Which of the following reactions of alkyl halides produces an isocyanide (isonitrile) as the major product? (P) $\mathrm{R}-\mathrm{X}+\mathrm{HCN}\to$ (Q) $\mathrm{R}-\mathrm{X}+\mathrm{AgCN}\to$ (R) $\mathrm{R}-\mathrm{X}+\mathrm{KCN}\to$ (S) $\mathrm{R}-\mathrm{X}+\mathrm{NaCN}\stackrel{\mathrm{H}{}_2\mathrm{O}/\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}}{\to }$ Choose the most appropriate answer.

Solution & Explanation

Cyanide is an ambident nucleophile. With ionic $\mathrm{KCN}$/$\mathrm{NaCN}$, the free $\mathrm{CN}{}^{-}$ attacks through the more nucleophilic carbon, giving alkyl cyanides (nitriles, $\mathrm{R}-\mathrm{CN}$). With $\mathrm{AgCN}$, the bond is largely covalent so the nitrogen lone pair attacks, giving the isocyanide $\mathrm{R}-\mathrm{NC}$ as the major product. Hence only reaction (Q), $\mathrm{R}-\mathrm{X}+\mathrm{AgCN}$, gives the isonitrile.