Cyanohydrin formation: practice and mechanism
Cyanide attacks the carbonyl carbon with its carbon: OH and C≡N then sit on the same carbon.
Example
The cyanide ion attacks the carbonyl carbon; the alkoxide is then protonated. A cyanohydrin forms: OH and CN on the same carbon, and the chain is one carbon longer.
Mechanism step by step
- 1
Cyanide (from the KCN catalyst) attacks the carbonyl carbon, giving an alkoxide bearing CN.
- 2
HCN protonates the alkoxide and regenerates cyanide (catalytic cycle).
- 3
Product: cyanohydrin, OH and C≡N on the same carbon.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Cyanide attacks through its carbon atom, not through nitrogen. A C–C≡N group forms, not an isonitrile.
Often confused with: Grignard reaction · Imine formation
Frequently asked questions
What can cyanohydrins be turned into?
Hydrolysis gives α-hydroxy acids (e.g. lactic acid), reduction gives β-amino alcohols.
Why is KCN needed when HCN is present?
HCN is a weak acid and provides hardly any free CN⁻; only the cyanide ion is nucleophilic enough.