Question

$KI$ in acetone, undergoes $S_{N}2$ reaction with each $P,Q,R$ and $S$. The rates of the reaction vary as

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

Answer 1

Answer: (B)

PLAN Acetone is an aprotic solvent and can dissolve both the nucleophile and the substrate and thus $5^2$ reaction is favoured.
$S_{N}2\stackrel{1^{\circ }{2}^{\circ }{3}^{\circ }}{\leftarrow }$ Alkyl halides

$\to S_{N}1$

S		$1^{\circ }$ alkyl halide but $(C-Cl)\cdot B\cdot E$. is decreased by electron withdrawing $\left[C_6{H}_{5}CO\right]$ group, (a case of $I$ -effect) Thus, maximum rate in $S_{N}2$ reaction
Q		$2^{\circ }$ alkyl halide, rate is minimum
R	$C{H}_3-Cl$	$1^{\circ }$ alkyl halide
S		$1^{\circ }$ allylic halide but allylic $1^{\circ }$ carbocation is resonance stabilised in $S_N$ Ireaction

Thus, reactivity order is $S>P>R>Q$