Explain `SN^(2)` reaction

Explanation

Nucleophilic Bimolecular substitution Reaction `S_(N) 2` :
1) The nucleophilic substitution reaction in which rate depends upon concentration of both reactants is called `S_(N) 2` reaction .
2) It follows `2^(nd)` order kinetics . So it is called bimolecular reaction .
Eg : Methyl chloride reacts with hydroxide ion and forms methanol and chloride ion.
3) Here the rate of reaction depends upon the concentration of two reactants .

4) In the above mechanism the configuration of carbon atom under attack inverts in much the same way as an umbrella is turned inside out when caught in a strong wind . This process is called inversion of configuration .
5) In transition state the carbon atom is simultaneously bonded to the incoming nucleophile and out going group . it is very unstable .
6) The order of reactivity for `S_(N) 2` reactions follows :
`1^(@)`-alkyl halides `gt 2^(@)`-alkyl halides `gt 3^(@)` - alkyl halides .