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

(B) $\boldsymbol{S}>\boldsymbol{P}>\boldsymbol{R}>\boldsymbol{Q}$

(A) $\boldsymbol{P}>\boldsymbol{Q}>\boldsymbol{R}>\boldsymbol{S}$

(C) $\boldsymbol{P}>\boldsymbol{R}>\boldsymbol{Q}>\boldsymbol{S}$

(D) $\boldsymbol{R}>\boldsymbol{P}>\boldsymbol{S}>\boldsymbol{Q}$

JEE Advanced · 2013 · 02 Jun 2013 · Paper 1 · Haloalkanes and Haloarenes

K I

in acetone, undergoes

S_{N} 2

reaction with each of

P, Q, R

and

S

. The rates of the reaction vary as

(A) $P > Q > R > S$
(B) $S > P > R > Q$ ✓ Correct
(C) $P > R > Q > S$
(D) $R > P > S > Q$

Answer: (B)

Solution

S_{N} 2

reactions, the rate depends on steric hindrance at the carbon bearing the leaving group. Less hindered substrates react faster.

P: Primary alkyl chloride (least hindered)
Q: Secondary alkyl chloride (more hindered)
R: Allylic chloride (resonance stabilization, but still primary)
S: Benzylic chloride (resonance stabilization, primary)

Benzylic and allylic halides react faster than simple alkyl halides due to stabilization of the transition state. The order is:

S

(benzylic) >

P

(primary) >

R

(allylic) >

Q

(secondary).

JEE Advanced · 2013 · 02 Jun 2013 · Paper 1 · Single correct option

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JEE Advanced · 2013 · 02 Jun 2013 · Paper 1 · Haloalkanes and Haloarenes

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