What is Sn1 And Sn2.

S_N1 Reaction 

The S_N1 reaction is a substitution nucleophilic unimolecular reaction. It is a two-step reaction. In the first step, The carbon-halogen bond breaks heterolytically with the halogen retaining the previously shared pair of electrons. In the second step, the nucleophile reacts rapidly with the carbocation that was formed in the first step.

S_N2 Reaction

This reaction follows second order kinetics and the rate of reaction depends upon both haloalkane as well as participating nucleophile. Hence this reaction is known as substitution nucleophilic bimolecular reaction. In this reaction, the nucleophile attacks the positively charged carbon and the halogen leaves the group.

It is a one-step reaction. Both the formation of carbocation and exiting of halogen take place simultaneously. In this process, unlike the S_N1 mechanism, the inversion of configuration is observed. Since this reaction requires the approach of the nucleophile to the carbon bearing the leaving group, the presence of bulky substituents on or near the carbon atom has a dramatic inhibiting effect.