Topic 13 of 34
Cyanide attacks the carbonyl carbon, then oxygen is protonated
The carbonyl becomes a tetrahedral hydroxynitrile centre and the carbon chain gains one carbon.
A-Level 9476 (2026-2027)
Cyanide attacks the carbonyl carbon, then oxygen is protonated
The carbonyl becomes a tetrahedral hydroxynitrile centre and the carbon chain gains one carbon.
Use HCN with a small amount of KCN as catalyst. HCN alone supplies only a low concentration of CN-; the cyanide ion is the nucleophile. Its carbon end donates a lone pair to the δ+ carbonyl carbon, while the C=O π pair moves onto oxygen. Protonation of the resulting alkoxide by HCN produces the hydroxynitrile and regenerates CN-.
Nucleophilic addition to ethanal
A lone pair on the carbon end of N triple bond C minus points to the carbonyl carbon. At the same time the carbonyl pi bond points towards oxygen, producing an O-minus alkoxide rather than a carbon with five bonds.
- Form the alkoxide
CH3CHO + CN- → CH3CH(O-)CN. The negative charge is on oxygen.
- Transfer a proton from HCN
The oxygen lone pair goes to H in H-CN; the H-C bond pair returns to the cyanide carbon.
- Write the product
CH3CH(O-)CN + HCN → CH3CH(OH)CN + CN-. Overall, HCN adds across C=O.
An aldehyde RCHO gives RCH(OH)CN; a ketone R2CO gives R2C(OH)CN. Attack on either face of a planar carbonyl can form a racemic pair if a new chiral centre is created in an otherwise achiral system. Ethanal creates such a centre; propanone does not, because the product has two identical methyl groups.