19.2 Nitriles and hydroxynitriles

CN⁻ attacks a halogenoalkane in ethanol on heating, replacing X and forming a nitrile, R–C≡N. The cyanide carbon becomes part of the product skeleton.

This route is useful for chain extension: count the starting carbon atoms and add one for the nitrile carbon. The reaction is nucleophilic substitution, not elimination.

CH₃CH₂Br + KCN → CH₃CH₂CN + KBr. Bromoethane has two carbons; propanenitrile has three.

Do not write HCN as the carbon source or forget the extra carbon in the product.

Cyanide attacks the carbonyl carbon and protonation gives a molecule containing both –OH and –C≡N: a hydroxynitrile. KCN helps generate CN⁻ and heat supports the reaction.

The product has one more carbon than the carbonyl compound because the nitrile carbon is added. Aldehyde and ketone starting structures lead to different substitution patterns.

CH₃CHO + HCN → CH₃CH(OH)CN. The new carbon bears OH, CN, H and CH₃.

Hydroxynitrile formation is not nitrile hydrolysis and does not remove the carbonyl carbon.

Dilute acid or dilute alkali hydrolyses a nitrile through amide/carboxylate stages. Acid work-up gives the carboxylic acid; alkaline conditions initially give a carboxylate salt.

The nitrile carbon becomes the carboxyl carbon, so the chain is retained after the earlier KCN extension. State whether the reaction mixture is acidic or alkaline at the final step.

CH₃CH₂CN + 2H₂O → CH₃CH₂COOH + NH₃ under acid hydrolysis (simplified overall equation). In alkali, propanoate forms first and is then acidified.

Do not lose the nitrile carbon or report a carboxylate as the acid before acidification.