Dear Candidate,

The S N 1 reaction is a nucleophilic substitution unimolecular reaction. It is a 2 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. This kind of rxn. follows first order kinetics.

Wheras, the Sn2 reaction is 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 rxn. Both the formation of carbocation and exiting of halogen take place simultaneously. In this process, unlike the S _N 1 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.

Hope it helps.