Prepare carboxylic acids by nitrile hydrolysis
Heat a nitrile with dilute acid, or dilute alkali followed by acidification, to make a 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
Which functional group is hydrolysed in this route?
What you will learn
- Recognise the nitrile group
- Recall both hydrolysis conditions
- Track the nitrile carbon into the acid
Hydrolysis converts –CN to –COOH
Heat the nitrile with dilute acid, or with dilute alkali. After alkaline hydrolysis, acidify the carboxylate to obtain the carboxylic acid.
The nitrile carbon becomes the carboxyl carbon. Thus propanenitrile forms propanoic acid; the carbon count is unchanged.
Keep route conditions explicit
For the alkaline route, write the acidification step after hydrolysis. The intermediate is a carboxylate salt in alkaline solution.
Do not call nitrile hydrolysis a reduction: the –CN group is converted to a carboxylic acid by hydrolysis.
Pause and explain
What extra step follows alkaline nitrile hydrolysis to obtain the free acid?
Worked example
Outline a route from ethanenitrile to ethanoic acid.
Show the worked solution
- Heat the nitrile with dilute acid, or dilute alkali.
- If using alkali, form the ethanoate salt first.
- Acidify the carboxylate to release ethanoic acid.
Answer Heat ethanenitrile with dilute acid; alternatively, heat with dilute alkali then acidify the ethanoate. Both routes give ethanoic acid.
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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.
Convert propanenitrile to propanoic acid by an alkaline route.
Give me a hint
Hydrolyse first, then protonate the carboxylate.
Compare my reasoning
- Heat propanenitrile with dilute alkali.
- The product in alkaline solution is propanoate.
- Acidify to form propanoic acid.
Heat with dilute alkali, then acidify: propanenitrile → propanoate → propanoic acid.
Look for these in your work
- Dilute alkali used
- Acidification included
- Three-carbon chain retained
Why does ethanenitrile produce a two-carbon acid rather than a one-carbon acid?
Give me a hint
Count the carbon in the nitrile group.
Compare my reasoning
- Ethane has one carbon in CH₃ and one in –CN.
- Hydrolysis changes the nitrile group, not the carbon skeleton.
- The nitrile carbon becomes the acid carbon.
It forms ethanoic acid because the –CN carbon becomes the –COOH carbon; the two-carbon chain is retained.
Look for these in your work
- Both carbons counted
- Nitrile carbon tracked
- Correct acid named
A route lists dilute NaOH and heating but ends directly with a neutral carboxylic acid. What step is missing?
Give me a hint
Check the ionic form in alkaline solution.
Compare my reasoning
- The hydrolysis product is a carboxylate.
- The solution is still alkaline.
- Add dilute acid after hydrolysis.
Acidify the mixture to convert the carboxylate into the carboxylic acid.
Look for these in your work
- Intermediate identified
- Missing step named
- Purpose explained
Common mistakes
- Omitting acidification after alkaline hydrolysis
- Losing the nitrile carbon
- Using concentrated acid as the stated dilute hydrolysis condition
What can you explain now?
Which functional group is hydrolysed in this route?
Compare with the explanation
Nitrile –C≡N
The nitrile group becomes a carboxylic acid.
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Make it stick
What changes during nitrile hydrolysis, and when is acidification required?
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.