Explain the mechanism of the bimolecular nucleophilic substitution $(S_N2)$ reaction with the example of $CH_3Cl$.

(N/A) The $S_N2$ reaction mechanism was proposed by Edward $D$. Hughes and Sir Christopher Ingold in $1937$.
Key characteristics of the $S_N2$ reaction:
$(a)$ The reaction is bimolecular,meaning the rate depends on the concentration of both the substrate $(CH_3Cl)$ and the nucleophile $(OH^-)$.
$(b)$ It is a nucleophilic substitution reaction where the leaving group $(-Cl)$ is replaced by the nucleophile $(-OH)$ to form the product $CH_3OH$.
$(c)$ The reaction occurs in a single concerted step. As the nucleophile $(OH^-)$ attacks the carbon atom from the side opposite to the leaving group,the $C-Cl$ bond begins to break while the $C-OH$ bond begins to form. This leads to a transition state where the carbon is pentacoordinate. No intermediate is formed during this process.