Aldehydes and ketones react with potassium cyanide, KCN, followed by dilute acid, to form hydroxynitriles. These contain both an OH group and a nitrile group, C≡N, on the same carbon.The reaction is a nucleophilic addition. The cyanide ion, :CN−, is the nucleophile: its lone pair on carbon attacks the δ+ carbonyl carbon, the C=O π electrons move onto oxygen to give an O− intermediate, and this is then protonated by H+ from the dilute acid.
Overall, H and CN add across the C=O bond:CH3CHO + HCN → CH3CH(OH)CN
CH3COCH3 + HCN → (CH3)2C(OH)CN Naming: the nitrile carbon is counted in the chain and is carbon 1, so the OH is on carbon 2:
• from ethanal: 2-hydroxypropanenitrile
• from propanone: 2-hydroxy-2-methylpropanenitrileThe reaction is useful in synthesis because it increases the length of the carbon chain by one carbon atom. The nitrile group can then be converted into other functional groups.If the carbonyl compound is unsymmetrical, the product has a chiral centre and forms as a racemic mixture (see the optical isomerism topic).Worked example: name the product from propanal, CH3CH2CHO. Adding CN gives CH3CH2CH(OH)CN. The chain is four carbons including the nitrile carbon (butanenitrile), with OH on carbon 2: 2-hydroxybutanenitrile.
Overall, H and CN add across the C=O bond:CH3COCH3 + HCN → (CH3)2C(OH)CN
• from ethanal: 2-hydroxypropanenitrile
• from propanone: 2-hydroxy-2-methylpropanenitrileThe reaction is useful in synthesis because it increases the length of the carbon chain by one carbon atom. The nitrile group can then be converted into other functional groups.If the carbonyl compound is unsymmetrical, the product has a chiral centre and forms as a racemic mixture (see the optical isomerism topic).Worked example: name the product from propanal, CH3CH2CHO. Adding CN gives CH3CH2CH(OH)CN. The chain is four carbons including the nitrile carbon (butanenitrile), with OH on carbon 2: 2-hydroxybutanenitrile.