Reduce a carboxylic acid to a primary alcohol
Use LiAlH₄ to reduce the carboxyl group while retaining the carbon skeleton.
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 reducing agent is specified for converting a carboxylic acid into an alcohol?
What you will learn
- Recall LiAlH₄ as the specified reagent
- Predict primary alcohol products
- Retain the acid carbon in the product
The carbonyl carbon remains in the chain
LiAlH₄ reduces a carboxylic acid to a primary alcohol. The carbonyl carbon becomes the terminal carbon bearing –CH₂OH; the number of carbon atoms does not change.
For ethanoic acid: CH₃COOH + 4[H] → CH₃CH₂OH + H₂O. The equation shows the carbon product and a balanced hydrogen representation.
Choose the specified reducing agent
The stated syllabus route for reducing a carboxylic acid uses LiAlH₄. NaBH₄ is the route taught for aldehydes and ketones; do not substitute it in an acid-reduction answer.
Carboxylic-acid reduction is a stronger reduction than carbonyl reduction. Identify the starting functional group before selecting the reagent.
Pause and explain
What alcohol class is formed by reducing a carboxylic acid?
Worked example
Predict the product of reducing 2-methylpropanoic acid with LiAlH₄.
Show the worked solution
- Keep all four carbon atoms.
- Change –COOH into –CH₂OH at the same end of the chain.
- The product is a primary alcohol.
Answer 2-methylpropanoic acid → 2-methylpropan-1-ol; LiAlH₄.
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.
Give the product and reagent for reducing propanoic acid.
Give me a hint
Retain the three-carbon chain and reduce –COOH to –CH₂OH.
Compare my reasoning
- Propanoic acid contains three carbons.
- The acid carbon remains as the terminal alcohol carbon.
- Name the three-carbon primary alcohol.
Propanoic acid + LiAlH₄ → propan-1-ol.
Look for these in your work
- Carbon count retained
- Product named
- Reagent correct
A learner predicts ethanol from butanoic acid because the COOH group is removed. Correct the carbon count.
Give me a hint
The carboxyl carbon becomes part of the alcohol product.
Compare my reasoning
- Butanoic acid has four carbon atoms.
- Reduction changes the functional group, not the carbon skeleton.
- The product is butan-1-ol.
Butan-1-ol forms; all four carbon atoms remain.
Look for these in your work
- Starting count correct
- Carbon retention explained
- Product named
Why is NaBH₄ not the reagent to name for the syllabus carboxylic-acid reduction?
Give me a hint
Compare the listed reagents for carbonyls and carboxylic acids.
Compare my reasoning
- NaBH₄ is listed for aldehydes and ketones.
- LiAlH₄ is specified for carboxylic-acid reduction.
- State the reagent that matches the starting functional group.
Use LiAlH₄; the syllabus lists NaBH₄ for aldehydes and ketones, not for the carboxylic-acid route.
Look for these in your work
- Starting group identified
- Reagents distinguished
- Correct choice stated
Common mistakes
- Removing the carboxyl carbon
- Naming a secondary alcohol at the acid carbon
- Using NaBH₄ as the stated acid-reduction reagent
What can you explain now?
Which reducing agent is specified for converting a carboxylic acid into an alcohol?
Compare with the explanation
LiAlH₄
Lithium aluminium hydride reduces a carboxylic acid to a primary alcohol.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which carbon stays in the molecule when a carboxylic acid is reduced, and what group does it become?
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.