17.1.3—Reactions of hydrogen cyanide
- Syllabus
- 9701–2028–2029
- Objective
- 17.1.3
- Level
- AS
HCN adds to a carbonyl by nucleophilic attack followed by proton transfer
The carbonyl carbon is electron-poor. CN⁻ attacks it with a lone pair, the C=O π electrons move to oxygen, and protonation gives the hydroxynitrile.
Curly arrows must begin at the cyanide lone pair and the C=O π bond, then show proton transfer. The carbonyl polarity explains why this is nucleophilic addition.
For ethanal, CN⁻ attacks CH₃CHO to give an alkoxide intermediate, which is protonated to CH₃CH(OH)CN. KCN helps generate CN⁻ without being consumed overall.
Do not draw electrophilic addition or start a curly arrow at the carbonyl carbon; electron pairs originate from a lone pair or bond.
ConceptA-Level CAIE Chemistry AS