In a $\mathrm{S}_{\mathrm{N}} 2$ substitution reaction of the type $\mathrm{R}-\mathrm{Br}+\mathrm{Cl}^{-} \stackrel{\mathrm{DMF}}{\longrightarrow} \mathrm{R}-\mathrm{Cl}+\mathrm{Br}^{-}$ which one of the following has the highest relative rate?

• A

  $\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{Br}$
• B

  
• C

  
• Correct answer
  D

  $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}$

Solution

$\mathrm{S}_{\mathrm{N}} 2$ mechanism is followed in case of primary and secondary halides i.e., $\mathrm{S}_{\mathrm{N}}^{2}$ reaction is favoured by small groups on the carbon atom attached to halogens so

Hence, the correct answer is option: 'D'.