Sn2 is organic chemistry shorthand for substitution, nucleophilic, bimolecular. In an Sn2 reaction, a nucleophile such as ethoxide ion back-side attacks the substrate, forcing the substrate molecule through a pentacoordinate transition state, with the leaving group finally exiting the molecule. The stereochemistry at the target carbon atom is preserved (although inverted) during an Sn2 reaction.

Good leaving groups for Sn2 reactions include species that are poor bases as well as poor nucleophiles. Most of the halogens are fairly good leaving groups. Often used in organic synthesis is the tosylate leaving group.

Good substrates for Sn2 reactions are not sterically hindered. Tertiary substrates have too much steric hindrance, and will generally not undergo Sn2 under any conditions.

Reaction conditions, solvent, and the nucleophile must be chosen carefully, as the E2 reaction to form alkenes is always a competing reaction pathway.

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