Consider the reaction,

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}+\mathrm{NaCN}\to {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{CN}+\mathrm{NaBr}$

This reaction will be the fastest in

The reaction,

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Br}+\mathrm{NaCN}\to {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{CN}+\mathrm{NaBr}$

$\text{ follows }{\mathrm{S}}_{\mathrm{N}}2\text{ mechanism which is favoured by polar }$

$\text{ aprotic solvent i.e., }N,N^{\text{'}}-\text{ dimethylformamide (DMF). }$

moreover no rearrangement is observed, it is $S{N}^2$ mechanism