Reduce aldehydes and ketones to alcohols
Use NaBH₄ or LiAlH₄ to convert aldehydes into primary and ketones into secondary alcohols.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. State reagents and conditions, follow the stated reaction pathway, and distinguish each alcohol class and oxidation product. Written practice is self-checked
Before you begin
- Use functional-group formulae and reaction types from Topic 13, alkene reactions from Topic 14, and halogenoalkane substitution from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reagents are listed for reducing aldehydes and ketones?
What you will learn
- Recall NaBH₄ and LiAlH₄
- Predict aldehyde reduction products
- Predict ketone reduction products
Aldehydes give primary alcohols
NaBH₄ or LiAlH₄ reduces the C=O group in an aldehyde to –CH₂OH. For ethanal: CH₃CHO + 2[H] → CH₃CH₂OH.
The carbonyl carbon becomes the carbon bearing OH. An aldehyde has one carbon substituent on that carbon, so reduction makes a primary alcohol.
Ketones give secondary alcohols
Reduction of a ketone converts C=O into C–OH without changing the carbon skeleton. Propanone becomes propan-2-ol: CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃.
The carbonyl carbon in a ketone has two carbon neighbours, so its reduction product is a secondary alcohol. The syllabus accepts NaBH₄ or LiAlH₄ for aldehydes and ketones.
Do not confuse these reductions with oxidation by acidified dichromate.
Pause and explain
What class of alcohol forms when propanal is reduced?
Worked example
Predict the product when butanone is reduced by NaBH₄.
Show the worked solution
- Keep the four-carbon skeleton.
- Change the carbonyl C=O to C–OH.
- The carbonyl carbon is attached to two carbon groups, so the product alcohol is secondary.
Answer Butanone + 2[H] → butan-2-ol; NaBH₄ (or LiAlH₄).
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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 reduction equation for ethanal to ethanol using [H].
Give me a hint
Reduce the carbonyl to CH₂OH and balance hydrogen representation.
Compare my reasoning
- Ethanal is CH₃CHO.
- Add 2[H] to change C=O to C–OH.
- The product is CH₃CH₂OH.
CH₃CHO + 2[H] → CH₃CH₂OH; NaBH₄ or LiAlH₄.
Look for these in your work
- Formula correct
- [H] coefficient correct
- Alcohol product named
What alcohol forms when pentan-2-one is reduced, and is it primary, secondary or tertiary?
Give me a hint
Keep the carbonyl position and count its carbon neighbours.
Compare my reasoning
- Reduction does not move the carbonyl carbon.
- The OH remains on carbon 2.
- That carbon has two carbon neighbours, so it is secondary.
Pentan-2-ol, a secondary alcohol.
Look for these in your work
- Position retained
- Name correct
- Class justified
A student reduces 2-methylpropanal and writes 2-methylpropan-2-ol. Diagnose the error.
Give me a hint
The aldehyde group is at a chain end.
Compare my reasoning
- The aldehyde carbon is C1 and has one carbon neighbour.
- Reduction changes –CHO to –CH₂OH.
- The product is 2-methylpropan-1-ol, not the tertiary alcohol.
The product is 2-methylpropan-1-ol, a primary alcohol; reduction does not move –OH to C2.
Look for these in your work
- Carbonyl position identified
- Product position corrected
- Alcohol class stated
Common mistakes
- Calling an aldehyde reduction product secondary
- Changing the carbon skeleton
- Using an oxidising reagent for a reduction
What can you explain now?
Which reagents are listed for reducing aldehydes and ketones?
Compare with the explanation
NaBH₄ or LiAlH₄
Either reducing agent can convert the carbonyl compound to an alcohol.
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Make it stick
Why do aldehyde and ketone reductions produce different alcohol classes?
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.