Alkyl Halides - Result Question 14
####14. The increasing order of reactivity of the following halides for the $S _N 1$ reaction is
(2017 Main) I. $CH _3 CH(Cl) CH _2 CH _3$
II. $CH _3 CH _2 CH _2 Cl$
III. $p-H _3 CO-C _6 H _4-CH _2 Cl$
(a) (III) $<$ (II) $<$ (I)
(b) (II) $<$ (I) $<$ (III)
(c) (I) $<$ (III) $<$ (II)
(d) (II) $<$ (III) $<$ (I)
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Solution:
- (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.
$$ R-X \rightleftharpoons R^{\oplus}+X^{\Theta} $$
Higher the stability of $R^{+}$(carbocation), higher would be the reactivity towards $S _N 1$ reaction.
$p-H _3 CO-C _6 H _4-CH _2{ }^{\oplus}$ is the most stable carbocation due to resonance and then $CH _3 \underset{\oplus}{CHCH _2} CH _3$ (2 $2^{\circ}$ carbocation) while $CH _3 CH _2 CH _2\left(1^{\circ}\right)$ is least stable.
Thus, the correct increasing order of the reactivity of the given halides towards the $S _N 1$ reaction is
$CH _3 CH _2 CH _2 Cl<\left.CH _3\right| _{Cl} ^{CHCH _2 CH _3}<p-H _3 COC _6 H _4 CH _2 Cl$ (I)
