Describe alkane substitution by chlorine and bromine
Describe UV-initiated halogen substitution in alkanes using ethane as the example.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is replaced in alkane halogenation?
What you will learn
- Recognise substitution of an alkane hydrogen
- Use chlorine or bromine and ultraviolet light
- Write a monohalogenation equation
A hydrogen atom can be replaced by a halogen
Alkanes react with chlorine or bromine under ultraviolet light by free-radical substitution. In ethane, one hydrogen can be replaced to form chloroethane or bromoethane, together with a hydrogen halide.
C₂H₆ + Cl₂ → C₂H₅Cl + HCl is a balanced example. UV light provides energy to initiate the radical chain; it is a condition, not a reactant in the balanced equation.
Substitution can continue
The first halogenoalkane can undergo further substitution, so a reaction mixture can contain products with more than one halogen atom and multiple positional isomers for longer chains.
The single-substitution equation is a representative step. Avoid promising a pure monohalogenated product unless the question supplies conditions or separation evidence.
Pause and explain
Which condition initiates alkane substitution by Cl₂ or Br₂?
Worked example
Describe the reaction of ethane with bromine in ultraviolet light and give a balanced equation for one substitution.
Show the worked solution
- Identify this as free-radical substitution.
- Replace one ethane hydrogen with bromine.
- The displaced hydrogen forms HBr.
Answer C₂H₆ + Br₂ → C₂H₅Br + HBr, in ultraviolet light.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the balanced equation for one hydrogen in ethane being replaced by chlorine.
Give me a hint
Replace H with Cl and account for the displaced H in the other product.
Compare my reasoning
- Ethane loses one H to become C₂H₅Cl.
- Cl₂ supplies the replacing Cl and the HCl chlorine.
- Check C, H and Cl counts.
C₂H₆ + Cl₂ → C₂H₅Cl + HCl (UV light).
Look for these in your work
- Replacement shown
- HCl included
- Atoms balanced
Why might chlorination of ethane produce more than one organic product?
Give me a hint
Consider what can happen after the first substitution.
Compare my reasoning
- The first product still contains C–H bonds.
- Further substitution can replace another hydrogen.
- Different hydrogens in later products can lead to different substituted structures.
Further substitution can continue, producing compounds with multiple chlorine atoms and potentially different structures.
Look for these in your work
- Remaining C–H noted
- Further reaction described
- Mixture possibility explained
A student writes C₂H₆ + Cl₂ → C₂H₅Cl. Identify the missing product.
Give me a hint
Count chlorine and hydrogen atoms in the equation.
Compare my reasoning
- The product side has one chlorine in C₂H₅Cl, but two enter as Cl₂.
- The displaced hydrogen also needs a product.
- Combine that hydrogen with the remaining chlorine atom.
HCl is missing: C₂H₆ + Cl₂ → C₂H₅Cl + HCl.
Look for these in your work
- Atom imbalance found
- Displaced H accounted for
- Equation corrected
Common mistakes
- Calling the reaction electrophilic addition
- Omitting the hydrogen halide coproduct
- Putting ultraviolet light in the equation as a consumed reagent
What can you explain now?
What is replaced in alkane halogenation?
Compare with the explanation
A hydrogen atom is replaced by a halogen atom
Substitution replaces an alkane hydrogen with Cl or Br.
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Make it stick
Why is ultraviolet light needed in the alkane–halogen substitution example?
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