Question

The increasing order of reactivity of the following halides for the $S_{N}1$ reaction is
$\left(I\right).C{H}_{3}CH\left(Cl\right)C{H}_{2}C{H}_3$
$\left(II\right).C{H}_{3}C{H}_{2}C{H}_{2}Cl$
$\left(III\right).p-H_{3}CO-C_6{H}_4-C{H}_{2}Cl$

• A. $\left(III\right)<\left(II\right)<\left(I\right)$
• B. $\left(II\right)<\left(I\right)<\left(III\right)$
• C. $\left(I\right)<\left(III\right)<\left(II\right)$
• D. $\left(II\right)<\left(III\right)<\left(I\right)$

Answer 1

The correct option is B $\left(II\right)<\left(I\right)<\left(III\right)$

(i) The rate of $S_{N}1$ reaction depends only upon the concentration of the alkyl halide.
(ii) $S_{N}1$ reaction proceeds through the formation of carbocation. The reactivity is decided by ease of dissociation of alkyl halide.
$R-X\rightleftharpoons R^{⨁}+X^{⊝}$
Higher the stability of $R^{+}$ (carbocation), higher would be the reactivity towards $S_{N}1$ reaction.
$p-H_{3}CO-C_6{H}_4-C{H}_2^{\oplus }$ is the most stable carbocation due to resonanance ond then $C{H}_3\stackrel{+}{CH}C{H}_{2}C{H}_3$ $(2^{\circ }$carbocation) while $C{H}_{3}C{H}_2\stackrel{\oplus }{C}{H}_2\left(1^{\circ }\right)$ is least stable. Thus, the correct increasing order of the reactivity of the given halides towards the $S_{N}1$ reaction is: