Alkyl halides reacts with alcoholic KCN to give

SN₂ reaction. Replacement of halogen by Cyanide ion.

KCN is predominantly ionic and provides cyanide ions in its solution. In the presence of alcoholic KCN alkyl halides gives alkyl cyanides. 

$\mathrm{RX} + \mathrm{KCN} \to \mathrm{RCN}+\mathrm{KBX}$