Q.Explain SN2 reaction mechanism.

SN2 is a one-step, concerted backside attack by the nucleophile on the carbon bearing the leaving group, going through a pentacoordinate transition state and inverting the stereochemistry at that carbon.

SN2 stands for Substitution, Nucleophilic, Bimolecular — "bimolecular" because the rate-determining step involves collision of two species: the substrate (alkyl halide, R-X) and the nucleophile (Nu-).

Mechanism, step by step:

1. The nucleophile (Nu-, electron-rich) approaches the carbon bearing the leaving group (X) from the side directly OPPOSITE to X (backside attack) — this is because the leaving group's electron cloud repels the approaching nucleophile from its own side, and backside attack allows the nucleophile's lone pair to overlap with the empty, antibonding sigma* orbital of the C-X bond most effectively.
2. As the nucleophile begins forming a new bond to carbon, the C-X bond simultaneously begins to break — both processes happen together, in a single concerted step (there is no discrete intermediate carbocation, unlike SN1).
3. At the midpoint of the reaction, a transition state is formed in which the carbon is briefly bonded to FIVE groups (bonded partially to both Nu and X, plus the three other original substituents) arranged in a trigonal bipyramidal-like geometry — this pentacoordinate transition state is the defining feature of SN2.
4. As the reaction completes, the C-X bond fully breaks (X- leaves) and the C-Nu bond fully forms, with the three other substituents on carbon flipping to the opposite side — like an umbrella turning inside out. This is called inversion of configuration (Walden inversion). …