Choose substitution or elimination from the conditions
Use solvent, nucleophile and product clues to distinguish halogenoalkane substitution from elimination.
Section 15 · Halogen compounds
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which hydroxide medium favors alcohol substitution?
What you will learn
- Select aqueous hydroxide for alcohol formation
- Select ethanolic hydroxide and heat for alkene formation
- Explain the bond changes
Aqueous hydroxide favors substitution
With aqueous NaOH and heat, OH⁻ replaces X and forms an alcohol. The C–X bond is replaced by C–O while the carbon skeleton stays the same.
For bromoethane, the product is ethanol: C₂H₅Br + NaOH(aq) → C₂H₅OH + NaBr.
Ethanolic hydroxide favors elimination
With ethanolic NaOH and heat, X and a hydrogen on a neighboring carbon are removed, making C=C and forming an alkene.
For bromoethane, CH₃CH₂Br → CH₂=CH₂. Read both solvent and product request before choosing; reaction conditions can interact with structure and should not be described as universal for every substrate.
Pause and explain
A question asks for an alkene from bromoethane. Which route should you choose?
Worked example
Choose conditions to make ethanol from bromoethane, then conditions to make ethene.
Show the worked solution
- Ethanol requires replacing Br by OH: use aqueous NaOH and heat.
- Ethene requires eliminating HBr and forming C=C.
- Use ethanolic NaOH and heat for elimination.
Answer Ethanol: aqueous NaOH and heat. Ethene: ethanolic NaOH and heat.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Complete a decision table for bromoethane: conditions for ethanol versus ethene.
Give me a hint
Match each product to bond substitution or elimination.
Compare my reasoning
- Ethanol replaces Br with OH.
- Use aqueous NaOH and heat.
- Ethene removes HBr; use ethanolic NaOH and heat.
Ethanol: aqueous NaOH, heat. Ethene: ethanolic NaOH, heat.
Look for these in your work
- Products matched
- Aqueous substitution route correct
- Ethanolic elimination route correct
A learner is given ethanolic NaOH and heat but answers ethanol. Identify the likely expected product and explain.
Give me a hint
Use the solvent and bond change.
Compare my reasoning
- Ethanolic hydroxide favors elimination.
- Br and a neighboring H are removed.
- A C=C forms, giving ethene from bromoethane.
Ethene is expected from bromoethane under ethanolic NaOH and heat because HX is eliminated.
Look for these in your work
- Condition interpreted
- Elimination described
- Product stated
Explain why solvent is a useful clue but not a replacement for checking the substrate and requested product.
Give me a hint
Conditions influence pathway, while structure determines which bond changes are possible.
Compare my reasoning
- Aqueous versus ethanolic conditions favor different routes.
- Elimination requires a neighboring hydrogen.
- The target product and substrate connectivity still constrain the answer.
Solvent suggests substitution or elimination, but the substrate must have the required atoms and the predicted product must match the question.
Look for these in your work
- Solvent clue recognized
- Structural requirement included
- Question target applied
Common mistakes
- Choosing solely from the base name and ignoring solvent
- Predicting alcohol when the target asks for an alkene
- Claiming the same route forms both products as the only outcome
What can you explain now?
Which hydroxide medium favors alcohol substitution?
Compare with the explanation
Aqueous NaOH
Aqueous hydroxide favors replacement of X by OH.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which details do you check before predicting substitution or elimination?
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.