenerates radicals, and two radicals join to stop the chain (termination).
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
| Stage | What happens | Example step |
|---|---|---|
| Initiation | UV light splits a halogen molecule by homolytic fission, making radicals. | Cl2 → 2 Cl• |
| Propagation | A radical reacts to make a product and a new radical — the chain keeps going. | Cl• + CH4 → •CH3 + HCl |
| The new radical attacks another halogen molecule, regenerating the first radical. | •CH3 + Cl2 → CH3Cl + Cl• | |
| Termination | Two radicals combine — radicals are removed, so the chain stops. | •CH3 + Cl• → CH3Cl |
How to tell the stages apart: - Initiation — a radical is created from a molecule (count goes 0 → 2 radicals). Always needs UV. - Propagation — a radical is used and a new radical is made (one in, one out): the number of radicals stays the same. - Termination — two radicals combine into one molecule (count goes down): no radical is left.
Why a chain reaction: Each propagation step regenerates a radical, so one initiation event can trigger thousands of propagation cycles. That is why radical substitution is called a chain reaction — and why a mixture of products (CH3Cl, CH2Cl2, …) forms.
How this is tested: R3.3 turns up as a quick Paper 1A MCQ ('which equation is a propagation step?', or 'which diagram shows heterolytic fission?') and as a short Paper 2 write-an-equation or distinguish question.
The classic Paper 2 asks are: write the initiation equation for a UV reaction (and name the bond breaking as homolytic), and distinguish homolytic from heterolytic fission — for the marks you must say where the two bonding electrons go in each.
Equation marks the markers want: Show the dot (•) on every radical, balance the equation, and label the stage if asked. For 'distinguish', make the contrast explicit: one electron to each atom → radicals vs both electrons to one atom → ions.
IB-style question — initiation step (a)
In UV light, chlorine reacts with ethane (C2H6) by radical substitution. (a) Write an equation for the initiation step and state the type of bond breaking involved. [2]
How to score the marks
- Mark 1 — the equation. UV light splits the chlorine molecule into two chlorine radicals: Cl2 → 2 Cl• (the dot shows the unpaired electron on each radical).
- Mark 2 — the bond breaking. Each Cl atom keeps one of the two shared electrons, so this is homolytic fission.
Final answer
Cl2 → 2 Cl• (initiation); the Cl–Cl bond undergoes homolytic fission.
IB-style question — propagation steps (b)
(b) Write the two propagation steps for the reaction of chlorine radicals with ethane to form chloroethane, C2H5Cl. [2]
How to score the marks
- Step 1 — radical attacks the alkane. A chlorine radical removes a hydrogen atom from ethane, making an ethyl radical and HCl: Cl• + C2H6 → •C2H5 + HCl.
- Step 2 — radical regenerates a chlorine radical. The ethyl radical reacts with a chlorine molecule, giving the product and a new chlorine radical: •C2H5 + Cl2 → C2H5Cl + Cl•.
- Each step uses one radical and makes one radical, so the chain continues.
Final answer
Cl• + C2H6 → •C2H5 + HCl, then •C2H5 + Cl2 → C2H5Cl + Cl•.