Question

Explain the mechanism of ${S_N}2$ reaction with one example.

Answer 1

${S_N}2$ reaction mechanism requires the attack of nucleophiles from the back side of the carbon atom, resulting in a stereochemical position opposite to the leaving group. It is an example of stereospecific reaction.

Complete step-by-step solution:
The ${S_N}2$ reaction is a nucleophilic substitution reaction where one bond is broken, and another is formed simultaneously. The two reacting species that are involved in the rate determining step of the reaction. Substitution nucleophilic by molecular reaction is also referred to as associative substitution and interchange mechanism.
This reaction is a second-order reaction, in which the rate determining step depends on concentration of nucleophile and the substrate. The reaction proceeds through a backside attack of nucleophiles on the substrate at an angle of $180^0$ to the carbon-leaving bond. The carbon-nucleophile bond forms and carbon-leaving bond breaks simultaneously via a transition state involved.
Now, the leaving group is pushed out of the transition state forming the required product on the opposite side of the carbon-nucleophile bond. The product formed is an inversion of the tetrahedral geometry at the central atom.
There are two ways in which a nucleophile can attack the substrate. A front side attack results in retention of stereochemical configuration in the product. A backside attack results in the inversion of stereochemical configuration in the product. As we get a hundred percent inversion in stereochemical configuration of ${S_N}2$ reactions, it occurs through a backside attack.
For example,

Note: Primary substrates undergo nucleophilic substitution easily as their back is unhindered. Nucleophilicity increases with a more negative charge and speeds up the rate of reaction. Polar aprotic solvents do not hinder the nucleophile and acetone is one such good solvent.