Describe nucleophilic addition to a carbonyl
Show CN⁻ attack at C=O followed by protonation of O⁻ by HCN, regenerating the catalyst.
Section 17 · Carbonyl compounds
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
Where does the cyanide nucleophile attack in carbonyl addition?
What you will learn
- Start curly arrows at electron pairs
- Show C=O π-bond movement
- Explain proton transfer and catalyst regeneration
Cyanide attacks the electrophilic carbonyl carbon
CN⁻ donates a lone pair to the δ⁺ carbonyl carbon. At the same time, the C=O π pair moves to oxygen, producing an alkoxide intermediate.
Draw a full curly arrow from a lone pair on carbon in CN⁻ to the carbonyl carbon, and a second arrow from the C=O bond to O. Arrows begin at electrons, not at a positive target.
The alkoxide takes a proton from HCN
The O⁻ intermediate accepts H⁺ from HCN to form the hydroxynitrile. The electrons from the H–C bond return to carbon, regenerating CN⁻ as catalyst.
This is nucleophilic addition: a nucleophile adds to the electrophilic C=O carbon and the π bond opens. KCN supplies CN⁻; HCN supplies the proton and is a reactant.
Pause and explain
Where does the first curly arrow in the mechanism begin?
Worked example
Describe the two stages of HCN addition to ethanal.
Show the worked solution
- CN⁻ lone pair attacks the carbonyl carbon as the π pair moves to O.
- This forms CH₃CH(O⁻)CN.
- O⁻ takes H⁺ from HCN to form CH₃CH(OH)CN and regenerate CN⁻.
Answer CN⁻ attacks C=O; the π electrons move to O. The alkoxide then takes H⁺ from HCN, forming 2-hydroxypropanenitrile and regenerating CN⁻.
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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.
Name both electron-pair movements in the first step of HCN addition to propanone.
Give me a hint
Track the CN⁻ lone pair and the carbonyl π pair.
Compare my reasoning
- The CN⁻ lone pair forms a C–C bond to the carbonyl carbon.
- The C=O π pair moves to oxygen.
- The intermediate is an alkoxide.
CN⁻ lone pair → carbonyl carbon; C=O π pair → O, forming (CH₃)₂C(O⁻)CN.
Look for these in your work
- First arrow source and target
- Second arrow source and target
- Intermediate identified
What happens to CN⁻ after O⁻ is protonated by HCN?
Give me a hint
Follow the H–C bond electrons during proton transfer.
Compare my reasoning
- O⁻ takes H⁺ from HCN.
- The H–C bond pair remains with carbon as CN⁻.
- The catalyst is regenerated.
CN⁻ is regenerated when the H–C bond pair returns to carbon during proton transfer.
Look for these in your work
- Proton donor identified
- Bond-electron path stated
- Catalyst regenerated
A diagram shows CN⁻ attack but no arrow from C=O to O. What charge or intermediate problem results?
Give me a hint
Consider where the π electrons move when the new C–C bond forms.
Compare my reasoning
- The carbonyl carbon would exceed its bonding capacity if the π bond stayed unchanged.
- The C=O pair must move to oxygen.
- That produces O⁻ for the next proton-transfer step.
Move the C=O π pair to O as CN bonds to carbon, giving the alkoxide intermediate.
Look for these in your work
- Bond capacity checked
- Arrow corrected
- Intermediate charge explained
Common mistakes
- Starting a curly arrow at an atom with no electron pair
- Leaving the C=O double bond unchanged after CN attack
- Forgetting the protonation step or catalyst regeneration
What can you explain now?
Where does the cyanide nucleophile attack in carbonyl addition?
Compare with the explanation
At the carbonyl carbon
The C=O carbon is electrophilic.
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Make it stick
Why do two nucleophilic-addition steps form a hydroxynitrile and regenerate CN⁻?
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.