Bromoethane is warmed with alcoholic $\mathrm{KCN}$. The main organic product is

Potassium cyanide is largely ionic, so the carbon atom, which carries the lone pair that is more available for bond formation in the free ion, attacks the alkyl carbon in SN2. This gives the nitrile $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CN}$, not the isocyanide.