In which case, formation of butane nitrile is possible 

${\mathrm{C}}_3{\mathrm{H}}_7\mathrm{Br}+\mathrm{KCN}\to {\mathrm{C}}_3{\mathrm{H}}_7\mathrm{CN}+\mathrm{KBr}$

In IUPAC system, the carbon of functional group is also taken in numbering. So, ${\mathrm{C}}_3{\mathrm{H}}_7\mathrm{CN}$ is butane nitrile.