Question

Which one of the following compounds can behave as a good substrate for both $S{N^1}_{}$ & $S{N^2}$ reactions.
A. $C{H_3}Cl$
B. $C{H_3}C{H_2}C{H_2}Cl$
C. $C{H_2}CHC{H_2}Cl$
D. ${(C{H_3})_3}CCl$

Answer 1

In $S{N^1}_{}$ reaction the first carbocation is formed by removing the leaving group. Then in the next step we see that the carbocation tends to be attacked by the nucleophile. Then in further steps we see that the protonated nucleophile undergoes the process of deprotonation for the desired product.

Complete step by step answer:
Let us first know what are $S{N^1}_{}$ and $S{N^2}$ reactions.
$S{N^1}_{}$ is a unimolecular nucleophilic substitution reaction which takes place in two steps and follows first order kinetics.
$S{N^2}$ is a bimolecular nucleophilic substitution reaction which takes place in a single step with the formation of 5 membered cyclic transition steps.
Now let us see the effect of substrate on these reactions.
In $S{N^1}_{}$ reaction the rate determining step is the formation of carbocation. Since tertiary carbocation is more stable than primary and secondary carbocation, hence this reaction prefers carbocation. The order of reactivity as follows.
${3^ \circ }$ carbocation > ${2^ \circ }$ carbocation > ${1^ \circ }$ carbocation
In $S{N^2}$ reaction the attacking and leaving of groups take place at the same time. Therefore, there is formation of 5 membered transition states. Hence the less sterically hindered substrate has been chosen. These reactions therefore prefer sterically less hindered groups. The order of reactivity as follows,
$C{H_3}X$> ${1^ \circ }$ carbon > ${2^ \circ }$ carbon > ${3^ \circ }$carbon
Hence the substrate which can be ideal for both $S{N^1}_{}$ &$S{N^2}$ is the compound containing secondary carbon. This is because it is favoured by both mechanisms. Hence the correct options can be option 2 and option3 but in option 3 the carbocation formed will be stabilised by resonance.

So, the correct answer is Option C.

Note: The step for the rate determination in these reactions depends on the interaction between the species which is the nucleophile and the organic compound. In the $S{N^2}$reaction we see that the nucleophile tends to attack the nucleophile from the back side of the carbon atom. This$S{N^2}$ reaction is the example of stereospecific reaction.