Write a note on `S_(N)2` mechanism.

Explanation

The reaction in which the reaction rate depends both on the concentration of substrate and nucleophile is called `S_(N)2` reaction or substitution nucleophilic biomolecular reactions. For example, reaction between `CH_(3)-Cl` and hydroxide ion to yield methanol and chloride ion follows a second order kinetics.

The incoming nucleophile interacts with the alkyl halide causing the carbon - halide bond to break and a new bond is formed between carbon and attacking nucleophile. Here it is C - O bond formed between C and - OH. These two processes take place simultaneously in a single step and no intermediate is formed.
In a reaction, the bond between incoming nucleophile and the carbon atom starts forming, the bond between carbon atom and leaving group weakens and as a result, the carbon - hydrogen bonds of the substrate start moving away from the nuclephile. In a transition state all the three C - H bonds are in the same plane and the attacking and leaving nucleophiles are partially attached to the carbon. Thus, in a transition state carbon is bonded to five atoms simultaneoulsy. Such structure is unstable and cannot be isolated.
During the reaction, as the attacking nucleophile approaches closer to the carbon, C - H bonds still keep on moving in the same direction till attacking nucleophile attaches the carbon and leaving group leaves the carbon. This is known as inverted in a same way as an umbrella is inerted in a same way as umbrella is inverted inside out when caught in a strong wind.