6.3.3Additional lessons
Halogenation of Alkanes — IB Chemistry SL Revision Notes & Topical Questions
5 min read 12 parts Lesson 96 of 98
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.
Section 1 of 12Why Alkanes Need Radicals
The reaction proceeds as a chain reaction in three stages.
Unreactive
- Unreactive Alkanes have strong, non-polar C–C and C–H bonds, so they do not react with polar reagents.
Reaction type
- Reaction type Alkanes react with halogens by free-radical substitution when UV light is present.
Substitution
- Substitution A hydrogen atom is replaced by a halogen atom and a hydrogen halide is formed.
Overall equation
- Overall equation For methane and chlorine: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g).
Section 2 of 12The Mechanism for Methane and Chlorine
1
Initiation: UV light breaks the Cl–Cl bond homolytically, Cl₂ → 2Cl•.
2
Propagation 1: a chlorine radical removes a hydrogen atom, CH₄ + Cl• → •CH₃ + HCl.
3
Propagation 2: the methyl radical attacks a chlorine molecule, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating Cl•.
4
Termination: two radicals combine to end the chain, for example Cl• + Cl• → Cl₂.
5
Other terminations include •CH₃ + Cl• → CH₃Cl and •CH₃ + •CH₃ → C₂H₆.
Section 3 of 12Further Substitution and Products
Further Substitution and Products
Multiple substitution
- Multiple substitution CH₃Cl can react further to give CH₂Cl₂, CHCl₃ and CCl₄.
Mixture of products
- Mixture of products A mixture is formed, which must be separated by distillation.
Excess alkane
- Excess alkane A large excess of alkane makes monosubstitution the most likely outcome.
By-products
- By-products Termination between two alkyl radicals gives longer alkanes, such as C₂H₆.
Reactivity
- Reactivity Bromine reacts more slowly than chlorine under the same conditions with UV light.
Section 4 of 12Evaluating the Reaction
Evaluating the Reaction
Selectivity
- Selectivity Free-radical substitution is not selective, so the hydrogen atoms are replaced at random.
Safety
- Safety Chlorine and bromine are toxic, so reactions are done in a fume cupboard.
Conditions
- Conditions UV light or sunlight and room temperature are needed; heating alone is not used in school work.
Use of products
- Use of products Chloroalkanes are used as solvents and intermediates, but CFCs cause ozone depletion.
Section 5 of 12Worked Example
Question Write the initiation, propagation and one termination step for the reaction of ethane with bromine in UV light.
- InitiationBr₂ → 2Br• (homolytic fission by UV light).
- PropagationC₂H₆ + Br• → •C₂H₅ + HBr, then •C₂H₅ + Br₂ → C₂H₅Br + Br•.
- Terminationfor example •C₂H₅ + •C₂H₅ → C₄H₁₀.
Answer Overall: C₂H₆ + Br₂ → C₂H₅Br + HBr.
Section 6 of 12Exam Checklist & Tips
6.3.3 – Key Points for the Exam
- Free-radical substitution A reaction where a radical chain mechanism replaces an atom in a molecule.
- Initiation The first step, forming radicals from a non-radical.
- Propagation Steps where a radical reacts and another radical is formed.
- Termination Steps in which two radicals combine and the chain stops.
Common Mistakes
- Leaving out the dots on radicals in the mechanism.
- Showing a radical reacting in termination with a molecule, rather than another radical.
- Writing UV light as a catalyst; it supplies energy.
- Forgetting that propagation regenerates the chlorine radical.
Section 7 of 12Why Alkanes Need Radicals
The reaction proceeds as a chain reaction in three stages.
Unreactive
- Unreactive Alkanes have strong, non-polar C–C and C–H bonds, so they do not react with polar reagents.
Reaction type
- Reaction type Alkanes react with halogens by free-radical substitution when UV light is present.
Substitution
- Substitution A hydrogen atom is replaced by a halogen atom and a hydrogen halide is formed.
Overall equation
- Overall equation For methane and chlorine: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g).
Section 8 of 12The Mechanism for Methane and Chlorine
1
Initiation: UV light breaks the Cl–Cl bond homolytically, Cl₂ → 2Cl•.
2
Propagation 1: a chlorine radical removes a hydrogen atom, CH₄ + Cl• → •CH₃ + HCl.
3
Propagation 2: the methyl radical attacks a chlorine molecule, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating Cl•.
4
Termination: two radicals combine to end the chain, for example Cl• + Cl• → Cl₂.
5
Other terminations include •CH₃ + Cl• → CH₃Cl and •CH₃ + •CH₃ → C₂H₆.
Section 9 of 12Further Substitution and Products
Further Substitution and Products
Multiple substitution
- Multiple substitution CH₃Cl can react further to give CH₂Cl₂, CHCl₃ and CCl₄.
Mixture of products
- Mixture of products A mixture is formed, which must be separated by distillation.
Excess alkane
- Excess alkane A large excess of alkane makes monosubstitution the most likely outcome.
By-products
- By-products Termination between two alkyl radicals gives longer alkanes, such as C₂H₆.
Reactivity
- Reactivity Bromine reacts more slowly than chlorine under the same conditions with UV light.
Section 10 of 12Evaluating the Reaction
Evaluating the Reaction
Selectivity
- Selectivity Free-radical substitution is not selective, so the hydrogen atoms are replaced at random.
Safety
- Safety Chlorine and bromine are toxic, so reactions are done in a fume cupboard.
Conditions
- Conditions UV light or sunlight and room temperature are needed; heating alone is not used in school work.
Use of products
- Use of products Chloroalkanes are used as solvents and intermediates, but CFCs cause ozone depletion.
Section 11 of 12Worked Example
Question Write the initiation, propagation and one termination step for the reaction of ethane with bromine in UV light.
- InitiationBr₂ → 2Br• (homolytic fission by UV light).
- PropagationC₂H₆ + Br• → •C₂H₅ + HBr, then •C₂H₅ + Br₂ → C₂H₅Br + Br•.
- Terminationfor example •C₂H₅ + •C₂H₅ → C₄H₁₀.
Answer Overall: C₂H₆ + Br₂ → C₂H₅Br + HBr.
Section 12 of 12Exam Checklist & Tips
6.3.3 – Key Points for the Exam
- Free-radical substitution A reaction where a radical chain mechanism replaces an atom in a molecule.
- Initiation The first step, forming radicals from a non-radical.
- Propagation Steps where a radical reacts and another radical is formed.
- Termination Steps in which two radicals combine and the chain stops.
Common Mistakes
- Leaving out the dots on radicals in the mechanism.
- Showing a radical reacting in termination with a molecule, rather than another radical.
- Writing UV light as a catalyst; it supplies energy.
- Forgetting that propagation regenerates the chlorine radical.
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