Question

Arrange the following organic halides in correct order of reactivity towards $S{N}_2$ displacement.
$\underset{\text{ (P)}}{(\mathrm{CH}{}_3){}_2\mathrm{C}(\mathrm{Br})\mathrm{CH}{}_2\mathrm{CH}{}_3},\underset{\text{ (Q)}}{\mathrm{Br}\mathrm{CH}{}_2(\mathrm{CH}{}_2){}_3\mathrm{CH}{}_3},\underset{\text{ (R)}}{\mathrm{CH}{}_3\mathrm{CH}(\mathrm{Br})(\mathrm{CH}{}_2){}_2\mathrm{CH}{}_3}$

• A. P > Q > R
• B. R > P >Q
• C. P > R > Q
• D. Q > R > P

Answer 1

Answer: (D)

$3^{\circ }$ alkyl halide (highly sterically hindered)



$1^{\circ }$ alkyl halide (least sterically hindered)



$(R)2^{\circ }$ alkyl halide

Rate of $S_{N}2$ reaction $\propto \frac{1}{\text{ sterically hindrance }}$

$\therefore$ Rate is $1^{\circ }>2^{\circ }>3^{\circ }$ alkyl halide.

Hence, $Q>R>P$ is correct answer.