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1.9.6Unit 1. Structure, Bonding and Introduction to Organic Chemistry

The Free Radical Substitution Mechanism — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 36 of 36 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
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Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Part 1 of 6The Overall Reaction

The mechanism has three stagesinitiation, propagation and termination.

Reagents
  • Reagents Methane reacts with chlorine in the presence of ultraviolet light, a substitution in which chlorine replaces hydrogen.
Overall equation
  • Overall equation CH₄ + Cl₂ → CH₃Cl + HCl is the simplest net change, producing chloromethane and hydrogen chloride.
Conditions
  • Conditions UV light or sunlight is needed because it supplies enough energy to break the Cl–Cl bond.
Type
  • Type It is a substitution because one atom is replaced by another, and a chain reaction because radicals are regenerated.

Part 2 of 6The Three Stages

1
Initiation: UV light splits chlorine by homolytic fission, Cl₂ → 2Cl•.
2
Propagation, first step: a chlorine radical removes hydrogen from methane, Cl• + CH₄ → •CH₃ + HCl.
3
Propagation, second step: the methyl radical reacts with chlorine, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating a chlorine radical.
4
Termination: two radicals combine and the chain stops, for example Cl• + Cl• → Cl₂.
5
Other termination steps are •CH₃ + Cl• → CH₃Cl and •CH₃ + •CH₃ → C₂H₆.

Part 3 of 6Multiple Substitution

Multiple Substitution
Further reaction
  • Further reaction The chloromethane formed still has C–H bonds, so chlorine radicals can replace more hydrogens.
Products
  • Products CH₂Cl₂, CHCl₃ and CCl₄ can all form, giving a mixture that must be separated.
Limiting substitution
  • Limiting substitution A large excess of methane makes it more likely that Cl• collides with methane rather than with chloromethane.
Ethane
  • Ethane C₂H₆ forms in a termination step, which is evidence supporting the proposed mechanism.

Part 4 of 6Related Examples

Related Examples
Bromine
  • Bromine Alkanes react with bromine by the same mechanism, though more slowly than with chlorine.
Larger alkanes
  • Larger alkanes Longer alkanes give a mixture of isomeric products because any hydrogen can be replaced.
Why not in the dark
  • Why not in the dark Without UV light no radicals form, so the reaction does not start at room temperature.
Equation writing
  • Equation writing Every propagation step must contain a radical on each side and the pair of steps must add up to the overall reaction.

Part 5 of 6Worked Example

Question Write the propagation steps for the bromination of ethane.

  1. A bromine radical removes a hydrogen atom from ethane: Br• + C₂H₆ → •C₂H₅ + HBr.
  2. The ethyl radical attacks a bromine molecule: •C₂H₅ + Br₂ → C₂H₅Br + Br•.
  3. The bromine radical is regenerated, so the cycle can repeat many times.

Answer Br• + C₂H₆ → •C₂H₅ + HBr, then •C₂H₅ + Br₂ → C₂H₅Br + Br•.

Part 6 of 6Exam Checklist & Tips

1.9.6 – Key Points for the Exam

  • Initiation The step that creates radicals from non-radical molecules.
  • Propagation Steps in which a radical reacts to produce another radical, continuing the chain.
  • Termination A step in which two radicals combine to form a stable molecule, ending the chain.
  • Substitution A reaction in which one atom or group is replaced by another.
  • Chain reaction A reaction in which radicals are continually regenerated.

Common Mistakes

  • Showing a termination step as a reaction between a radical and a non-radical molecule.
  • Writing propagation steps that do not produce a new radical.
  • Using curly arrows with full heads in homolytic steps.
  • Forgetting to mention UV light as the condition.

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