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9476 / 2027
Organic Chemistry

Lesson 8 of 34

Radical substitution is a chain reaction

Initiation makes radicals; propagation regenerates a radical; termination removes them.

A-Level 9476 (2026-2027)

  • I can describe the free-radical substitution mechanism.Syllabus

    Syllabus 9476, 11.3(k). Describe the free-radical substitution mechanism.

  • I can describe alkane chemistry using ethane.Syllabus

    Syllabus 9476, 11.4(a). Describe alkane chemistry using ethane.

Radical substitution is a chain reaction

Initiation makes radicals; propagation regenerates a radical; termination removes them.

Ethane reacts with chlorine in ultraviolet light at room temperature to form chloroethane and HCl. The UV light splits Cl-Cl evenly, one electron to each atom. This is homolytic fission, and it makes radicals, not ions. One chlorine radical can keep many propagation steps going before a termination step removes it.

Initiation: one bonding electron goes to each chlorine atom

Two single-barbed arrows start at the Cl-Cl bond and end at separate chlorine atoms. The product is two chlorine radicals, each with one unpaired electron.

Single-barbed fishhooks track individual electrons. UV supplies energy for Cl2 → 2Cl•.
The complete ethane/chlorine chain
StageEquationWhat it achieves
InitiationCl2 → 2Cl•, under UV.Creates radicals from a non-radical molecule.
Propagation 1Cl• + CH3CH3 → HCl + CH3CH2•.Abstracts H and creates an ethyl radical.
Propagation 2CH3CH2• + Cl2 → CH3CH2Cl + Cl•.Forms the C-Cl product and regenerates the chain carrier.
Termination example 1Cl• + Cl• → Cl2.Two radicals combine; no new radical.
Termination example 2CH3CH2• + Cl• → CH3CH2Cl.Two radicals combine; no new radical.
Termination example 3CH3CH2• + CH3CH2• → CH3CH2CH2CH3.Two ethyl radicals form butane.

Adding the propagation steps cancels the ethyl and chlorine radicals. The overall equation is CH3CH3 + Cl2 → CH3CH2Cl + HCl. More hydrogens can also be replaced, so a mixture of products forms. Using excess ethane makes it less likely that a radical attacks the product. Bromination in UV light follows the same three stages with bromine radicals.

Check your understanding

Is CH3CH2* + Cl2 -> CH3CH2Cl + Cl* a termination step, since it makes the wanted product?

  1. No. A new radical forms, so the chain keeps going; it is a propagation step.
  2. No. It is initiation, because the Cl-Cl bond is broken to make chlorine radicals.
  3. Yes. The organic product appears in this step, so the chain reaction ends here.
Show the answer

No. A new radical forms, so the chain keeps going; it is a propagation step.

Classify a step by what happens to radicals. Propagation uses one radical and makes another. Termination removes radicals by joining two of them.

Exam-style question

Ethane reacts with bromine in ultraviolet light to form bromoethane. Name and describe the mechanism, including one termination step. [3]

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