Choose a route to prepare a carboxylic acid
Select oxidation or hydrolysis by inspecting the starting functional group and required work-up.
Section 18 · Carboxylic acids and derivatives
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
Which route suits an aldehyde when the target is a carboxylic acid?
What you will learn
- Match starting group to reaction
- Choose conditions
- Include any required work-up
Start by identifying the functional group
Primary alcohol or aldehyde: acidified K₂Cr₂O₇/KMnO₄ and reflux. Nitrile: dilute acid or dilute alkali and heat; acidify after alkaline hydrolysis. Ester: dilute acid or alkali and heat; acidify the alkaline product.
All three routes can give a carboxylic acid, but they use different chemistry and conditions.
Check the carbon count and work-up
Oxidation retains the chain. Nitrile hydrolysis retains the nitrile carbon as the acid carbon. Ester hydrolysis returns the acyl side as acid; the alcohol side becomes an alcohol.
A written route is incomplete if it omits reflux for full oxidation, heating for hydrolysis, or acidification after an alkaline route.
Pause and explain
Which starting group can be hydrolysed to a carboxylic acid?
Worked example
Choose a route from propanal to propanoic acid and from propanenitrile to propanoic acid.
Show the worked solution
- Propanal is an aldehyde, so oxidise it.
- Propanenitrile is hydrolysed.
- State reflux for oxidation and acidification for the alkaline hydrolysis option.
Answer Propanal: acidified K₂Cr₂O₇/KMnO₄, reflux. Propanenitrile: dilute acid and heat, or dilute alkali and heat then acidify.
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.
Select a route from methyl propanoate to propanoic acid.
Give me a hint
Identify the ester and choose hydrolysis.
Compare my reasoning
- Heat with dilute acid, or dilute alkali.
- Acid hydrolysis gives the acid and methanol.
- After alkaline hydrolysis, acidify the propanoate.
Hydrolyse methyl propanoate with dilute acid and heat; alternatively dilute alkali and heat, then acidify.
Look for these in your work
- Ester recognized
- Hydrolysis chosen
- Conditions/work-up stated
A three-carbon nitrile must become the corresponding acid. Explain why acidification may be needed.
Give me a hint
Consider the product in alkali.
Compare my reasoning
- Alkaline hydrolysis forms a carboxylate.
- Carboxylate is the conjugate base.
- Acidification protonates it to the acid.
After dilute-alkali hydrolysis, acidify the carboxylate to obtain the free carboxylic acid.
Look for these in your work
- Intermediate named
- Work-up reasoned
- Product stated
Why is reflux appropriate for converting butanal to butanoic acid but not for isolating butanal from butan-1-ol?
Give me a hint
Distinguish the two target products.
Compare my reasoning
- Acid formation needs continued oxidation.
- Aldehyde isolation needs removal as it forms.
- Use reflux for acid and distillation for aldehyde.
Reflux promotes further oxidation to the acid; distillation removes butanal when the aldehyde is the target.
Look for these in your work
- Targets distinguished
- Apparatus matched
- Reason explained
Common mistakes
- Using a ketone as the stated oxidation starting group
- Omitting work-up
- Applying a single route to every functional group
What can you explain now?
Which route suits an aldehyde when the target is a carboxylic acid?
Compare with the explanation
Oxidation under reflux
Acidified oxidant and reflux continue aldehyde oxidation.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How can the starting functional group reveal the acid-preparation route and its work-up?
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.