Write a note on $S_{N}2$ mechanism.

(N/A) The reaction in which the reaction rate depends on the concentration of both the substrate and the nucleophile is called an $S_{N}2$ reaction or substitution nucleophilic bimolecular reaction.
For example,the reaction between $CH_{3}Cl$ and the hydroxide ion $(OH^{-})$ to yield methanol $(CH_{3}OH)$ and the chloride ion $(Cl^{-})$ follows second-order kinetics:
$\text{Rate} = k[CH_{3}Cl][OH^{-}]$
The incoming nucleophile interacts with the alkyl halide,causing the carbon-halide bond to break as a new bond forms between the carbon and the attacking nucleophile. Here,a $C-O$ bond is formed between the $C$ atom and the $-OH$ group. These two processes occur simultaneously in a single step,and no intermediate is formed.
During the reaction,the bond between the incoming nucleophile and the carbon atom starts forming,while the bond between the carbon atom and the leaving group weakens. As a result,the carbon-hydrogen bonds of the substrate start moving away from the nucleophile. In the transition state,all three $C-H$ bonds are in the same plane,and both the attacking and leaving nucleophiles are partially attached to the carbon. Thus,in the transition state,the carbon is bonded to five atoms simultaneously. Such a structure is unstable and cannot be isolated.
As the attacking nucleophile approaches the carbon,the $C-H$ bonds continue to move in the same direction until the nucleophile attaches to the carbon and the leaving group departs. This results in the inversion of configuration,similar to an umbrella turning inside out in a strong wind.