Alkyl halide is converted into alkyl isocyanide by reaction with

We need to identify which reagent converts an alkyl halide into an alkyl isocyanide. The key distinction lies in the ambident nature of the cyanide ion ($$CN^-$$), which can bond through either the carbon atom or the nitrogen atom. Bonding through carbon ($$C$$) gives an alkyl cyanide (nitrile) $$R-CN$$, while bonding through nitrogen ($$N$$) yields an alkyl isocyanide $$R-NC$$.

According to the HSAB (Hard-Soft Acid-Base) principle, $$NaCN$$ and $$KCN$$ are ionic cyanides that provide free $$CN^-$$ ions in solution; these ions attack preferentially through the carbon end, forming alkyl cyanides (nitriles). By contrast, $$AgCN$$ is a covalent cyanide in which silver is bonded to the carbon end of $$CN$$, leaving the nitrogen end free to attack the alkyl halide and form an alkyl isocyanide.

The reaction with $$AgCN$$ is:

$$R-X + AgCN \rightarrow R-NC + AgX$$

This pathway produces an alkyl isocyanide because the nitrogen atom of $$CN$$ attacks the carbon of the alkyl halide. Therefore, the correct answer is Option 4: AgCN.