# In a $S_N2$ substitution reaction of the type $R-B+CI^- \underrightarrow {DMF} R-CI+Br^+,$ which one of the following has the highest reactivity rate :

$\begin {array} {1 1} (1)\;CH_3CH_3Br & \quad (2)\;CH_3-CH_2-CH_2Br \\ (3)\;abc & \quad (4)\;abc \end {array}$