Extend a carbon chain through a hydroxynitrile
Add HCN to a carbonyl compound to form a hydroxynitrile, then use the nitrile group in a further route.
Section 36 · Organic synthesis
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 36. Routes use reactions listed in the syllabus and state the relevant reagents and conditions. Check every intermediate and possible by-product; no route implies a practical procedure. Written practice is self-checked
Before you begin
- Use the functional-group reactions from Topics 13–19 and 29–35, including their reagents, conditions, products and limitations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What product class forms when an aldehyde reacts with HCN using KCN catalyst and heat?
What you will learn
- Form a hydroxynitrile from an aldehyde or ketone
- Track the carbon introduced by HCN
- Plan a next step from the nitrile group
Add HCN across a carbonyl
Aldehydes and ketones react with HCN, using KCN as catalyst and heat, to form hydroxynitriles. The carbonyl carbon becomes bonded to both OH and CN in the product.
The cyanide carbon becomes part of the nitrile group, so the product has one additional carbon compared with the carbonyl substrate. Preserve the original carbon skeleton and identify any new stereochemical issue only where the supplied structure makes it relevant.
Use the nitrile as a route branch
The –CN group in a hydroxynitrile can be hydrolysed to –COOH, giving a hydroxycarboxylic acid, or reduced to –CH₂NH₂ using a syllabus reducing system.
Choose the next step from the target. Hydrolysis and reduction are alternatives; they do not both occur in the same route unless a later target requires another transformation.
Pause and explain
Where does the carbon from HCN appear in the hydroxynitrile product?
Worked example
Plan a route from ethanal to 2-hydroxypropanoic acid.
Show the worked solution
- Add HCN to ethanal using KCN as catalyst and heat.
- This forms 2-hydroxypropanenitrile.
- Hydrolyse –CN to –COOH and retain –OH.
Answer CH₃CHO → CH₃CH(OH)CN → CH₃CH(OH)COOH. The first step adds one carbon; nitrile hydrolysis retains it as the acid carbon.
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.
Predict the hydroxynitrile from propanone and HCN, and state the carbon-count change.
Give me a hint
The carbonyl carbon receives CN and OH; include HCN's carbon.
Compare my reasoning
- Keep the three-carbon propanone skeleton.
- Form OH and CN on the former carbonyl carbon.
- Count the nitrile carbon as a fourth carbon.
2-hydroxy-2-methylpropanenitrile forms; the product has one more carbon than propanone.
Look for these in your work
- Carbonyl carbon retained
- OH/CN placement correct
- Carbon count increased
Continue the route from 2-hydroxypropanenitrile to 2-hydroxypropanoic acid.
Give me a hint
Hydrolyse the nitrile while retaining the existing alcohol group.
Compare my reasoning
- Use dilute acid for nitrile hydrolysis, or dilute alkali followed by acidification.
- The nitrile carbon becomes the carboxyl carbon.
- Keep the –OH group on carbon 2.
Hydrolyse –CN and, if alkaline hydrolysis is used, acidify; the product is 2-hydroxypropanoic acid.
Look for these in your work
- Hydrolysis reagent stated
- Nitrile carbon retained
- Existing OH retained
A student says HCN converts ethanal into 2-hydroxyethanoic acid in one step. Identify both errors.
Give me a hint
Remember the first product is a nitrile and includes HCN's carbon.
Compare my reasoning
- HCN addition first forms a hydroxynitrile.
- Hydrolysis is a separate step to the acid.
- The added cyanide carbon makes the acid three-carbon, not two-carbon.
The sequence is ethanal → 2-hydroxypropanenitrile → 2-hydroxypropanoic acid; HCN addition and nitrile hydrolysis are separate steps and add one carbon overall.
Look for these in your work
- Intermediate restored
- Separate step added
- Carbon count corrected
Common mistakes
- Adding CN to the wrong carbon
- Losing the carbonyl carbon
- Forgetting KCN is a catalyst in HCN addition
- Assuming nitrile hydrolysis removes the added carbon
What can you explain now?
What product class forms when an aldehyde reacts with HCN using KCN catalyst and heat?
Compare with the explanation
A hydroxynitrile
HCN adds across the carbonyl to give adjacent OH and CN groups.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How does HCN addition extend a carbonyl molecule, and how can the nitrile direct the next step?
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.