Question

Which of the following is not stereospecific?
$A.$ $S{N^1}$
$B.$ $S{N^2}$
$C.$ Electrophilic substitution
$D.$ Addition $B{r_2}$ to ethylene in $CC{l_4}$

Answer 1

Stereospecific is defined as the reaction in which the stereochemistry of the reactant controls the outcome of the reaction. In general we can say that one stereoisomer of a certain reactant produces one stereoisomer of a certain product whereas different stereoisomers of the same reactant produce a different stereoisomer of the same product. For example the addition of molecular bromine to $2 - butene$ .

Complete step-by-step answer:
In the given question we have to find which of the reactions is not stereospecific. Let's see them one by one.
For $S{N^1}$ reaction, the $S{N^1}$ reaction proceeds through two steps. In the first step the carbocation is formed from the removal of the leaving group. Then the carbocation is attacked by the nucleophile. Finally deprotonation of the protonated nucleophile takes place to give the required product. This is a unimolecular reaction, thus the order of the reaction is one. The rate of the reaction depends upon the carbocation. The reaction is non-stereospecific as (attack by nucleophile can occur from both sides).
The reaction proceeds through the transition state formed . This is a bimolecular reaction thus the rate of the reaction depends on both the reactant. in this reaction mechanism the basic configuration of the product is changed so it is stereospecific(Walden Inversion of configuration ).
For electrophilic reaction is stereospecific reaction as (Anti-periplanar geometry preferred, syn-periplanar geometry is possible. In this reaction all bonds form and break at the same time. It is also bimolecular –rate depends on concentration of both the base and substrate. This reaction is favoured by a strong base.
Addition $B{r_2}$ to ethylene in $CC{l_4}$ is also a stereospecific reaction. The most interesting feature of this reaction is that the product follows a very predictable stereochemical pattern. For example in the reaction of cyclohexene with $B{r_2}$ , the two bromine atoms add to opposite faces of the alkenes (anti stereochemistry).

Thus, the correct option is $\left( A \right)$ .

Note: The main difference between $S{N^1}$ reaction and $S{N^2}$ reaction given below in the table.

$S{N^1}$ reactions	$S{N^2}$ reactions
These reactions are unimolecular reaction.	These reactions are bimolecular reactions.
These reactions are non-stereospecific.	These reactions are stereospecific.
Rate of reaction depends only on one reactant.	Rate of reaction depends on both the reactants.