Add hydrogen cyanide to an aldehyde
Use HCN, KCN catalyst and heat to add CN and OH across an aldehyde carbonyl.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Distinguish aldehydes from ketones by structure and evidence; state reagent, condition and observation. Written practice is self-checked
Before you begin
- Use carbonyl structures, alcohol oxidation and reduction from Topics 13 and 16, nucleophiles and curly arrows, and the organic reaction patterns from Topic 15.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What two groups add across a carbonyl in HCN addition?
What you will learn
- Recall HCN, KCN catalyst and heat
- Predict a hydroxynitrile
- Recognise the additional nitrile carbon
The carbonyl carbon gains CN and OH
An aldehyde reacts with HCN in the presence of KCN catalyst and heat to form a hydroxynitrile (cyanohydrin). The C=O carbon gains a CN group and an OH group.
Ethanal forms 2-hydroxypropanenitrile: CH₃CHO + HCN → CH₃CH(OH)CN. The carbon from CN is added to the product skeleton, so the nitrile parent has one more carbon than the aldehyde chain.
Name the nitrile carbon in the parent chain
The carbon in –C≡N is counted as part of the nitrile parent. CH₃CH(OH)CN has three carbon atoms and is 2-hydroxypropanenitrile.
Use KCN as catalyst with HCN and heat. This is nucleophilic addition to C=O, unlike CN⁻ substitution of a halogenoalkane, which also adds one carbon but replaces X.
Pause and explain
How many carbon atoms are in 2-hydroxypropanenitrile?
Worked example
Predict the hydroxynitrile from propanal and state the conditions.
Show the worked solution
- Add CN to the carbonyl carbon.
- Convert C=O to C–OH.
- Count the nitrile carbon when naming.
Answer CH₃CH₂CHO + HCN → CH₃CH₂CH(OH)CN; 2-hydroxybutanenitrile, KCN catalyst and heat.
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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.
Write the hydroxynitrile from ethanal and name it.
Give me a hint
Add CN and OH at the former carbonyl carbon.
Compare my reasoning
- Ethanal contributes two carbon atoms.
- The CN carbon adds one more.
- Place OH and CN on the former carbonyl carbon.
CH₃CHO + HCN → CH₃CH(OH)CN; 2-hydroxypropanenitrile.
Look for these in your work
- Structure correct
- Nitrile carbon counted
- Name correct
A student names CH₃CH₂CH(OH)CN as 2-hydroxypropanenitrile. Check the carbon count.
Give me a hint
Count all carbons, including –CN.
Compare my reasoning
- CH₃CH₂ contributes two carbons.
- The carbonyl carbon is the third.
- The nitrile carbon is the fourth, so the product parent is butanenitrile.
It is 2-hydroxybutanenitrile; the nitrile carbon is included in the four-carbon parent.
Look for these in your work
- All carbons counted
- Parent length corrected
- Hydroxy position stated
Why is HCN addition to an aldehyde called nucleophilic addition rather than substitution?
Give me a hint
Check whether a leaving group is replaced.
Compare my reasoning
- CN⁻ attacks the electrophilic carbonyl carbon.
- The C=O π bond opens and no group leaves from the carbon skeleton.
- CN and H are added across the carbonyl rather than replacing a leaving group.
It is addition because CN adds to C=O and no substituent is displaced; the carbonyl π bond becomes C–O.
Look for these in your work
- Nucleophile identified
- Bond change stated
- No leaving group noted
Common mistakes
- Forgetting the carbon from CN
- Writing a nitrile with no OH group
- Using KCN without HCN catalyst conditions
What can you explain now?
What two groups add across a carbonyl in HCN addition?
Compare with the explanation
CN and OH
The carbonyl carbon gains CN and oxygen is protonated to OH.
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Make it stick
What conditions and structural changes turn an aldehyde into a hydroxynitrile?
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