Prepare an alkene by eliminating hydrogen halide
Use ethanolic sodium hydroxide and heat to eliminate HX from a halogenoalkane and form an alkene.
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
Which conditions favor elimination from a halogenoalkane to form an alkene?
What you will learn
- Recognise elimination as an alkene preparation
- Select ethanolic NaOH and heat
- Write a balanced example using bromoethane
Elimination forms a carbon–carbon double bond
When a halogenoalkane is heated with ethanolic sodium hydroxide, H and X are removed from adjacent carbon atoms. A C=C bond forms between them.
For bromoethane, CH₃CH₂Br + NaOH → CH₂=CH₂ + NaBr + H₂O. The ethanol is the solvent and heat is supplied. Aqueous hydroxide instead favors substitution to an alcohol in the syllabus examples.
Distinguish substitution and elimination conditions
Elimination removes atoms or groups to make a multiple bond; substitution replaces the leaving group with another group.
Check both reagent medium and product. If the question specifies ethanolic NaOH and heat, predict the alkene route rather than the aqueous substitution route.
Pause and explain
What happens to the carbon skeleton in elimination of HX?
Worked example
Write the equation and conditions for preparing ethene from bromoethane.
Show the worked solution
- Remove H from one carbon and Br from its neighbor.
- Form C=C between the two carbon atoms.
- Add the sodium and displaced hydrogen/bromine products.
Answer CH₃CH₂Br + NaOH → CH₂=CH₂ + NaBr + H₂O; ethanolic NaOH, heat.
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Test this topic →From guided practice to a new situation
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 elimination equation for 2-bromopropane using ethanolic NaOH.
Give me a hint
Remove H and Br from neighboring carbons and show a propene product.
Compare my reasoning
- The three-carbon skeleton is retained.
- A C=C forms between adjacent carbons.
- Balance sodium, bromine, oxygen and hydrogen.
CH₃CHBrCH₃ + NaOH → CH₃CH=CH₂ + NaBr + H₂O (ethanol, heat).
Look for these in your work
- Three-carbon chain retained
- Alkene formed
- Equation balanced
Choose between ethanolic and aqueous NaOH to make ethene from bromoethane; justify.
Give me a hint
Use the target product and the reaction type.
Compare my reasoning
- The target has a C=C bond.
- Removal of HBr is elimination.
- Ethanolic NaOH with heat favors this syllabus route.
Use ethanolic NaOH and heat to eliminate HBr and form ethene.
Look for these in your work
- Alkene target recognised
- Elimination named
- Conditions selected
A learner writes bromoethane + NaOH(aq) → ethene. Explain the mismatch.
Give me a hint
Compare the stated solvent with the intended pathway.
Compare my reasoning
- Aqueous hydroxide favors nucleophilic substitution in the syllabus examples.
- Substitution replaces Br with OH and forms ethanol.
- Use ethanolic NaOH and heat for alkene elimination.
The aqueous conditions favor substitution to ethanol; ethanolic NaOH and heat are used for elimination to ethene.
Look for these in your work
- Aqueous route identified
- Substitution product stated
- Elimination conditions supplied
Common mistakes
- Using aqueous NaOH for the elimination example
- Keeping the C–Br bond and also forming C=C
- Omitting water or sodium halide from the equation
What can you explain now?
Which conditions favor elimination from a halogenoalkane to form an alkene?
Compare with the explanation
Ethanolic NaOH and heat
The alcohol solvent and heat are the stated elimination conditions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do the solvent and product distinguish elimination from substitution of a halogenoalkane?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.