Which of the following is the correct order of decreasing $\mathrm{SN}^{2}$ reactivity? ($\mathrm{X}=\mathrm{a}$ halogen)

(B) $\mathrm{RCH}_{2} \mathrm{X}>\mathrm{R}_{2} \mathrm{CHX}>\mathrm{R}_{3} \mathrm{CX}$

(A) $\mathrm{RCH}_{2} \mathrm{X}>\mathrm{R}_{3} \mathrm{CX}>\mathrm{R}_{2} \mathrm{CHX}$

(C) $\mathrm{R}_{3} \mathrm{CX}>\mathrm{R}_{2} \mathrm{CHX}>\mathrm{RCH}_{2} \mathrm{X}$

(D) $\mathrm{R}_{2} \mathrm{CHX}>\mathrm{R}_{3} \mathrm{CX}>\mathrm{RCH}_{2} \mathrm{X}$

JEE Main · 2007 · 29 Apr 2007 · Offline · Haloalkanes and Haloarenes

Which of the following is the correct order of decreasing

SN^{2}

reactivity?
(

X = a

halogen)

(A) $RCH_{2} X > R_{3} CX > R_{2} CHX$
(B) $RCH_{2} X > R_{2} CHX > R_{3} CX$ ✓ Correct
(C) $R_{3} CX > R_{2} CHX > RCH_{2} X$
(D) $R_{2} CHX > R_{3} CX > RCH_{2} X$

Answer: (B)

Solution

S_{N} 2

substitution goes through a crowded transition state in which the nucleophile attacks the carbon from the side opposite the halogen. Added bulk around that carbon hinders the approach, so the rate falls as the carbon becomes more substituted. The order of decreasing reactivity is therefore primary

RC H_{2} X

, then secondary

R_{2} CHX

, then tertiary

R_{3} CX

JEE Main · 2007 · 29 Apr 2007 · Offline · Single correct option

Practise more: all JEE Main PYQs on Haloalkanes and Haloarenes →

JEE Main · 2007 · 29 Apr 2007 · Offline · Haloalkanes and Haloarenes

Solution

More JEE Main questions on Haloalkanes and Haloarenes