6.3.3Additional lessons
Halogenation of Alkanes — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 149 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4The Overall Reaction
Alkanes are unreactive to most reagents but react with halogens under UV light.
Reactants
- Reactants An alkane such as methane reacts with chlorine or bromine in the presence of UV light.
Substitution
- Substitution A hydrogen atom is replaced by a halogen atom, so CH₄ + Cl₂ → CH₃Cl + HCl.
Further products
- Further products More hydrogen atoms can be replaced, giving CH₂Cl₂, CHCl₃ and CCl₄ in a mixture.
Mechanism type
- Mechanism type The reaction proceeds by a free-radical chain mechanism with three stages.
Part 2 of 4The Three Stages
1
Initiation: UV light breaks the halogen molecule homolytically, Cl₂ → 2Cl•.
2
Propagation step 1: a chlorine radical removes a hydrogen atom, Cl• + CH₄ → •CH₃ + HCl.
3
Propagation step 2: the methyl radical attacks a chlorine molecule, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating Cl•.
4
Termination: two radicals combine, such as Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl, or •CH₃ + •CH₃ → C₂H₆.
5
Because propagation regenerates the radical, thousands of product molecules can form from one initiation step.
Part 3 of 4Features of the Reaction
Features of the Reaction
Mixture of products
- Mixture of products Further substitution and termination steps give a mixture, so yields of a single product are low.
Control
- Control Using excess alkane makes it more likely that a chlorine radical meets methane rather than chloromethane.
Bromine
- Bromine Bromine reacts in the same way but more slowly, and the red-brown colour fades as the reaction proceeds.
Termination product
- Termination product Ethane forms in a termination step, which is evidence supporting the radical mechanism.
| Stage | Radicals | Example equation |
|---|---|---|
| Initiation | Made | Cl₂ → 2Cl• |
| Propagation | Used and remade | Cl• + CH₄ → •CH₃ + HCl |
| Termination | Removed | Cl• + •CH₃ → CH₃Cl |
Part 4 of 4Exam Checklist & Tips
6.3.3 – Key Points for the Exam
- Free-radical substitution A mechanism in which radicals replace an atom in a molecule through a chain reaction.
- Initiation The step that forms radicals from a molecule.
- Propagation Steps in which a radical reacts and another radical is produced.
- Termination Steps in which two radicals combine and the chain stops.
Common Mistakes
- Writing a radical without the dot in propagation steps.
- Putting both reactions of propagation as a single step.
- Forgetting UV light as the condition, or using aqueous conditions.
- Thinking the reaction gives only one product when a mixture forms.
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