JNTU Pharm. D I Year PHARMACEUTICAL ORGANIC CHEMISTRY Previous Question Paper (June/July - 2019 - R08)

The S_N2 reaction (Substitution Nucleophilic Bimolecular) is a concerted, one-step process where the nucleophile attacks the electrophilic carbon atom from the side opposite to the leaving group. As the nucleophile forms a new bond, the bond to the leaving group breaks simultaneously, passing through a pentacoordinate transition state where the central carbon is bonded to five groups. The rate of the reaction depends on the concentration of both the substrate and the nucleophile, following second-order kinetics: Rate = k[Substrate][Nucleophile].

S_N2 reactions proceed with Walden inversion (inversion of configuration). Because the nucleophile attacks from the backside (180 degrees away from the leaving group), the spatial arrangement of the groups attached to the chiral carbon is inverted, similar to an umbrella turning inside out in a strong wind. If the starting material is optically active, the product will have the opposite configuration.