The order of the rate of S_N1 reactivity for | vedclass

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(B) The rate of $S_N1$ reaction depends on the stability of the carbocation intermediate formed after the departure of the leaving group.$I$ and $II$

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$I > II > IV > III$

Vedclass · Answer

(B) The rate of $S_N1$ reaction depends on the stability of the carbocation intermediate formed after the departure of the leaving group.$I$ and $II$ are $p$-methoxybenzyl halides. The $-OCH_3$ group at the para position provides strong $+M$ (mesomeric) effect,which stabilizes the benzylic carbocation significantly.Comparing $I$ ($p$-methoxybenzyl bromide) and $II$ ($p$-methoxybenzyl chloride),the bromide ion $(Br^-)$ is a better leaving group than the chloride ion $(Cl^-)$,so $I$ reacts faster than $II$.$IV$ is benzyl chloride,which forms a stable benzylic carbocation but lacks the additional $+M$ stabilization from the $-OCH_3$ group,so it is less reactive than $I$ and $II$.$III$ is $p$-chloroanisole,where the $Cl$ atom is directly attached to the benzene ring. $S_N1$ reaction at this position would form an unstable aryl cation,making it the least reactive.Thus,the order is $I > II > IV > III$.

The order of the rate of $S_N1$ reactivity for the following compounds is:

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