Question

Explain why do haloalkanes give alkyl cyanides when treated with KCN but give alkyl isocyanides with silver cyanide?

Explain why:

Alkyl halides give alkyl cyanides on treatment with potassium cyanide but give alkyl isocyanides when treated with silver cyanide.

Answer 1

The different products formed by the reaction of haloalkanes with KCN and AgCN result from the nature of the cyanide source and the ambident character of the cyanide ion. KCN is ionic, allowing a free cyanide ion to attack via carbon, while AgCN is covalent, forcing the attack to occur via the nitrogen atom.

1. Reaction with KCN (Potassium Cyanide): The free cyanide ion can attack from either the carbon or nitrogen atom. However, the attack primarily happens through the carbon atom because the carbon-carbon bond formed (R – C ≡ N) is stronger and more stable than the carbon-nitrogen bond (R – N ≡ C).
   \[\ce{CH3CH2Br + KCN -> CH3CH2-C ≡ N + KBr}\]
2. Reaction with AgCN (Silver Cyanide): AgCN is less soluble, and the CN⁻ ion is bonded to Ag⁺, forming an ambident nucleophile. In this case, the nitrogen end of the cyanide ion preferentially attacks the carbon of the haloalkane, forming alkyl isocyanides (R – N ≡ C)
   \[\ce{CH3CH2Br + AgCN -> CH3CH2-N ≡ C + AgBr}\]