Benzoin condensation: practice and mechanism
Cyanide reverses the polarity of the aldehyde carbon: it becomes nucleophilic and attacks a second aldehyde molecule. Two benzaldehydes are joined to an α-hydroxy ketone (benzoin).
Example
Cyanide adds to an aromatic aldehyde. The cyanohydrin anion reverses the polarity: the former carbonyl carbon is now nucleophilic (acyl anion equivalent) and attacks a second aldehyde molecule. After loss of cyanide an α-hydroxy ketone forms.
Mechanism step by step
- 1
Cyanide attacks the carbonyl carbon of benzaldehyde; protonation gives the cyanohydrin.
- 2
The H on the former carbonyl carbon is now acidic (CN and phenyl stabilise the anion). The carbanion attacks a second benzaldehyde.
- 3
Cyanide is released again and benzoin (an α-hydroxy ketone) forms. Cyanide acts as a catalyst.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Cannizzaro reaction · Cyanohydrin formation
Frequently asked questions
What does umpolung mean?
Normally the carbonyl carbon is an electrophile. Cyanide temporarily makes it nucleophilic so it can attack a carbonyl group itself.
Why only aromatic aldehydes?
The phenyl ring stabilises the carbanion. Aliphatic aldehydes would rather undergo an aldol reaction with base. In nature thiamine (vitamin B1) plays the role of cyanide.