Question

Haloalkanes react with $KCN$ to form alkyl cyanides as main product while $AgCN$ form isocyanides as the chief product. Explain.

Answer 1

Reaction of alkyl halide with $KCN$

$KCN$ is ionic in nature and provides cyanide ions in solution. Although both carbon and nitrogen atoms are in a position to donate electron pairs, the attack takes place mainly
through carbon atom and not through nitrogen atom since $C-C$ bond is more stable than $C-N$ bond.

Involved reaction is shown below:

$R-X+K^{+}C{N}^{-}\underset{reaction}{\overset{substitution}{\to }}$
$Alkylhalide$

$R-CN+KX$
$Cyanide$

Reaction of alkyl halide with $AgCN$

$AgCN$ is mainly covalent in nature. Here only nitrogen is free to donate electron pair,hence it forms isocyanide as the main product.

Involved reaction is shown below:

$R-X+AgCN\underset{reaction}{\overset{substitution}{\to }}$
$Alkylhalide$

$R-NC+AgX$
$Isocyanide$

Hence, here isocyanide is formed as major product.