CBSE Class 12-science Answered

The term S_N2 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 S_N2 reaction thus leads to a predictable configuration of the stereocenter - it proceeds with inversion (reversal of the configuration).

In S_N1 reactions means nucleophilic substitution unimolecular. The reaction rate depends only on the concentration of the reactant and not on the nucleophile.

In the S_N1 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.