3.3.3—Radical substitution
- Syllabus
- First assessment 2025
- Objective
- 3.3.3
- Level
- HL
Free-Radical Substitution
Initiation creates radicals; propagation abstracts H from an alkane and then regenerates the halogen radical; termination combines radicals. A mixture can form because substitution may occur at different positions.
C2H6+Cl⋅→C2H5⋅+HCl;C2H5⋅+Cl2→C2H5Cl+Cl⋅
For methane chlorination, initiation forms 2Cl· from Cl₂ under UV. Propagation uses Cl· + CH₄ → HCl + CH₃· and CH₃· + Cl₂ → CH₃Cl + Cl·; termination combines two radicals. Further substitution creates a mixture, so the mechanism does not guarantee only CH₃Cl.
Writing Chain-Substitution Equations
Assessment in practice
Representative question
Question 1
[Maximum number: 4]
Explain the reaction mechanism by writing equations for each step.
One initiation step:
Two propagation steps:
One termination step:
One initiation step:
Cl2→2⋅Cl
Two propagation steps:
C2H6+⋅Cl→⋅C2H5+HCl
- C2H5+Cl2→C2H5Cl+⋅Cl
One termination step:
- C2H5+⋅C2H5→C4H10
OR
- C2H5+⋅Cl→C2H5Cl - Cl+⋅Cl→Cl2
Radical Chain Summary
Retrieve the route: locate the unpaired electron, split the bond homolytically, initiate with UV or heat, propagate by single-electron steps and terminate by radical combination.
Every propagation step must regenerate a radical, and every radical symbol and single-electron movement must be shown where required.