Haloalkanes react with potassium cyanide (KCN) to give alkyl cyanide, but gives alkyl isocyanide with silver cyanide (Ag CN).

Explanation

KCN is an ionic solid which ionises in polar solvents like ethanol to give `CN^(-)` ions.
`K^(+) CN^(-) hArr K^(+) + CN^(-)`
Attack of CN on alkyl halides gives Alkyl cyanide as the main product.
`R-X + CN^(-) to R - C -= N +X^(-)`
On the other hand AgCN is a covalent solid in which Ag-CN bond has a large covalent character. As such, it does not give `CN^(-)` ions to act as neucleophiles. Here `Ag-C=overset(* * )N` itself act as a nucleophile with the lone pair of electrons on nitrogen acting as electrophilic centre,

As such the main product obtained in this case is an isocyanide.