nucleophilic substitution

Nucleophilic substitution (SN) is a class of chemical reactions in which a nucleophile replaces a functional group within another electrophile. The most general form of the reaction may be given as \[\ce{Nu{:} + R-LG -> R-Nu + LG{:}}\].

The electron pair from the nucleophile (\[\ce{Nu}\]) attacks the substrate \[\ce{R-LG}\] and bonds with it. Simultaneously, the leaving group (\[\ce{LG}\]) departs with an additional electron pair.

Nucleophilic substitution generally does not occur at sp2-hybridised carbons, either by the SN2 or SN1 pathway.