Prepare carboxylic acids by ester hydrolysis
Heat an ester with dilute acid or dilute alkali; acidify after alkaline hydrolysis to release the carboxylic acid.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Track the carbon skeleton, name reagents and conditions, and show products and observations. Written practice is self-checked
Before you begin
- Use alcohol oxidation and carbonyl chemistry from Topics 16–17, functional-group names, acid–base equations and balanced reaction formulae.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the functional-group reaction when an ester forms an acid and alcohol?
What you will learn
- Identify the ester group
- Use dilute hydrolysis conditions and heat
- Distinguish acid and alkaline work-up
Hydrolysis breaks the ester link
Heat the ester with dilute acid or dilute alkali. Acid hydrolysis gives a carboxylic acid and an alcohol; alkaline hydrolysis gives a carboxylate salt and an alcohol.
Acidify the alkaline mixture to convert the carboxylate into the free carboxylic acid. State the work-up when the question asks for the acid product.
Track the two sides of the ester
The acyl side becomes the carboxylic acid or carboxylate; the alkyl side attached through oxygen becomes an alcohol.
Hydrolysis adds the elements of water across the ester bond. Do not confuse hydrolysis with esterification, which forms an ester from an acid and alcohol.
Pause and explain
What must follow alkaline ester hydrolysis to obtain the free acid?
Worked example
Describe an alkaline route from methyl propanoate to propanoic acid.
Show the worked solution
- Heat the ester with dilute alkali.
- This forms propanoate and methanol.
- Acidify the propanoate to obtain propanoic acid.
Answer Heat methyl propanoate with dilute alkali, then acidify: propanoate and methanol form, followed by propanoic acid after acidification.
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.
What forms when ethyl ethanoate is heated with dilute acid?
Give me a hint
Split the ester into its acid and alcohol sides.
Compare my reasoning
- The acyl side is ethanoate/ethanoic acid.
- The alkyl side is ethyl, which gives ethanol.
- Acid hydrolysis uses dilute acid and heat.
Ethanoic acid and ethanol.
Look for these in your work
- Acid product correct
- Alcohol product correct
- Conditions stated
A learner heats an ester with dilute NaOH and isolates sodium ethanoate. How can ethanoic acid be obtained?
Give me a hint
The salt is a carboxylate.
Compare my reasoning
- Keep the hydrolysis product in solution.
- Add dilute acid after heating.
- The acid protonates ethanoate.
Acidify the sodium ethanoate solution to obtain ethanoic acid.
Look for these in your work
- Work-up specified
- Carboxylate identified
- Acid product justified
How does the reaction direction differ between hydrolysis and esterification?
Give me a hint
Compare starting functional groups and products.
Compare my reasoning
- Hydrolysis starts with an ester.
- It produces an acid and an alcohol.
- Esterification starts with an acid and alcohol and forms the ester.
Hydrolysis breaks an ester into acid/alcohol products; esterification combines an acid and alcohol to form an ester and water.
Look for these in your work
- Starting groups compared
- Products compared
- Water role stated
Common mistakes
- Forgetting heat
- Calling the alkaline product a free acid before acidification
- Confusing hydrolysis with ester formation
What can you explain now?
What is the functional-group reaction when an ester forms an acid and alcohol?
Compare with the explanation
Hydrolysis
Hydrolysis breaks the ester using water under acidic or alkaline conditions.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do conditions and work-up distinguish acid hydrolysis from alkaline ester hydrolysis?
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.