3.7.2Physical Chemistry
Chlorination of Alkanes — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 91 of 102
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 5Conditions and Overall Reaction
Conditions and Overall Reaction
Reagents
- Reagents An alkane such as methane reacts with chlorine gas, Cl₂.
Conditions
- Conditions Ultraviolet light is needed to supply the energy to break the Cl–Cl bond;
- the alkane does not react in the dark.
Overall equation
- Overall equation CH₄ + Cl₂ → CH₃Cl + HCl, in which one hydrogen atom is substituted by chlorine.
Mechanism type
- Mechanism type A free-radical chain reaction, called free-radical substitution.
Part 2 of 5The Three Stages
Using methane as the example, with • showing an unpaired electron.
1
Initiation: UV light splits chlorine homolytically, Cl₂ → 2Cl•, forming two chlorine radicals.
2
Propagation step 1: a chlorine radical removes hydrogen, CH₄ + Cl• → •CH₃ + HCl.
3
Propagation step 2: the methyl radical reacts with chlorine, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating a chlorine radical.
4
The chlorine radical produced in step 2 starts another cycle, which is why the reaction is a chain reaction.
5
Termination: two radicals combine to end the chain, for example Cl• + •CH₃ → CH₃Cl or •CH₃ + •CH₃ → C₂H₆.
Part 3 of 5Features of Each Stage
Initiation
- Initiation A molecule forms two radicals, and this is the only step that needs UV light.
Propagation
- Propagation Always two steps, each with a radical and a molecule reacting to give a new radical and a new molecule.
Termination
- Termination Two radicals meet and form a molecule, so no radicals remain to continue the chain.
Ethane example
- Ethane example C₂H₆ + Cl• → •C₂H₅ + HCl, then •C₂H₅ + Cl₂ → C₂H₅Cl + Cl•.
Part 4 of 5Limitations of the Reaction
Limitations of the Reaction
Further substitution
- Further substitution The product CH₃Cl can react again to form CH₂Cl₂, CHCl₃ and CCl₄ in later cycles.
Control
- Control Using excess alkane makes it more likely that a chlorine radical meets an alkane molecule rather than a product.
Mixtures
- Mixtures Longer alkanes can be substituted at different carbon atoms, giving a mixture of isomers that is hard to separate.
By-products
- By-products Termination steps can produce traces of other alkanes such as ethane, so the product is never pure.
Part 5 of 5Exam Checklist & Tips
3.7.2 – Key Points for the Exam
- Free radical A species with an unpaired electron.
- Initiation The step in a chain reaction in which radicals are first formed.
- Propagation Steps in which a radical reacts with a molecule to form a new radical and a new molecule.
- Termination A step in which two radicals combine to form a molecule and the chain ends.
- Substitution A reaction in which one atom or group is replaced by another.
Common Mistakes
- Showing UV light as being needed in the propagation steps; it is required for initiation only.
- Writing a propagation step that produces two radicals, or one with no radical, instead of one radical in and one radical out.
- Forgetting the dot on radicals in the equations.
- Giving a termination step that has a radical among the products.
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