i didn't understood the meaning of SN1 and SN2 rxn mechanisms
The term SN2 means that two molecules are involved in the actual transition state:
The departure of the leaving group occurs simultaneously with the backside attack by the nucleophile.
The SN2 reaction thus leads to a predictable configuration of the stereocenter - it proceeds with inversion (reversal of the configuration).
In SN1 reactions means nucleophilic substitution unimolecular. The reaction rate depends only on the concentration of the reactant and not on the nucleophile.
In the SN1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization.