In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
However, if the substrate is a chiral molecule, inversion of configuration is more prominent. When the α-carbon undergoing substitution is chiral, the stereochemistry of the product is reversed. Molecules with R configuration result in products with S stereochemistry and vice-versa. Similarly, an inversion of configuration of the product is observed in case of a cyclic substrate. Substrates in cis form becomes products with a trans form. Thus, SN2 reactions are said to be stereospecific reactions.
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