Aldehydes and ketones react with hydrogen cyanide, HCN, to form hydroxynitriles. HCN adds across the C=O double bond: the CN group bonds to the carbonyl carbon and the H bonds to the oxygen, forming an OH group.Conditions: HCN in the presence of potassium cyanide, KCN, at room temperature. HCN is a weak acid and only slightly ionised, so KCN is added to provide a high concentration of cyanide ions, CN−, which are the attacking nucleophile. HCN is extremely toxic, so the reaction is carried out in a fume cupboard.Ethanal:
CH3CHO + HCN → CH3CH(OH)CN
The product is 2-hydroxypropanenitrile.Propanone:
CH3COCH3 + HCN → (CH3)2C(OH)CN
The product is 2-hydroxy-2-methylpropanenitrile.The reaction is useful in synthesis because it adds one carbon atom to the chain. The nitrile group can then be hydrolysed to a carboxylic acid; for example 2-hydroxypropanenitrile gives 2-hydroxypropanoic acid (lactic acid).Worked example: butanal, CH3CH2CH2CHO, has 4 carbon atoms. With HCN it forms CH3CH2CH2CH(OH)CN, 2-hydroxypentanenitrile, with 5 carbon atoms.When the product carbon has four different groups (as with any aldehyde except methanal), it is chiral and a racemic mixture forms. The reason for this is explained with the mechanism in the next lesson.
The product is 2-hydroxypropanenitrile.Propanone:
The product is 2-hydroxy-2-methylpropanenitrile.The reaction is useful in synthesis because it adds one carbon atom to the chain. The nitrile group can then be hydrolysed to a carboxylic acid; for example 2-hydroxypropanenitrile gives 2-hydroxypropanoic acid (lactic acid).Worked example: butanal, CH3CH2CH2CHO, has 4 carbon atoms. With HCN it forms CH3CH2CH2CH(OH)CN, 2-hydroxypentanenitrile, with 5 carbon atoms.When the product carbon has four different groups (as with any aldehyde except methanal), it is chiral and a racemic mixture forms. The reason for this is explained with the mechanism in the next lesson.