What is the stereochemistry of Sn2

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Course of the reaction

During the reaction, the nucleophile approaches the substrate from the rear.

The non-bonding π-orbital of the nucleophile (free electron pair) interacts with the antibonding σ * -orbital of the C-ligand bond of the substrate.

As the antibonding orbital is occupied, the C-ligand bond is weakened. As the new Nu-C bond is formed, the bond length of the C-ligand bond increases. As the length of the carbon ligand bond increases, the other groups on the carbon atom begin to fold over like an umbrella.

In the transition state the central carbon atom is sp2Hybridized and three remaining substitutes are arranged in a planar manner. The negative charge is localized on the nucleophile and the leaving group. The product is finally formed by the complete elimination of the leaving group.

When the three remaining substitutes are flipped over during the reaction, the configuration at the central carbon is inverted (Walden reversal). If this central carbon is a center of chirality and if one of the two enantiomers is assumed, a pure enantiomer with the mirror-image absolute configuration is obtained. If the nucleophile that has entered has the same priority as the leaving group, one arises (R)Molecule(S) -Molecule and vice versa. It should be noted, however, that if the priority of nucleophiles and leaving group is different, the application of the priority rules for the (R / S)- or Cahn-Ingold-Prelog nomenclature (CIP nomenclature) sometimes in a reaction from a (R)-Connection also back on (R)-Enantiomer is created. Here, too, the absolute configuration has been reversed, but the naming is retained due to the other priority of the new substitute.