Form hydroxynitriles from carbonyl compounds
Add HCN across an aldehyde or ketone C=O using KCN as catalyst and heat.
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. Classification of amines is not tested at AS Level. Cyanides, hydrogen cyanide and pressurised reactions are hazardous: these examples teach exam chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use nucleophilic substitution and addition, carbonyl compounds, functional-group names and balanced equations from Topics 13–18.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which functional group is converted when a hydroxynitrile forms?
What you will learn
- Recall HCN, KCN catalyst and heat
- Predict the C(OH)(CN) product from an aldehyde or ketone
- Track the new carbon in the hydroxynitrile
Add HCN across the carbonyl bond
An aldehyde or ketone reacts with HCN using KCN as catalyst and heat to form a hydroxynitrile. The C=O carbon receives both new groups: oxygen becomes –OH and the carbon of HCN forms the –C≡N bond.
The former carbonyl carbon remains in the molecule, and HCN adds one carbon. For ethanal, CH₃CHO becomes CH₃CH(OH)C≡N, 2-hydroxypropanenitrile. For propanone, (CH₃)₂CO becomes (CH₃)₂C(OH)C≡N.
Separate this route from making a nitrile by substitution
A halogenoalkane plus KCN makes a nitrile by substitution. A carbonyl compound plus HCN, with KCN as catalyst, makes a hydroxynitrile by nucleophilic addition.
HCN and cyanide are highly toxic. Only use these examples to understand examination chemistry; they are not practical directions for handling or generating cyanide reagents.
Pause and explain
What two groups attach to the former carbonyl carbon?
Worked example
Predict the product when ethanal reacts with HCN in the presence of KCN and heat.
Show the worked solution
- The carbonyl carbon is the carbon in CH₃CHO.
- Add CN to that carbon and convert C=O into C–OH.
- Include the HCN carbon when naming the nitrile.
Answer CH₃CH(OH)C≡N, 2-hydroxypropanenitrile.
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.
Draw or name the product from propanone and HCN with KCN catalyst and heat.
Give me a hint
Change the carbonyl carbon from C=O to C(OH)(CN).
Compare my reasoning
- Keep both methyl groups attached to the former carbonyl carbon.
- Add OH and CN to that carbon.
- Name the nitrile including its CN carbon.
(CH₃)₂C(OH)C≡N, 2-hydroxy-2-methylpropanenitrile.
Look for these in your work
- Carbonyl skeleton retained
- OH and CN on the same carbon
- Nitrile carbon counted
A student writes CH₃CH(OH)CHO as the product from ethanal and HCN. Correct it.
Give me a hint
HCN adds its carbon and changes the carbonyl bond; it does not leave the aldehyde group intact.
Compare my reasoning
- The aldehyde carbon becomes bonded to OH.
- HCN supplies a carbon bonded as –C≡N.
- The product has a hydroxynitrile group.
CH₃CH(OH)C≡N, 2-hydroxypropanenitrile.
Look for these in your work
- C=O converted
- CN group added
- Product correctly named
Compare the products of bromoethane + KCN and ethanal + HCN/KCN catalyst.
Give me a hint
Compare substrate and reaction type, then identify the product group in each case.
Compare my reasoning
- Bromoethane undergoes substitution.
- Ethanal undergoes nucleophilic addition across C=O.
- The latter retains an OH group alongside CN.
Bromoethane + KCN forms propanenitrile by substitution and adds one carbon. Ethanal + HCN/KCN forms 2-hydroxypropanenitrile by addition, also adding one carbon and converting C=O to C–OH.
Look for these in your work
- Both substrates distinguished
- Reaction types distinguished
- Products and carbon changes compared
Common mistakes
- Leaving the C=O unchanged
- Putting OH on a different carbon
- Using KCN as the stoichiometric product instead of a catalyst
- Forgetting that HCN adds a carbon
What can you explain now?
Which functional group is converted when a hydroxynitrile forms?
Compare with the explanation
Aldehyde or ketone C=O
HCN adds across the carbonyl bond.
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Make it stick
How do the substrate, reaction type and product distinguish cyanide substitution from hydroxynitrile formation?
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.