Substitute a halogen with aqueous hydroxide
Use aqueous NaOH and heat to replace X with OH and form an alcohol.
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 nucleophile forms an alcohol from a halogenoalkane in aqueous NaOH?
What you will learn
- Identify hydroxide as the nucleophile
- Write a substitution equation
- State aqueous NaOH and heat
Hydroxide replaces the halogen
Aqueous NaOH with heat converts a halogenoalkane to an alcohol. OH⁻ donates a lone pair to the carbon attached to X while the C–X bond pair moves to the halogen.
For bromoethane: C₂H₅Br + NaOH(aq) → C₂H₅OH + NaBr. In ionic form, C₂H₅Br + OH⁻ → C₂H₅OH + Br⁻.
Solvent changes the product route
Aqueous hydroxide favors nucleophilic substitution to an alcohol. Ethanolic hydroxide with heat favors elimination to an alkene, treated in a separate lesson.
Always read the solvent and the requested product. The same base name does not make the two conditions interchangeable.
Pause and explain
Which product forms from bromoethane and aqueous NaOH on heating?
Worked example
Write the equation and conditions for converting 1-bromopropane to propan-1-ol.
Show the worked solution
- OH⁻ replaces Br at carbon 1.
- The carbon skeleton remains three carbons.
- Include aqueous NaOH and heat.
Answer CH₃CH₂CH₂Br + NaOH(aq) → CH₃CH₂CH₂OH + NaBr; 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.
Write a molecular and ionic equation for bromoethane substitution by hydroxide.
Give me a hint
Show OH entering and Br leaving.
Compare my reasoning
- The molecular reagent is NaOH(aq).
- Ethanol and NaBr form.
- Cancel spectator Na⁺ for the ionic equation.
Molecular: C₂H₅Br + NaOH → C₂H₅OH + NaBr. Ionic: C₂H₅Br + OH⁻ → C₂H₅OH + Br⁻.
Look for these in your work
- Molecular equation balanced
- Ionic equation balanced
- Spectator removed
A learner heats 2-bromopropane with NaOH in water and predicts propene as the main syllabus route. Correct the prediction.
Give me a hint
Use the solvent to select substitution or elimination.
Compare my reasoning
- The solvent is water, so these are aqueous conditions.
- Aqueous hydroxide substitutes Br with OH.
- The alcohol product is propan-2-ol.
Aqueous NaOH favors substitution to propan-2-ol; ethanolic NaOH and heat are used for alkene elimination.
Look for these in your work
- Solvent read
- Reaction type corrected
- Product named
How many moles of propan-2-ol can form from 0.25 mol of 2-bromopropane with excess aqueous NaOH?
Give me a hint
Use the 1:1 substitution equation.
Compare my reasoning
- One mole of halogenoalkane forms one mole of alcohol.
- Hydroxide is in excess.
- Apply the mole ratio to the limiting halogenoalkane.
0.25 mol propan-2-ol.
Look for these in your work
- Equation ratio used
- Limiting reagent identified
- Amount stated
Common mistakes
- Using ethanolic NaOH when the target is an alcohol
- Adding OH without removing Br
- Changing the carbon skeleton
What can you explain now?
Which nucleophile forms an alcohol from a halogenoalkane in aqueous NaOH?
Compare with the explanation
OH⁻
Hydroxide donates a lone pair and replaces X.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do aqueous and ethanolic hydroxide conditions lead to different halogenoalkane products?
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.