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

Explanation

The cyanide ion `(CN^(-))` is an ambident nucleophile because it can attack electrophillic centres through two different nucleophilic centres (C and N).

In the presence of AgCN, the reaction proceeds by the `S_(N)1` mechanism because `Ag^(+)` ion coordinates with halogen atom and promotes the formation of a carbocation by precipitating silver halide. Attack on the carbocation then takes place through the more electornegative and more electrons dense nitrogen atom (more positive charge centre is attacked by the more negative charge centre). As a result, alkyl isocyanide `(R-NC)` is obtained prodominantly.

In the presence of KCN, on the other hand, the reaction proceds by the `S_(N)2` mechanism because unlike silver ion `K^(+)` ion cannot promote ionisation of the carbon-halogen (C-X) bond. In this case, the cyanide ion attacks R-X through the more polarisable (larger in size and less electronegative) and more nucleophillc carbon atom (less positive charge centre is attacked by less negative one). As a result, an alkyl cyanide `(R-CN)` is obtained predominantly.
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