Question

Which of the following is the correct order of decreasing $S_N2$ reactivity?

• A. $\ce{R_2CH X > R_3C X > RCH_2 X}$
• B. $\ce{RCH_2 X > R3C X > R2CH X}$
• C. $\ce{RCH2 X > R2CH X > R3C X}$
• D. $\ce{R3C X > R2CH X > RCH2 X}$ (X is a halogen)

Answer 1

Answer: (C)

$\ce{RCH2 X > R2CH X > R3C X}$

Answer 2

In $S_{N^2}$ mechanism transition state is pentavelent. For bulky alkyl group it will have sterical hinderance and smaller alkyl group will favour the $S_{N^2}$ mechanism. So the decreasing order of reactivity of alkyl halides is ${RCH2X > R2CHX > R3CX}$