Reduce an aldehyde to a primary alcohol
Use NaBH₄ or LiAlH₄ to reduce –CHO to a terminal –CH₂OH group.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Distinguish aldehydes from ketones by structure and evidence; state reagent, condition and observation. Written practice is self-checked
Before you begin
- Use carbonyl structures, alcohol oxidation and reduction from Topics 13 and 16, nucleophiles and curly arrows, and the organic reaction patterns from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reagents reduce an aldehyde to an alcohol?
What you will learn
- Recall the two reducing agents
- Write the aldehyde reduction
- Identify a primary alcohol product
Reduce the carbonyl without changing the chain
NaBH₄ or LiAlH₄ reduces an aldehyde to a primary alcohol. The carbonyl carbon gains hydrogen and becomes terminal –CH₂OH.
Ethanal + 2[H] → ethanol. The carbon skeleton is retained; the aldehyde oxygen remains as the alcohol oxygen.
Reduction is the reverse functional-group change
Reduction adds hydrogen or removes oxygen in the organic structure. Do not use acidified dichromate, which oxidises alcohols and aldehydes.
The product class follows the carbonyl structure: aldehydes yield primary alcohols; ketones yield secondary alcohols.
Pause and explain
What class of alcohol forms when an aldehyde is reduced?
Worked example
Reduce 2-methylpropanal and classify the alcohol formed.
Show the worked solution
- Keep all four carbon atoms.
- Change –CHO to –CH₂OH at the same end.
- A terminal OH-bearing carbon with one carbon neighbour is primary.
Answer 2-methylpropanal + 2[H] → 2-methylpropan-1-ol; NaBH₄ or LiAlH₄. The product is primary.
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 the reduction equation for propanal to propan-1-ol.
Give me a hint
Reduce –CHO to –CH₂OH and retain the three-carbon chain.
Compare my reasoning
- Propanal has three carbons.
- Add 2[H] to the carbonyl group.
- Name the terminal alcohol product.
CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH; NaBH₄ or LiAlH₄.
Look for these in your work
- Product correct
- Carbon count retained
- Reagent stated
A learner reduces butanal and calls butan-2-ol the product. Correct the position.
Give me a hint
The aldehyde group starts at the end of the chain.
Compare my reasoning
- Aldehyde carbon is C1.
- Reduction changes –CHO to –CH₂OH without moving it.
- The product is butan-1-ol.
Butanal reduces to butan-1-ol, not butan-2-ol.
Look for these in your work
- Starting group located
- OH position corrected
- Product named
Why does aldehyde reduction not shorten the carbon chain?
Give me a hint
Only the carbonyl functional group changes.
Compare my reasoning
- No carbon–carbon bond is broken.
- The aldehyde carbon remains in the alcohol.
- The atom count stays the same for carbon.
The carbonyl carbon becomes the alcohol carbon; reduction retains the entire carbon skeleton.
Look for these in your work
- Bond change stated
- Carbon retention explained
- Conclusion justified
Common mistakes
- Moving the OH group to a secondary carbon
- Using an oxidising agent
- Changing the carbon count
What can you explain now?
Which reagents reduce an aldehyde to an alcohol?
Compare with the explanation
NaBH₄ or LiAlH₄
Either listed reducing agent converts the aldehyde carbonyl.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which carbon becomes –CH₂OH when an aldehyde is reduced?
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.