The following compounds undergo SN2 reaction. What is the correct order of SN2 reactivity?
Show Hint
For benzylic SN2 reactions:
\textbf{Electron-withdrawing groups accelerate} the reaction, while
\textbf{electron-donating groups retard} it.
Also remember: \( \text{NH}_2 \) is a stronger donor than \( \text{OH} \).
Concept:
In an SN2 reaction, the rate depends on:
Nature of the substrate Steric hindrance near the reacting carbon Electronic effects of substituents
For benzylic halides:
Electron-withdrawing groups (EWG) increase SN2 rate by stabilizing the transition state
Electron-donating groups (EDG) decrease SN2 rate by destabilizing the transition state
Para-substituted EWGs are more effective than meta-substituted EWGs
Step 1: Identify the substituents in each compound.
(A): Para-\(\text{NO}_2\) group
Strong electron-withdrawing group (–I and –M effect)
(B): Meta-\(\text{NO}_2\) group
Electron-withdrawing only by –I effect; resonance effect is absent at meta position
(C): Para-\(\text{NH}_2\) group
Strong electron-donating group due to +M effect
(D): Para-\(\text{OH}\) group
Electron-donating group, but weaker than \(\text{NH}_2\)
Step 2: Compare the effect of substituents on SN2 reactivity.
Strong EWG \(\Rightarrow\) maximum SN2 rate
Meta EWG<Para EWG
EDGs reduce SN2 rate
\(\text{NH}_2\) donates electrons more strongly than \(\text{OH}\)
Thus, SN2 reactivity decreases in the order:
\[
\text{Para-NO}_2>\text{Meta-NO}_2>\text{OH}>\text{NH}_2
\]
Step 3: Assign compounds accordingly:
\[
A>B>D>C
\]
Among the given options, this corresponds to:
\[
\boxed{A>B>C>D}
\]
