Question

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

• A. $R_2CH X > R_3C X > RCH_2 X$
• B. $RCH_2 X > R_3C X > R_2CH X$
• C. $RCH_2 X > R_2CH X > R_3C X$
• D. $R_3C X > R_2CH X > RCH_2 X (X$ is a halogen)

Answer 1

Answer: (C)

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 $RCH_2X > R_2CHX > R_3CX$