Question

Which of the following is correct order regarding nucleophilic substitution?

• A. $ArSN:I>II$
• B. $S_{N}1:IV>III$
• C. $ArSN:II>I$
• D. $S_{N}2:III>IV$

Answer 1

The correct option is C $ArSN:II>I$

For $S_{N}1$ mechanism, the order of reactivity is dependent on the stability of the intermediate carbocation formed by the cleavage of $C-X$ bond. The $3^{\circ }$ carbocation (formed from $III$) will be more stable than its $2^{\circ }$ counter part (formed from $IV$) .
For $S_{N}1$ mechanism, steric factor plays important role.
$II$ will be more reactive than $I$ towards $ArSN$ due to the presence of the electron withdrawing ${\text{NO}}_2$ group.